How close would the sun have to get to Earth for there to be severe consequences?

How close would the sun have to get to Earth for there to be severe consequences?

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According to the Earth gets 3 million miles closer to the Sun than its usual distance of about 93 million miles on average.

My question is, how close would the earth have to be to the sun for there to be problems with our survival?

I'm going to use Gm (1 gigameter = $1 imes10^9$m) and degrees Celsius for this answer.

By getting closer to the Sun, probably the most major problem would be the increase in temperature.

A while ago I wrote a program that calculates the effective surface temperature of a planet. Having dug it back out, I've played around with the values a bit. It's important to note that the effective temperature is not the same as the actual surface temperature, since it doesn't account for Earth's atmosphere. If Earth had no atmosphere, it would be the correct actual temperature.

At our current distance of (avg.) 149.6Gm, the effective temperature is 257K, or -16°C (some websites cite other values ±2°C). NASA cites an actual average temperature of 15°C. Assuming a linear relationship between effective temperature and actual temperature, we can assume that $T_{actual} = T_{eff}+31 pm 2$.

So, what would happen if we moved, say, 10Gm closer?

Well, the effective surface temp is now -7°C, and I'm guessing the actual temperature would be about 24°C. This is an increase of 9°C. We would probably still be able to survive, but with difficulties. For starters, the sea level would rise by over 40m, leading the world to look (at the best) like this. Not an easy situation, but nevertheless survivable. Of course, there would be other effects like an increase in extreme weather, the mass extinction of species, and probably a whole load of things that are hard to foresee, but I'm not going to try to predict them.

So, what about a move towards the Sun of 20Gm?

The effective temperature is now ~3°C, and the surface temperature is about 34°C. Things start to get bad now. All of the ice caps have melted. Once fertile areas are now barren deserts. Surviving is hard, but possible, although famine is now a major problem across much of the world (especially around the equator). The Gulf Stream may have stopped, oddly enough cooling down some of western Europe and all of Britain. It's not looking good.

What about 50Gm?

The effective temperature is 42°C; the actual one is around 73°C. Where the Sun's habitable zone begins and ends is disputed, but now, at 0.65AU away from the sun, it's quite likely that we're not in it. It's very hard to predict what happens now. Humans would likely have to stay underground to remain alive, and food would be a major issue. The ecosystem would be pretty much destroyed, having not had a chance to adapt to the new temperature.

For fun, if we moved 100Gm closer, we'd be at about 205°C. Ouch. We're closer than Mercury now, and look how that planet's coping. Mercury ranges from -173°C to 427°C depending on various factors. Survival without massive life support is not possible.

Any closer, and things just get worse.

Hope this is a suitable answer for your question!

The Earth passes within 91.4 million miles for just a brief time, then it moves back further out and 6 months later it's 94.5. That 3.4% and change closer works out to 7% more solar energy, but that's just for closest and furthest points. Over the closest and furthest month, the variation is smaller and (perhaps obviously), it averages out over the entire year.

There's two ways to address to this question - how eccentric the Earth's orbit can get which would make the perihelion closer and aphelion further, or, the 2nd way, how much closer can you make the semi-major axis, which defines orbital period.

Earth's present eccentricity is 0.017 and decreasing (Wikipedia article above). For simple approximation and low eccentricity orbits, the eccentricity 0.017 translates into (doubling it) a 0.034 (or 3.4%) variation perihelion to aphelion, which works out to a (1.034^2), about a 7% variation in solar energy.

Earth's peak eccentricity of 0.0679, (this time I'll do the math), closest point (1-0.0679) = 0.9321 and farthest (1+0.0679)= 1.0679. The ratio 1.0679/0.9321 = about 14.5% which works into a 31.3% variation in solar energy closest to farthest. 14.5% and 31% energy variation may sound like a lot, but consider that the winter/summer solar energy variation in latitudes away from the equator can be considerably over 100%. So, 31% isn't world ending, in fact, the Earth handles those 31% days without too much trouble, though they can trigger or end ice ages depending on how it lines up with other Milankovich cycles. We should also remember that it's 31% hottest to coldest, it's about 14.5% hottest to average, and again, just for one day then that number begins to drop.

14.5% size change isn't as much as it sounds either, that's about the variation between the largest full moon or supermoon and the smallest (micromoon), though the average full moon variation is between perigee and apogee 11% and 12% in diameter. How often do you look at a full moon and say "that's bigger than the full moon 4 months ago". That's not to say nobody notices, but many of us wouldn't recognize a supermoon without being told when to look for it, though it's fairly obvious if we could see them side to side.

So what effect does Earth's periodic 0.0679 eccentricity have? Apart from triggering or perhaps ending an ice age, not that much… You could push Earth's eccentricity pretty far without much danger to Earth, though past a certain point, crashing into Venus might become a concern.

Lets have some fun and push Earth's eccentricity up to 0.15 (that's about half way between Mars' and Mercury's eccentricity.

Perihelion 0.85% of the semi-major axis, Aphelion 1.15%. At perihelion the solar intensity would be (1/.85)^2 = 38% stronger than normal. That might cause scorching heat in some places where the Perihelion lined up with summer, and you might get some bitter cold weather when the Aphelion lines up with winter (note, this doesn't always happen, perihelion slowly cycles around the calendar every 26,000 years or so). But when it lines up you might see some seasonal-weather extreemes, but Earth could (I think) survive even a 0.15 eccentricity. The views of Venus at Perihelion would be impressive too, Venus would stay in the sky longer and be larger and brighter.

Push Earth's eccentricity to about .26 and Earth and Venus would get uncomfortabaly/scary close and might crash into each other. Fun to think about, but a potential planet killer.

But apart from creating potentially greater variation in seasons, some very wild weather and perhaps triggering an ice age (perhaps the biggest ice age since snow-ball Earth) or accelerating climate change (it all depends on how the 3 Milankovich cycles line up with each other), You could push Earth's orbit surprisingly close to the Sun, half way to Venus or even closer at perihelion, without ending life on the planet.

Now, if you adjust Earth's semi-major axis, which makes the orbit faster or slower, and changes the length of the year - with that, there's much less variation. @JThistle covered this. If you push Earth's semi-major axis just a few percentage points closer to the sun, Earth would heat up and it would get bad quickly. In fact, I think his estimates are conservative. There are already some pretty good estimates out there on this, just google "what will the Earth be like in 500 million years or a billion years" for a few articles. Here's one. The sun is growing slowly larger and more luminous, about 1% every 100 million years. That will be a significant problem as soon as 500-600 million years from now, perhaps less. Likewise, just a 2.5%-3% push closer to the sun would have a similar effect. You can't reduce Earth's semi-major axis by much without causing serious problems. Just 1% might be enough to melt Greenland over time, raise sea levels and prevent future ice ages.

Similarly if you push Earth just 1-2% further away, Earth could enter permanent ice ages (fortunately we can fix that by burning oil and coal), but sans man made greenhouse gas, just 1%-2% further away would trigger perhaps a permanent ice age for the next several million years and perhaps, dangerously low CO2 levels, at least until Antarctica drifts north enough for it's ice to melt. It would be nice to be told to burn oil for the "good of the planet" though. :-)

This is all very ballpark, but the Earth is, surprisingly, at a very good distance from the sun and just a few % one way or the other could be very bad, though in 15-25 million years when Antarctica is brushing up against South America and is no longer covered in ice, when that happens, Earth will have greater wiggle room away from the sun, but not closer to the sun.

Precise mathematical answers to this question are obviously impossible. I'm just giving approximations.

Without any significant change in orbital distance, there have been a variety of long-term (millions of years) tropic or ice ages, and transitions from one to another led to rather a lot of extinctions.
In the same vein, a nice big solar eruption could wreak havoc independent of orbital distance changes.

I"m not arguing with the other answers; just pointing out that time scale matters.

Giant planet at large distance from sun-like star puzzles astronomers

A direct image of the exoplanet YSES 2b (bottom right) and its star (centre). The star is blocked by a so-called coronagraph. Credit: ESO/SPHERE/VLT/Bohn et al.

A team of astronomers led by Dutch scientists has directly imaged a giant planet orbiting at a large distance around a sun-like star. Why this planet is so massive and how it got to be there is a mystery. The researchers will publish their findings in the journal Astronomy & Astrophysics.

The planet in question is YSES 2b, located 360 light years from Earth in the direction of the southern constellation of Musca (Latin for The Fly). The gaseous planet is six times heavier than Jupiter, the largest planet in our solar system. The newly discovered planet orbits 110 times more distant from its star than the Earth does from the sun (or 20 times the distance between the sun and Jupiter). The accompanying star is only 14 million years old and resembles our sun in its childhood.

The large distance from the planet to the star presents a puzzle to astronomers because it does not seem to fit either of the two most well-known models for the formation of large gaseous planets. If the planet had grown in its current location far from the star by means of core accretion, it would be too heavy because there is not enough material to make a huge planet at this large distance from the star. If the planet was created by so-called gravitational instability in the planetary disk, it appears to be not heavy enough. A third possibility is that the planet formed close to the star by core accretion and then migrated outwards. Such a migration, however, would require the gravitational influence of a second planet, which the researchers have not yet found.

The astronomers will continue to investigate the surroundings of this unusual planet and its star in the near future and hope to learn more about the system, and they will continue to search for other gaseous planets around young, sun-like stars. Current telescopes are not yet large enough to carry out direct imaging of earth-like planets around sun-like stars.

Lead researcher Alexander Bohn (Leiden University): "By investigating more Jupiter-like exoplanets in the near future, we will learn more about the formation processes of gas giants around sun-like stars."

The planet YSES 2b was discovered with the Young suns Exoplanet Survey (YSES). This survey already provided the first direct image of a multi-planet system around a sun-like star in 2020. The researchers made their observations in 2018 and 2020 using the Very Large Telescope of the European Southern Observatory (ESO) in Chile. They used the telescope's SPHERE instrument for this. This instrument was co-developed by the Netherlands and can capture direct and indirect light from exoplanets.

Solar Orbiter makes first close approach to the sun

ESA's Sun-explorer Solar Orbiter reached its first perihelion, the point in its orbit closest to the star, on 15 June 2020, getting as close as 77 million kilometres to the star's surface. Credit: ESA/Medialab

ESA's sun-exploring mission Solar Orbiter has made its first close approach to the star on June 15, getting as close as 77 million kilometers to its surface, about half the distance between the sun and Earth.

In the week following this first perihelion, the point in the orbit closest to the sun, the mission scientists will test the spacecraft's ten science instruments, including the six telescopes on-board, which will acquire close-up images of the sun in unison for the first time. According to ESA's Solar Orbiter Project Scientist Daniel Müller, the images, to be released in mid-July, will be the closest images of the sun ever captured.

"We have never taken pictures of the sun from a closer distance than this," Daniel says. "There have been higher resolution close-ups, e.g. taken by the four-meter Daniel K. Inouye Solar Telescope in Hawaii earlier this year. But from Earth, with the atmosphere between the telescope and the sun, you can only see a small part of the solar spectrum that you can see from space."

NASA's Parker Solar Probe, launched in 2018, makes closer approaches. The spacecraft, however, doesn't carry telescopes capable of looking directly at the sun.

"Our ultraviolet imaging telescopes have the same spatial resolution as those of NASA's Solar Dynamic Observatory (SDO), which takes high-resolution images of the sun from an orbit close to Earth. Because we are currently at half the distance to the sun, our images have twice SDO's resolution during this perihelion," says Daniel.

Solar Orbiter’s first close approach to the sun. Credit: European Space Agency

The primary objective of these early observations is to prove that Solar Orbiter's telescopes are ready for future scientific observations.

"For the first time, we will be able to put together the images from all our telescopes and see how they take complementary data of the various parts of the sun including the surface, the outer atmosphere, or corona, and the wider heliosphere around it," says Daniel.

The scientists will also analyze data from the four in-situ instruments that measure properties of the environment around the spacecraft, such as the magnetic field and the particles making up the solar wind.

"This is the first time that our in-situ instruments operate at such a close distance to the sun, providing us with a unique insight into the structure and composition of the solar wind," says Yannis Zouganelis, ESA's Solar Orbiter Deputy Project Scientist. "For the in-situ instruments, this is not just a test, we are expecting new and exciting results."

Solar Orbiter, launched on 10 February this year, is completing its commissioning phase on 15 June and will commence its cruise phase, which will last until November 2021. During the main science phase that follows, the spacecraft will get as close as 42 million kilometers to the sun's surface, which is closer than the planet Mercury.

Animation showing the trajectory of Solar Orbiter around the sun, highlighting the gravity assist manoeuvres that will enable the spacecraft to change inclination to observe the sun from different perspectives. Credit: European Space Agency

The spacecraft will reach its next perihelion in early 2021. During the first close approach of the main science phase, in early 2022, it will get as close as 48 million kilometers.

Solar Orbiter operators will then use the gravity of Venus to gradually shift the spacecraft's orbit out of the ecliptic plane, in which the planets of the Solar System orbit. These fly-by maneuvers will enable Solar Orbiter to look at the sun from higher latitudes and get the first ever proper view of its poles. Studying the activity in the polar regions will help the scientists to better understand the behavior of the sun's magnetic field, which drives the creation of the solar wind that in turn affects the environment of the entire Solar System.

Since the spacecraft is currently 134 million kilometers from Earth, it will take about a week for all perihelion images to be downloaded via ESA's 35-m deep-space antenna in Malargüe, Argentina. The science teams will then process the images before releasing them to the public in mid-July. The data from the in-situ instruments will become public later this year after a careful calibration of all individual sensors.

"We have a nine-hour download window every day but we are already very far from Earth so the data rate is much lower than it was in the early weeks of the mission when we were still very close to Earth," says Daniel. "In the later phases of the mission, it will occasionally take up to several months to download all the data because Solar Orbiter really is a deep space mission. Unlike near-Earth missions, we can store a lot of data on-board and downlink it when we are closer to home again and the data connection is much better."

Humongous flare from sun's nearest neighbor breaks records

Artist's conception of the violent stellar flare from Proxima Centauri discovered by scientists in 2019 using nine telescopes across the electromagnetic spectrum, including the Atacama Large Millimeter/submillimeter Array (ALMA). Powerful flares eject from Proxima Centauri with regularity, impacting the star's planets almost daily. Credit: NRAO/S. Dagnello

Scientists have spotted the largest flare ever recorded from the sun's nearest neighbor, the star Proxima Centauri.

The research, which appears today in The Astrophysical Journal Letters, was led by the University of Colorado Boulder and could help to shape the hunt for life beyond Earth's solar system.

CU Boulder astrophysicist Meredith MacGregor explained that Proxima Centauri is a small but mighty star. It sits just four light-years or more than 20 trillion miles from our own sun and hosts at least two planets, one of which may look something like Earth. It's also a red dwarf, the name for a class of stars that are unusually petite and dim.

Proxima Centauri has roughly one-eighth the mass of our own sun. But don't let that fool you.

In their new study, MacGregor and her colleagues observed Proxima Centauri for 40 hours using nine telescopes on the ground and in space. In the process, they got a surprise: Proxima Centauri ejected a flare, or a burst of radiation that begins near the surface of a star, that ranks as one of the most violent seen anywhere in the galaxy.

"The star went from normal to 14,000 times brighter when seen in ultraviolet wavelengths over the span of a few seconds," said MacGregor, an assistant professor at the Center for Astrophysics and Space Astronomy (CASA) and Department of Astrophysical and Planetary Sciences (APS) at CU Boulder.

The team's findings hint at new physics that could change the way scientists think about stellar flares. They also don't bode well for any squishy organism brave enough to live near the volatile star.

"If there was life on the planet nearest to Proxima Centauri, it would have to look very different than anything on Earth," MacGregor said. "A human being on this planet would have a bad time."

The star has long been a target for scientists hoping to find life beyond Earth's solar system. Proxima Centauri is nearby, for a start. It also hosts one planet, designated Proxima Centauri b, that resides in what researchers call the habitable zone—a region around a star that has the right range of temperatures for harboring liquid water on the surface of a planet.

But there's a twist, MacGregor said: Red dwarves, which rank as the most common stars in the galaxy, are also unusually lively.

"A lot of the exoplanets that we've found so far are around these types of stars," she said. "But the catch is that they're way more active than our sun. They flare much more frequently and intensely."

Artist's conception of a violent stellar flare erupting on neighboring star, Proxima Centauri. The flare is the most powerful ever recorded from the star, and is giving scientists insight into the hunt for life in M dwarf star systems, many of which have unusually lively stars. Artist's conception of a violent stellar flare erupting on neighboring star, Proxima Centauri. The flare is the most powerful ever recorded from the star, and is giving scientists insight into the hunt for life in M dwarf star systems, many of which have unusually lively stars. Credit: NRAO/S. Dagnello

To see just how much Proxima Centauri flares, she and her colleagues pulled off what approaches a coup in the field of astrophysics: They pointed nine different instruments at the star for 40 hours over the course of several months in 2019. Those eyes included the Hubble Space Telescope, the Atacama Large Millimeter Array (ALMA) and NASA's Transiting Exoplanet Survey Satellite (TESS). Five of them recorded the massive flare from Proxima Centauri, capturing the event as it produced a wide spectrum of radiation.

"It's the first time we've ever had this kind of multi-wavelength coverage of a stellar flare," MacGregor said. "Usually, you're lucky if you can get two instruments."

The technique delivered one of the most in-depth anatomies of a flare from any star in the galaxy.

The event in question was observed on May 1, 2019 and lasted just 7 seconds. While it didn't produce a lot of visible light, it generated a huge surge in both ultraviolet and radio, or "millimeter," radiation.

"In the past, we didn't know that stars could flare in the millimeter range, so this is the first time we have gone looking for millimeter flares," MacGregor said.

Those millimeter signals, MacGregor added, could help researchers gather more information about how stars generate flares. Currently, scientists suspect that these bursts of energy occur when magnetic fields near a star's surface twist and snap with explosive consequences.

In all, the observed flare was roughly 100 times more powerful than any similar flare seen from Earth's sun. Over time, such energy can strip away a planet's atmosphere and even expose life forms to deadly radiation.

That type of flare may not be a rare occurrence on Proxima Centauri. In addition to the big boom in May 2019, the researchers recorded many other flares during the 40 hours they spent watching the star.

"Proxima Centauri's planets are getting hit by something like this not once in a century, but at least once a day if not several times a day," MacGregor said.

The findings suggest that there may be more surprises in store from the sun's closest companion.

"There will probably be even more weird types of flares that demonstrate different types of physics that we haven't thought about before," MacGregor said.

Deadly stars: Our sun could also be superflare star

Earth is often struck by solar eruptions. These eruptions consist of energetic particles that are hurled away from the Sun into space, where those directed towards Earth encounter the magnetic field around our planet. When these eruptions interact with Earth's magnetic field they cause beautiful auroras. A poetic phenomenon that reminds us, that our closest star is an unpredictable neighbor.

When the Sun pours out gigantic amounts of hot plasma during the large solar eruptions, it may have severe consequences on Earth. Solar eruptions are, however, nothing compared to the eruption we see on other stars, the so-called 'superflares'. Superflares have been a mystery since the Kepler mission discovered them in larger numbers four years ago.

Questions arose: Are superflares formed by the same mechanism as solar flares? If so, does that mean that the Sun is also capable of producing a superflare?

An international research team led by Christoffer Karoff from Aarhus University, Denmark, has now provided answers to some of these questions. These alarming answers are published in Nature Communications.

The dangerous neighbor

The Sun is capable of producing monstrous eruptions that can break down radio communication and power supplies here on Earth. The largest observed eruption took place in September 1859, where gigantic amounts of hot plasma from our neighboring star struck Earth.

On 1 September 1859, astronomers observed how one of the dark spots on the surface of the Sun suddenly lit up and shone brilliantly over the solar surface. This phenomenon had never been observed before and nobody knew what was to come. On the morning of September 2, the first particles from, what we now know was an enormous eruption on the Sun, reached Earth.

The 1859 solar storm is also known as the "Carrington Event." Auroras associated with this event could be seen as far south as Cuba and Hawaii, telegraph system worldwide went haywire, and ice core records from Greenland indicate that Earth's protective ozone layer was damaged by the energetic particles from the solar storm.

The cosmos, however, contains other stars and some of these regularly experience eruptions that can be up to 10,000 times larger than the Carrington event.

Solar flares occur when large magnetic fields on the surface of the Sun collapse. When that happens, huge amounts of magnetic energy are released. Christoffer Karoff and his team have use observations of magnetic fields on the surface of almost 100,000 stars made with the new Guo Shou Jing telescope in China to show that these superflares are likely formed via the same mechanism as solar flares.

"The magnetic fields on the surface of stars with superflares are generally stronger than the magnetic fields on the surface of the Sun. This is exactly what we would expect, if superflares are formed in the same way as solar flares" explains Christoffer Karoff.

Can the Sun create a superflare?

It does therefore not seem likely that the Sun should be able to create a superflare, its magnetic field is simply to weak. However.

Out of all the stars with superflares that Christoffer Karoff and his team analyzed, around 10% had a magnetic field with a strength similar to or weaker than the Sun's magnetic field. Therefore, even though it is not very likely, it is not impossible that the Sun could produce a superflare.

"We certainly did not expect to find superflare stars with magnetic fields as week as the magnetic fields on the Sun. This opens the possibility that the Sun could generate a superflare -- a very frightening thought" elaborates Christoffer Karoff.

If an eruption of this size was to strike Earth today, it would have devastating consequences. Not just for all electronic equipment on Earth, but also for our atmosphere and thus our planet's ability to support life.

Trees hid a secret

Evidence from geological archives has shown that the Sun might have produced a small superflare in AD 775. Here, tree rings show that anomalously large amounts of the radioactive isotope 14C were formed in Earth's atmosphere. 14C is formed when cosmic-ray particles from our galaxy, the Milky Way, or especially energetic protons from the Sun, formed in connection with large solar eruptions, enter Earth's atmosphere.

The studies from the Guo Shou Jing telescope support the notion that the event in AD 775 was indeed a small superflare, i.e. a solar eruption 10-100 times larger that the largest solar eruption observed during the space age.

"One of the strengths of our study is that we can show how astronomical observations of superflares agree with Earth-based studies of radioactive isotopes in tree rings." Explains Christoffer Karoff.

In this way, the observations from the Guo Shou Jing telescope can be used to evaluate how often a star with a magnetic field similar to the Sun would experience a superflare. The new study shows that the Sun, statistically speaking, should experience a small superflare every millennium. This is in agreement with idea that the event in AD 775 and a similar event in AD 993 were indeed caused by small superflares on the Sun.

It is no coincidence that the new Guo Shou Jing telescope in China was used for this study. In order to measure the magnetic fields, Christoffer Karoff and his team used a spectrum for every star of the 100,000 stars available for this analysis. A spectrum shows the colors, or wavelengths, of the light from the stars. Here, certain short ultraviolet wavelengths can be used to measure the magnetic fields around the stars.

The problem is, however, that conventional telescopes are only capable of obtaining one spectrum of a single star at a time. Therefore, if the observations were to be made with another telescope, such as the Nordic Optical Telescope on La Palma -- a telescope the research group has used before -- it would require 15-20 years of continuous observations.

The Guo Shou Jing telescope, or LAMOST as it is also called, is optimized for obtaining spectra of up to 4,000 stars simultaneously, as 4,000 optical fibers are connected to the telescope. This makes it possible to observe 100,000 stars in only a few weeks and it is this special capability that has made it possible to generate the new results.

Ask Ethan: What Happens When Stars Pass Through Our Solar System?

70,000 years ago, a brown dwarf pair known as Scholz's Star, right on the precipice of igniting . [+] hydrogen fusion in its core, passed through the Solar System's Oort cloud. Unlike the illustration, however, it still wouldn't have been visible to human eyes.

We like to think of our Solar System as a stable, mostly quiet place. Sure, we'll find that the planets and other bodies in their orbits will kick around a comet or asteroid every once in a while, but for the most part, things are stable. Even the occasional interstellar visitor doesn't pose much of a risk, at least, not to the integrity of worlds like our own. But our entire Solar System is orbiting through the galaxy, and that means it has hundreds of billions of chances to have a close interaction with another star. How often do we actually get one, and what are the potential consequences? That's what our Patreon supporter Paweł Zuzelski wants to know (edited for English), as he asks:

How bad would it be if a star passed near the Sun? How close/large would it have to be to pose serious danger? How likely would such an event be?

The possibilities range from the mundane, where a few Oort cloud objects get thrown around, to the catastrophic, such as a collision with or ejection of an entire planet. Let's take a look at what actually transpires.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, large and . [+] small Magellanic Clouds, and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. All told, the Milky Way contains some 200 billion stars over its disk-like extent.

Our best estimates are that there are between 200 and 400 billion stars in our own Milky Way galaxy. Although stars come in a huge variety of sizes and masses, the majority of stars (around 3 out of every 4) are red dwarf stars: somewhere between 8% and 40% the mass of our Sun. These stars have similarly smaller physical sizes than our Sun: on average, about 25% the Sun's diameter. And finally, we know roughly how big the Milky Way is: a disk around 2,000 light years thick, around 100,000 light years across, and with a central bulge that's around 5,000-8,000 light years in radius.

Finally, relative to the Sun, the typical star moves at a speed of around 20 km/s: about 1/10th of the speed that the Sun (and all the stars) orbit through the Milky Way itself.

Although the Sun orbits within the plane of the Milky Way some 25,000-27,000 light years from the . [+] center, the orbital directions of the planets in our Solar System do not align with the galaxy at all.

Science Minus Details /

These are the stats on the stars within our galaxy. There are plenty of details, caveats, and nuances that we are ignoring here, like the density changing with respect to whether we're in a spiral arm or not, the fact that there are more stars towards the center than the outskirts (and our Sun is mid-way towards the edge), the inclination of the orbits in our Solar System with respect to the galaxy, and slight changes dependent on whether we're in the center of the galactic plane or not. But the reason we can ignore them is that just from the above approximation, these numbers allow us to calculate how often stars from the galaxy come within a specific distance of our Sun, and therefore, how often we can expect a close encounter of various impacts.

The distances between the Sun and many of the nearest stars shown here are accurate, but each star . [+] -- even the largest ones here -- would be less than one-one millionth of a pixel in diameter if this were to scale.

Andrew Z. Colvin / Wikimedia Commons

The way we compute it is very simple: we calculate the number density of stars, the cross-section we're interested in (defined by how close you want another star to get to our own), and the speed with which the stars move relative to each other, and then multiply them all together to get the collision rate. This method of computation for a collision rate is useful for everything from particle physics to condensed matter physics (for the experts, this is basically the Drude model), and it applies just as readily to astrophysics. If we assume there are 200 billion stars in the Milky Way, that the stars are evenly distributed throughout the disk (and we ignore the bulge), and that 20 km/s is the speed that the stars move relative to one another, here's what we get if we plot the interaction rate vs. the distance from the Sun.

A plot of how frequently stars within the Milky Way is likely to pass within a certain distance of . [+] our Sun. This is a log-log plot, with distance on the y-axis and how long you typically need to wait for such an event to happen on the x-axis.

It tells us that, on average, the closest we can expect a star to have come to the Sun over the history of the Universe is around 500 A.U., or about ten times the distance from the Sun to Pluto. It tells us that, once every billion years, we can expect a star to come within about 1,500 A.U. of the Sun, close to the edge of the scattered Kuiper belt. And more frequently, about once every 300,000 years or so, we'll get a star that comes within about a light year of us.

A logarithmic view of our Solar System, extending out all the way to the next-nearest stars, shows . [+] the extend of the asteroid belt Kuiper belt, and Oort cloud. While stars passing through the Oort cloud may be common, they are exceedingly unlikely to have passed by any closer than that.

This is good for the long-term stability of the planets in our Solar System, for certain. It tells us that over our Solar System's 4.5 billion year history, the odds that a star would come as close to any of the planets as our Sun is to Pluto is approximately 1-in-10,000 the odds that a star would come as close to a planet as the Sun is to Earth (which would severely disrupt an orbit and cause an ejection) is less than 1-in-1,000,000,000. It means that the likelihood of another star in the galaxy passing by us and causing us severe difficulties is terrifically low. We cannot bet on us losing the cosmic lottery, and the odds are that we haven't lost it so far, and won't for the foreseeable future.

Orbits of the inner and outer planets, all obeying Kepler's laws. The odds of a passing star coming . [+] within any appreciable distance of even Pluto are extremely low.

NASA / JPL-Caltech / R. Hurt, modified by E. Siegel

But there have probably been upwards of 40,000 times that a star has passed through the Oort cloud (defined as 1.9 light years from the Sun), disrupting a large number of icy bodies in the process. Stars are interesting when they pass through the Solar System like this, because of the combination of two factors:

  1. Oort cloud objects are very loosely bound to the Solar System, meaning that a very small gravitational tug is enough to alter their orbits significantly.
  2. Stars are very massive, so a star that passes the same distance from an object as that object is from the Sun can "kick" it enough to alter its orbit.

This tells us that whenever we do experience a close encounter with a passing star, we're at an increased risk, for perhaps the next few million years, of a collision with an incoming object from the Oort cloud.

The Kuiper belt is the location of the greatest number of known objects in the Solar System, but the . [+] Oort cloud, fainter and more distant, not only contains many more, but is more likely to be perturbed by a passing mass like another star. Note that all Kuiper belt and Oort cloud objects move at extremely small speeds relative to the Sun.

In other words, the effects of a passing star won't have an observable effect on what icy, comet-like bodies come into the inner Solar System until another 20 or so additional stars have had a close encounter with our own! This is problematic, because the last star system that passed near our own Sun, Scholz's star (which did so 70,000 years ago), is already 20 light years away from us. However, there is a potentially optimistic thing that comes from this analysis: as we come to better map out and understand the stars and their motions within the nearest 500 light years, we can better predict when-and-where rogue, incoming Oort cloud objects are likely to arise. If we're concerned with planetary defense from objects hurled inwards by passing stars, this kind of knowledge is the obvious next step.

WISEPC J045853.90+643451.9, shown in green, is the first ultra-cool brown dwarf discovered by NASA's . [+] Wide-field Infrared Survey Explorer, or WISE. This star is located about 20 light years away to survey the entire sky and get the stars that may have passed within the Sun's vicinity to cause potential Oort cloud storms today, we will have to go out to about 500 light years.

It will require building wide-field surveying telescopes capable of seeing faint stars out to great distances. NASA's Wide-field Infrared Survey Explorer (WISE) mission was the prototype for this, but the distances over which it could observe the faintest, most common stars was severely limited by its size and observing time. An all-sky, infrared space telescope could map the neighborhood around us out, telling us what's likely to arrive, on what timescales, from what directions, and what star caused these perturbations in the Oort cloud objects. Gravitational interactions are always occurring, as even though there's a great distance between stars in space, the Oort cloud is huge, and we literally have all the time in the world for objects to pass by and affect us. Given enough chances, everything you can imagine will occur.

Sun Will Vaporize Earth Unless We Can Change Our Orbit

New calculations by University of Sussex astronomers predict that the Earth will be swallowed up by the Sun in about 7.6 billion years unless the Earth&rsquos orbit can be altered.

Dr Robert Smith, Emeritus Reader in Astronomy, said his team previously calculated that the Earth would escape ultimate destruction, although be battered and burnt to a cinder. But this did not take into account the effect of the drag caused by the outer atmosphere of the dying Sun.

He says: "We showed previously that, as the Sun expanded, it would lose mass in the form of a strong wind, much more powerful than the current solar wind. This would reduce the gravitational pull of the Sun on the Earth, allowing the Earth's orbit to move outwards, ahead of the expanding Sun.

&ldquoIf that were the only effect the Earth would indeed escape final destruction. However, the tenuous outer atmosphere of the Sun extends a long way beyond its visible surface, and it turns out the Earth would actually be orbiting within these very low density outer layers. The drag caused by this low-density gas is enough to cause the Earth to drift inwards, and finally to be captured and vaporised by the Sun.&rdquo

The new paper was written in collaboration with Dr Klaus-Peter Schroeder, previously at Sussex, who is now in the Astronomy Department of the University of Guanajuato in Mexico.

Life on Earth will have disappeared long before 7.6 billion years, however. Scientists have shown that the Sun's slow expansion will cause the temperature at the surface of the Earth to rise. Oceans will evaporate, and the atmosphere will become laden with water vapour, which (like carbon dioxide) is a very effective greenhouse gas. Eventually, the oceans will boil dry and the water vapour will escape into space. In a billion years from now the Earth will be a very hot, dry and uninhabitable ball.

Can anything be done to prevent this fate? Professor Smith points to a remarkable scheme proposed by a team at Santa Cruz University, who suggest harnessing the gravitational effects of a close passage by a large asteroid to "nudge" the Earth's orbit gradually outwards away from the encroaching Sun. A suitable passage every 6000 years or so would be enough to keep the Earth out of trouble and allow life to survive for at least 5 billion years, and possibly even to survive the Sun's red giant phase.

&ldquoThis sounds like science fiction,&rdquo says Professor Smith. &ldquoBut it seems that the energy requirements are just about possible and the technology could be developed over the next few centuries.&rdquo However, it is a high-risk strategy - a slight miscalculation, and the asteroid could actually hit the Earth, with catastrophic consequences. &ldquoA safer solution may be to build a fleet of interplanetary 'life rafts' that could manoeuvre themselves always out of reach of the Sun, but close enough to use its energy,&rdquo he adds.

Journal reference: "Astronomical engineering: a strategy for modifying planetary orbits". D G Korycansky of the University of California at Santa Cruz, with colleagues Greg Laughlin and Fred Adams: (Astrophysics & Space Science, 275, 349-366, 2001)

Story Source:

Materials provided by University of Sussex. Note: Content may be edited for style and length.

How close would the sun have to get to Earth for there to be severe consequences? - Astronomy

Collisions with Near Earth Objects

    Will the world be hit by an asteroid causing us all to die? I've read about the big rock that will collide with Earth by 2018. How true is that? What will be the consequences of the phenomenon? Please explain as this involves the future of life.
    - question from Bob & Amir

The asteroid Ida and its satellite Dactyl

There are many types of objects in the solar system that can and do regularly impact the Earth. Most of these objects are small particles of rock or dust left behind by comets or created by collisions between asteroids. These small particles are collectively known as meteoroids. Meteoroids fall into Earth's atmosphere daily but pose no threat since they usually burn up at high altitude to create meteors. Those meteoroids large enough to survive their fall through the atmosphere and impact Earth's surface are called meteorites, but these objects are still too small to do any significant damage. It is estimated that about one hundred tons of interplanetary material falls to the surface of the Earth each day, but most of this debris consists of particles of dust that go unnoticed.

The greatest threat comes from asteroids and comets. Asteroids are bodies composed primarily of rock and metals left over after the planets and moons formed. Though still very small in comparison to Earth, they are much larger than meteoroids and are typically tens of meters to tens of kilometers in diameter. Most asteroids formed between Mars and Jupiter but their orbits are often perturbed by collisions or the gravitational influence of planets causing them to spiral inward closer to Earth. Comets are also fairly small bodies containing rock but mixed with large amounts of ice and gases. Comets formed at the outer fringes of the solar system but are sometimes perturbed from their orbits by collisions or the strong gravitation of giant planets like Jupiter. These disturbances cause comets to fall inward toward the Sun and they occasionally pass inside the orbit of Earth to pose a threat of impact.

Even relatively large objects like these do collide with Earth fairly regularly. It is estimated that an object with a diameter of at least 165 ft (50 m) hits Earth about once a century. The most recent major impact came in 1908 when an object struck a sparsely inhabited region of central Siberia. It is unknown whether the object was an asteroid or a comet, but the increasing pressure on the body as it plunged through the atmosphere apparently caused it to explode about 4 to 6 miles (6 to 10 km) above the Earth's surface. The energy of the detonation was estimated between 10 and 20 megatons of TNT, comparable to the largest nuclear weapons ever tested, and knocked down more than 80 million trees across an area of over 830 square miles (2,150 sq km).

Trees downed by the 1908 Tunguska blast in Siberia

Megaton impacts like the 1908 Tunguska blast probably occur no more often than once a millennia, but smaller events happen more frequently. Gene Shoemaker estimated that an impact releasing as much energy as the atomic bomb dropped on Hiroshima, on the order of a few kilotons, occurs every year. Most of these detonations go unnoticed because they occur over the oceans, in uninhabited areas, or too high in the atmosphere to be seen from the ground. Although the public is generally unaware of such blasts, the US Air Force is capable of tracking them using early warning satellites designed to detect launches of nuclear missiles. The Air Force reported that 136 major explosions were detected in the upper atmosphere between 1975 and 1992.

A classification that has been devised for the most threatening asteroids is Potentially Hazardous Asteroids (PHAs). PHAs are selected based upon the object's potential to make a close approach to Earth and upon its size. Any asteroid that will never come closer to Earth than 0.05 astronomical units or is less than 500 ft (150 m) in diameter is excluded from the list of PHAs. Although an asteroid may be classified as a PHA, however, that does not mean that it will impact Earth but only that it has the potential to do so. These possible threats are regularly monitored to update their orbits and better quantify how dangerous they may be. To date, a total of 848 asteroids are considered PHAs.

Approximate frequency of impacts with different sizes of objects

    2014: In September 2003, the British government's Near Earth Object Information Centre announced that the asteroid 2003 QQ47 has a remote chance of 1 in 909,000 of hitting Earth on 21 March 2014.

Close approach of 99942 Apophis predicted in 2029

    2039: In 1999, Italian astronomers estimated that an asteroid named 1999 AN10 had a high probability of colliding with Earth in 2039. This body is about 0.6 miles (1 km) across. Further observations confirmed that 1999 AN10 has no chance of striking Earth in this century.

The threat becomes far more serious in the next tier covering index levels 5 through 7. Objects falling into this region are likely to make a very close approach to Earth and are large enough to do considerable damage should they impact. However, the probability of impact is unknown and further study is needed. Imminent collisions are classified at the most severe end the scale in indexes 8 through 10 ranging from localized destruction to regional destruction to a global catastrophe.

A representation of the Torino Impact Hazard Scale based on probability and energy of impact

The objects currently being monitored by NASA are listed on the Near Earth Object Program website. As of this writing, none of the more than 100 objects listed are rated higher than 0 on the Torino scale and none has a probability of impact higher than about 1 in 1,000. The site explains that NASA uses an automated collision monitoring system called Sentry that scans the catalog of known asteroids and identifies those that could potentially impact Earth within the next century. Another useful page is the NEO Earth Close Approaches list including objects that have recently made or soon will make close approaches to Earth.

NASA's goal is to discover at least 90% of all Near Earth Objects with a diameter of at least 1 kilometer within 10 years. Several independent teams are funded to conduct this search. Among these are MIT's Lincoln Near-Earth Asteroid Research (LINEAR), Near-Earth Asteroid Tracking (NEAT) operated by NASA's Jet Propulsion Laboratory, Spacewatch at the University of Arizona, the Lowell Observatory Near-Earth Object Search (LONEOS), and the Catalina Sky Survey (CSS). Foreign participation also includes Australia's Siding Spring Survey (SSS), the Japanese Spaceguard Association (JSGA), and Italy's Asiago DLR Asteroid Survey (ADAS).

As of 1 March 2007, a total of 4,569 NEOs had been discovered. This total includes 64 comets and 4,505 asteroids. Of the asteroids, 707 are at least a kilometer in diameter. The total of Potentially Hazardous Asteroids (PHAs) is 839 while 134 of these have a diameter of a kilometer or more.

Steep increase in discoveries of Near Earth Asteroids since the late 1990s

While there are currently no known objects posing an immediate threat of impact within at least the next 100 years, the question remains as to what could be done if an object on a collision course with Earth was found. Asteroid impacts are unique among natural disasters in that humans have the technological capability to prevent them from happening. Movies often portray dangerous missions using nuclear weapons to destroy these objects before they strike. The 1979 film Meteor, for example, depicts the US and USSR launching a swarm of nuclear missiles to destroy an asteroid headed for Earth. More recently, Deep Impact portrays a team of astronauts on a mission to plant a nuclear weapon within a comet on a collision course with Earth hoping to destroy the comet before it wipes out humanity. The same basic plot was recycled in the truly ridiculous Armageddon that replaces the comet with an "asteroid the size of Texas."

Unfortunately, these "action-packed, glued-to-your-seat blockbusters" are not very realistic. While detonating a nuclear device on or near an asteroid or comet might help nudge it out of Earth's path, the blast may do more harm than good. Research has suggested that the radiation created by a nuclear reaction can vaporize part of the surface of the object causing a layer of debris to blast off at high speed. This effect creates a thrust that deflects the object's course or slows it down slightly depending on where the blast is detonated. However, most scientists consider this kind of brute force approach a last resort. The greatest concern is that such an explosion might cause the asteroid or comet to break into multiple bodies. These fragments might still be on a collision course for Earth and would likely cause even more damage than the original body could.

There are many other more subtle methods of deflection that scientists prefer to nuclear detonations. Most of these approaches make use of natural phenomena that already influence the orbital trajectories of asteroids and comets. Collisions, for example, are often responsible for the paths asteroids and comets follow. It has been proposed that humans can simulate this effect by purposefully crashing sufficiently large spacecraft into the surface of an object to change its momentum. This approach is typically called a kinetic impact deflection. A related idea is to place a sufficiently massive spacecraft near an asteroid and use the mutual gravitational attraction between the two bodies to pull the asteroid into a non-threatening orbit. The primary disadvantage of these ideas is the large mass of the spacecraft needed to affect the orbit of an asteroid.

Total number of known Near Earth Asteroids and those more than a kilometer in diameter

Another natural phenomenon that could be exploited is called the Yarkovsky effect. This effect says that the thermal heating of the Sun acting on a rotating object in space creates a small thrust. Over long periods of time, this thrust gently nudges the object along a different path. The effect is probably miniscule for asteroids over a kilometer in size, but it does have a significant influence on smaller objects. Should an asteroid on the order of a hundred meters or so be on a path that will impact Earth, scientists have proposed placing materials on the object's surface that would reflect or absorb heat. The different thermal properties would change the amount of thrust generated by its surface causing the body's orbital trajectory to gradually change. A similar idea is to use mirrors to focus solar energy onto the surface of a body to vaporize a small region and create a thrust like that of the Yarkovsky effect. Another suggestion is to attach a solar sail that takes advantage of pressure created by the solar wind to deflect the object.

Still other researchers propose more conventional methods of applying thrust to an object headed for Earth. Chemical rockets or electrical propulsion systems like those used on spacecraft today could be attached to an asteroid or comet and deflect its path. A more advanced idea is to attach a device called a mass driver to the object. A mass driver drills into the surface and ejects material away from the body to create a thrust effect.

The one factor common to any strategy for deflecting an oncoming asteroid or comet is time. It will take at least a decade to implement any of the approaches described above, which is why organizations like NASA have put so many resources into studying the space around Earth for objects that may pose a threat. The more warning time decision makers and engineers have, the smaller the nudge to change an object's path will need to be.

Scars on Jupiter following the impact of Shoemaker-Levy 9 in 1994

Though impacts from objects large enough to cause significant destruction are generally quite rare, perhaps a more immediate threat facing humanity instead comes from small rocks that vaporize high in the atmosphere. Had the object that struck Tunguska in 1908 instead hit Siberia 50 or 60 years later, it is likely that the Soviet Union would have thought is was under nuclear attack. In the tense days of the Cold War, the Soviets might have launched a full-scale retaliation on the West resulting in global nuclear devastation. Though diminished, that threat still exists today. It is believed that only the United States currently has the ability to differentiate between a man-made nuclear explosion in the atmosphere and a naturally occurring meteor detonation. If a sufficiently large meteor were to impact or explode above a major city or military base, it is very possible that the blast could be mistaken for a nuclear attack and provoke a retaliatory strike.

In January 2000, a meteor only 15 ft (5 m) across entered the atmosphere and exploded over the town of Whitehorse in the Canadian Yukon. The blast created an electromagnetic pulse (EMP) similar to that of a high-altitude nuclear detonation and disabled a third of the region's electrical power grid. This kind of high-altitude blast is considered a likely first strike tactic that would blind a country's defenses to further attack or invasion. In the summer of 2001, another high-altitude explosion was detected over the Mediterranean Sea. This detonation produced a level of energy comparable to that of a nuclear weapon. In both cases, the US was able to determine that the explosions were natural phenomena and not nuclear attacks. If the same events were to occur in a region of world tension where countries possess nuclear weapons, the reactions of national leaders could have horrific consequences.
- answer by Justine Whitman, 4 March 2007

How close would the sun have to get to Earth for there to be severe consequences? - Astronomy

Changing Sun, Changing Climate?

Since it is the Sun's energy that drives the weather system, scientists naturally wondered whether they might connect climate changes with solar variations. Yet the Sun seemed to be stable over the timescale of human civilization. Attempts to discover cyclic variations in weather and connect them with the 11-year sunspot cycle, or other possible solar cycles ranging up to a few centuries long, gave results that were ambiguous at best. These attempts got a well-deserved bad reputation. Jack Eddy overcame this with a 1976 study that demonstrated that irregular variations in solar surface activity, a few centuries long, were connected with major climate shifts. The mechanism was uncertain, but plausible candidates emerged. The next crucial question was whether a rise in the Sun's activity could explain the global warming seen in the 20th century? By the 1990s, there was a tentative answer: minor solar variations could indeed have been partly responsible for some past fluctuations. but future warming from the rise in greenhouse gases far outweighed any solar effects. (1)

<= Simple models

<= Carbon dates

The Sun so greatly dominates the skies that the first scientific speculations about different climates asked only how sunlight falls on the Earth in different places. The very word climate (from Greek klimat , inclination or latitude) originally stood for a simple band of latitude. When scientists began to ponder the possibility of climate change, their thoughts naturally turned to the Sun. Early modern scientists found it plausible that the Sun could not burn forever, and speculated about a slow deterioration of the Earth's climate as the fuel ran out.In 1801 the great astronomer William Herschel introduced the idea of more transient climate connections. It was a well-known fact that some stars varied in brightness. Since our Sun is itself a star, it was natural to ask whether the Sun's brightness might vary, bringing cooler or warmer periods on Earth? As evidence of a connection between Sun and weather, Herschel pointed to periods in the 17th century, ranging from two decades to a few years, when hardly any sunspots had been observed. During those periods, he remarked,the price of wheat had been high, presumably reflecting spells of drought.(2)
Chasing Sunspot Cycles TOP OF PAGE
Speculation increased in the mid-19th century following the discovery that the number of spots seen on the Sun rose and fell in a regular 11-year cycle. It appeared that the sunspots reflected some kind of storminess on the Sun's surface — violent activity that strongly affected the Earth's magnetic field. Astronomers also found that some stars, which otherwise seemed quite similar to the Sun, went through very large variations. By the end of the century a small community of scientists was pursuing the question of how solar variability might relate to short-term weather cycles, as well as long-term climate changes. (3) Attempts to correlate weather patterns with the sunspot cycle were stymied, however, by inaccurate and unstandardized weather data, and by a lack of good statistical techniques for analyzing the data. Besides, it was hard to say just which of many aspects of weather were worth looking into.
At the end of the 19th century, most meteorologists held firmly that climate was stable overall, about the same from one century to the next. That still left room for modest cycles within the overall stability. A number of scientists looked through various data hoping to find correlations, and announced success. Enthusiasts for statistics kept coming up with one or another plausible cycle of dry summers or cold winters or whatever, in one or another region, repeating periodically over intervals ranging from 11 years to several centuries. Many of these people declined to speculate about the causes of the cycles they reported, but others pointed to the Sun. An example was a late 19th-century British school of "cosmical meteorology," whose leader Balfour Stewart grandly exclaimed of the Sun and planets, "They feel, they throb together." (4)
Confusion persisted in the early decades of the 20th century as researchers continued to gather evidence for solar variation and climate cycles. For example, Ellsworth Huntington, drawing on work by a number of others, concluded that high sunspot numbers meant storminess and rain in some parts of the world, resulting in a cooler planet. The "present variations of climate are connected with solar changes much more closely than has hitherto been supposed," he maintained. He went on to speculate that if solar disturbances had been magnified in the past, that might explain the ice ages. (5)
Meanwhile an Arizona astronomer, Andrew Ellicott Douglass, announced a variety of remarkable correlations between the sunspot cycle and rings in trees. Douglass tracked this into past centuries by studying beams from old buildings as well as Sequoias and other long-lived trees. Noting that tree rings were thinner in dry years, he reported climate effects from solar variations, particularly in connection with the 17th-century dearth of sunspots that Herschel and others had noticed. Other scientists, however, found good reason to doubt that tree rings could reveal anything beyond random regional variations. The value of tree rings for climate study was not solidly established until the 1960s.(6*)
Through the 1930s the most persistent advocate of a solar-climate connection was Charles Greeley Abbot of the Smithsonian Astrophysical Observatory. His predecessor, Samuel Pierpont Langley, had established a program of measuring the intensity of the Sun's radiation received at the Earth, called the "solar constant." Abbot pursued the program for decades. By the early 1920s, he had concluded that the solar "constant" was misnamed: his observations showed large variations over periods of days, which he connected with sunspots passing across the face of the Sun. According to his calculations, over a period of years when the Sun was more active it was brighter by nearly one percent. Surely this influenced climate! As early as 1913, Abbot announced that he could see a plain correlation between the sunspot cycle and cycles of temperature on Earth. (This only worked, however, if he took into account temporary cooling spells caused by the dust from volcanic eruptions.) Self-confident and combative, Abbot defended his findings against all objections, meanwhile telling the public that solar studies would bring wonderful improvements in weather prediction.(7*) He and a few others at the Smithsonian pursued the topic single-mindedly into the 1960s, convinced that sunspot variations were a main cause of climate change. (8)
Other scientists were quietly skeptical. Abbot's solar constant variations were at the edge of detectability if not beyond. About all he seemed to have shown for certain was that the solar constant did not vary by more than one percent, and it remained an open question whether it varied anywhere near that level. Perhaps Abbot was detecting variations not in the solar constant, but in the transmission of radiation through the atmosphere. (9) Still, if that varied with the sunspot cycle, it might by itself somehow change the weather.
Despite widespread skepticism, the study of cycles was popular in the 1920s and 1930s. By now there were a lot of weather data to play with, and inevitably people found correlations between sunspot cycles and selected weather patterns. Respected scientists and over-enthusiastic amateurs announced correlations that they insisted were reliable enough to make predictions.
Sooner or later, every prediction failed. An example was a highly credible forecast that there would be a dry spell in Africa during the sunspot minimum of the early 1930s. When that came out wrong, a meteorologist later recalled, "the subject of sunspots and weather relationships fell into disrepute, especially among British meteorologists who witnessed the discomfiture of some of their most respected superiors." Even in the 1960s, he said, "For a young [climate] researcher to entertain any statement of sun-weather relationships was to brand oneself a crank." (10) Specialists in solar physics felt much the same. As one of them recalled, "purported connections with. weather and climate were uniformly wacky and to be distrusted. there is a hypnotism about cycles that. draws all kinds of creatures out of the woodwork."(11) (This was a robust tradition: into the 21st century, enthusiasts with weird or incomprehensible theories of solar influences, backed up by selected weather data and intricate graphs, continued to show up at scientific meetings of meteorological societies.) By the 1940s, most meteorologists and astronomers had abandoned the quest for solar cycles in the weather. Yet some respected experts continued to suspect that a connection did exist, lurking somewhere in the data. (12)
Less prone to crank enthusiasm and scientific scorn, if equally speculative, was the possibility that the Sun could affect climate on much longer timescales. During the 1920s, a few people developed simple models that suggested that even a modest change in solar radiation might set off an ice age, by initiating self-sustaining changes in the polar ice. A leading British meteorologist, Sir George Simpson, believed the sequence of ice ages showed that the Sun is a variable star, changing its brightness over a cycle some 100,000 years long. (13) "There has always been a reluctance among scientists to call upon changes in solar radiation. to account for climatic changes," Simpson told the Royal Meteorological Society in a Presidential address of 1939. "The Sun is so mighty and the radiation emitted so immense that relatively short period changes. have been almost unthinkable." But none of the terrestrial causes proposed for ice ages was at all convincing, he said, and that "forced a reconsideration of extra-terrestrial causes." (14*)
Such thinking was still in circulation in the 1950s. The eminent astrophysicist Ernst Öpik wrote that none of the many explanations proposed for ice ages was convincing, so "we always come back to the simplest and most plausible hypothesis: that our solar furnace varies in its output of heat." Öpik worked up a theory for cyclical changes of the nuclear reactions deep inside the Sun. The internal fluctuations he hypothesized had a hundred-million-year timescale that seemed to match the major glacial epochs. Manwhile,within a given glacial epoch "a kind of 'flickering' of solar radiation" in the Sun's outer shell would drive the expansion and retreat of ice sheets. (15) When reviews and textbooks listed various possible explanations of ice ages and other long-term climate changes, ranging from volcanic dust to shifts of ocean currents, they often invoked long-term solar variation as a particularly likely cause. As a U.S. Weather Bureau expert put it, "the problem of predicting the future climate of Planet Earth would seem to depend on predicting the future energy output of the sun. " (16)
Searching for a Mechanism (1950s - Early 1970s)
Some people continued to pursue the exasperating hints that minor variations in the sunspot cycle influenced present-day weather. Interest in the topic was revived in 1949 by H.C. Willett, who dug out apparent relationships between changes in the numbers of sunspots and long-term variations of wind patterns. Sunspot variations, he declared, were "the only possible single factor of climatic control which might be made to account for all of these variations." Others thought they detected sunspot cycle correlations in the advance and retreat of mountain glaciers. Willett admitted that "the physical basis of any such relationship must be utterly complex, and is as yet not at all understood." But he pointed out an interesting possibility. Perhaps climate changes could be due to "solar variation in the ultraviolet of the sort which appears to accompany sunspot activity." As another scientist had pointed out a few years before, ultraviolet radiation from the explosive flares that accompany sunspots would heat the ozone layer high in the Earth's atmosphere, and that might somehow influence the circulation of the lower atmosphere. (17)
In the 1950s and 1960s, instruments on rockets that climbed above the atmosphere managed to measure the Sun's ultraviolet radiation for the first time. They found that the radiation did intensify during high sunspot years. However, ultraviolet light does not penetrate below the stratosphere. Meteorologists found it most unlikely that changes in the thin stratosphere could affect the layers below, which contain far more mass and energy. Still, the hypothesis of atmospheric influence remained alive, if far from healthy.
A few scientists speculated more broadly. Maybe weather patterns were affected by the electrically charged particles that the Sun sprayed out as "solar wind." More sunspots throw out more particles, and they might do something to the atmosphere. More indirectly, at times of high sunspot activity the solar wind pushes out a magnetic field that tends to shield the Earth from the cosmic rays that rain down from the universe beyond. When these rays penetrate the upper reaches of the atmosphere, they expend their energy producing sprays of charged particles. Therefore, more sunspots would mean fewer of these particles. Either way there might be an influence on the weather. Meteorologists gave these ideas some credence. (18*) But the solar wind and ultraviolet carried only a tiny fraction of the Sun's total energy output. If they did influence weather, it had to be through a subtle triggering mechanism that remained altogether mysterious. Anyway variations connected with sunspots seemed likely to bear only on temporary weather anomalies lasting a week or so (the timescale of variations in sunspot groups themselves), not on long-term climate change. (19)
People continued to report weather features that varied with the sunspot cycle of 11 years, or with the full solar magnetic cycle of 22 years (the magnetic polarity of sunspots reverses from one 11-year cycle to the next). There were also matches to possible longer solar variation cycles.(20) It was especially scientists in the Soviet Union who pursued such correlations. In the lead was a team under the Leningrad meteorologist Kirill Ya. Kondratyev, who sent balloons into the stratosphere to measure the solar constant. In 1970 his group claimed that the Sun's output varied along with the number of sunspots by as much as 2%. This drew cautious notice from other scientists. But the authors admitted that the conclusion would remain in doubt unless it could be verified by spacecraft entirely above the atmosphere. (21)
Another tentatively credible study came from a team led by the Danish glaciologist Willi Dansgaard. Inspecting layers of ancient ice in cores drilled from deep in the Greenland ice cap, they found cyclical variations. They supposed the Sun was responsible. For the cycle that they detected, about 80 years long, had already been reported by scientists who had analyzed small variations in the sunspot cycle. (22*) Another cycle with a length of about 180 years was also, the group suspected, caused by "changing conditions on the Sun." The oscillations were so regular that in 1970 Dansgaard's group boldly extrapolated the curves into the future. They began by matching their results with a global cooling trend that, as others reported, had been underway since around 1940. The group predicted the cooling would continue through the next one or two decades, followed by a warming trend for the following three decades or so. (23)
The geochemist Wallace Broecker was impressed. He "made a large leap of faith" (as he later put it) and assumed that the cycles were not just found in Greenland, but had a global reach. (24) He calculated that the global cooling trend since around 1940 could be explained by the way the two cycles both happened to be trending down. His combined curve would bottom out in the 1970s, then quickly head up. Greenhouse effect warming caused by human emissions of carbon dioxide gas ( CO 2 ) would come on top of this rise, making for a dangerously abrupt warming. (25)
(Later studies failed to find Dansgaard's cycles globally. If they existed at all, the cause did not seem to be the Sun, but quasi-cyclical shifts in the North Atlantic Ocean's surface warmth and winds. This was just another case of supposed global weather cycles that faded away as more data came in. It was also one of several cases where Broecker's scientific instincts were sounder than his evidence. Whatever caused the downturn in temperature since the 1940s, perhaps a combination of factors such as a surge of industrial pollutiion, it would eventually be overmatched by the steadily increasing greenhouse gases. Indeed warming did resume in the 1970s..)
By now it was clear that if you applied powerful statistical techniques to enough tree ring samples, you would sometimes turn up the 11-year solar cycle. Solar activity definitely had some kind of effect on climate in some places &mdash but nothing obviously strong or consistent. For exaample, the 1970s saw controversial claims that weather data and tree rings from various parts of the American West revealed a 22-year cycle of droughts, presumably driven by the solar magnetic cycle. Coming at a time of severe droughts in the West and elsewhere, these claims caught some public attention. (26*) Scientists were beginning to understand, however, that the planet's climate system could go through purely self-sustaining oscillations, driven by feedbacks between ocean temperatures and wind patterns. The patterns cycled 2010s-regularly by themselves on timescales ranging from a few years (like the important El Niño - Southern Oscillation in the Pacific Ocean) to several decades. That might help to explain at least some of the quasi-regular cycles that had been tentatively associated with sunspots.
All this helped to guarantee that scientists would continue to scrutinize any way that solar activity might influence climate, but always with a skeptical eye. If meteorologists had misgivings, most astronomers dismissed outright any thought of important solar variations on a timescale of hundreds or thousands of years. Surface features like sunspots might cycle over decades, but that was a weak, superficial, and short-term effect. As for the main energy flow, improved theories of the nuclear furnace deep within the Sun showed stability over many millions of years. Alongside this sound scientific reasoning there may have been a less rational component. "We had adopted a kind of solar uniformitarianism," solar physicist John (Jack) Eddy suggested in retrospect. "As people and as scientists we have always wanted the Sun to be better than other stars and better than it really is." (27)
Carbon-14 and Jack Eddy TOP OF PAGE
Evidence was accumulating that the Sun truly does change at least superficially from one century to another. Already in 1961 Minze Stuiver had moved in the right direction. Stuiver was concerned about peculiar variations in the amount of radioactive carbon-14 found in ancient tree rings. Carbon-14 is generated when cosmic rays from the far reaches of the universe strike the atmosphere. Stuiver noted how changes in the magnetic field of the Sun would change the flux of cosmic rays reaching the Earth. (28) He had followed this up in collaboration with the carbon-14 expert Hans Suess, confirming that the concentration of the isotope had varied over past millennia. They were not suggesting that changes in carbon-14 (or cosmic rays) altered climate rather, they were showing that the isotope could be used to measure solar activity in the distant past. For the development of this important technique, a good example of laboratory work and its attendant controversies, see the supplementary essay on Uses of Radiocarbon Dating.
In 1965 Suess tried correlating the new data with weather records, in the hope that carbon-14 variations "may supply conclusive evidence regarding the causes for the great ice ages." He focused on the bitter cold spell that historians had discovered in European writings about weather from the 15th through the 18th century (the "Little Ice Age"). That had been a time of relatively high carbon-14, which pointed to low solar activity. Casting a sharp eye on historical sunspot data, Suess noticed that the same centuries indeed showed a low count of sunspots. In short, fewer sunspots apparently made for colder winters. A few others found the connection plausible, but to most scientists the speculation sounded like just one more of the countless correlations that people had announced over the past century on thin evidence. (29*)
Meanwhile carbon-14 experts refined their understanding of how the concentration of the isotope had varied over past millennia. They could not decide on a cause for the shorter-term irregularities. Solar fluctuations were only one of half a dozen plausible possibilities. (30) The early 1970s also brought claims that far slower variations in the Earth's magnetic field correlated with climate. In cores of clay drawn from the seabed reaching back a million years, colder temperatures had prevailed during eras of high magnetism. The magnetic variations were presumably caused by processes in the Earth's interior rather than on the Sun, but the correlation suggested that cosmic rays really did influence climate. As usual the evidence was sketchy, however, and it failed to convince most scientists. (31)
In 1975 the respected meteorologist Robert Dickinson, of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, took on the task of reviewing the American Meteorological Society's official statement about solar influences on weather. He concluded that such influences were unlikely, for there was no reasonable mechanism in sight — except, maybe, one. Perhaps the electric charges that cosmic rays generated in the atmosphere somehow affected how dust and other aerosol particles coalesced. Perhaps that somehow affected cloudiness, since cloud droplets condensed on the nuclei formed by aerosol particles. This was just piling speculation on speculation, Dickinson hastened to point out. Scientists knew little about such processes, and would need to do much more research "to be able to verify or (as seems more likely) to disprove these ideas." For all his frank skepticism, Dickinson had left the door open a crack. One way or another, it was now at least physically conceivable that changes in sunspots could have something to do with changes in climate. Most experts, however, continued to believe the idea was not only unproven but preposterous. Interest might be piqued when someone reported a new correlation between solar changes and weather, but nobody was surprised when further data and analysis knocked it down. (32*)
In 1976, Eddy tied all the threads together in a paper that soon became famous. He was one of several solar experts in Boulder, where a vigorous community of astrophysicists, meteorologists, and other Earth scientists had grown up around the University of Colorado and NCAR. Yet Eddy was ignorant of the carbon-14 research — an example of the poor communication between fields that always impeded climate studies. He had won scant success in the usual sort of solar physics research, and in 1973 he lost his job as a researcher, finding only temporary work writing a history of NASA's Skylab. In his spare time he pored over old books. Eddy had decided to review historical naked-eye sunspot records, with the aim of definitively confirming the long-standing belief that the sunspot cycle was stable over the centuries.
Instead, Eddy found evidence that the Sun was by no means as constant as astrophysicists supposed. Especially intriguing was evidence suggesting that during the "Little Ice Age" of the 16th-17th centuries, sky-watchers had observed almost no sunspot activity. People clear back to Herschel had noticed this prolonged dearth of sunspots. A 19th-century German astronomer, G.W. Spörer, had been the first to document it, and a little later, in 1890, the British astronomer E. Walter Maunder drew attention to the discovery and its significance for climate. Other scientists, however, thought this was just another case of dubious numbers at the edge of detectability. Maunder's publications sank into obscurity. It was only by chance that while Eddy was working to prove the Sun was entirely stable, another solar specialist told him about Maunder's work.(33*)
"As a solar astronomer I felt certain that it could never have happened," Eddy later recalled. But hard historical work gradually persuaded him that the early modern solar observers were reliable — the absence of sunspot evidence really was evidence of an absence. Digging deeper, he found the inconstancy confirmed by historical sightings of auroras and of the solar corona at eclipses (both of which reflected activity on the Sun's surface). Once his attention was drawn to the carbon-14 record, he saw that it too matched the pattern. All the evidence pointed to long-sustained minimums and at least one maximum of solar activity in the past two thousand years. It was "one more defeat in our long and losing battle to keep the Sun perfect, or, if not perfect, constant, and if inconstant, regular. Why we think the Sun should be any of these when other stars are not," he continued, "is more a question for social than for physical science." (34)
As it happened, the ground had already been prepared by developments in astrophysics in the early 1970s. Physicists had built a colossal particle detector expressly to observe the elusive neutrinos emitted by the nuclear reactions that fueled the Sun. The experiment failed to find anywhere near the flux of neutrinos that theorists insisted should be reaching the Earth. Was it possible that deep within the Sun, production of energy was going through a lull? Perhaps the output of stars like the Sun really could wander up and down, maybe even enough to cause ice ages? The anomaly was eventually traced to neutrino physics, not solar physics. Meanwhile, however, it called into doubt the theoretical reasoning that said the Sun could not be a variable star. (35)
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Eddy's announcement of a solar-climate connection nevertheless met the customary skepticism. He pushed his arguments vigorously, stressing especially the Little Ice Age, which he memorably dubbed the "Maunder Minimum" of sunspots. The name he chose emphasized that he was not alone with his evidence. It is not unusual for a scientist to make a "discovery" that others had already announced fruitlessly. A scientific result cannot flourish in isolation, but needs support from other evidence and ideas. Eddy had gone some distance beyond his predecessors in historical investigation. More important, he could connect the sunspot observations with the carbon-14 record and the new doubts about solar stability. It also mattered that he worked steadily and persuasively to convince other scientists that the thing was true.
Pushing farther, Eddy drew attention to a spell of low carbon-14, and thus high solar activity, during the 11th-12th centuries. Remarks in medieval manuscripts showed that these centuries had been unusually warm in Europe. It was far from proven that those were times of higher temperatures all around the globe. However, scientists were (as usual) particularly impressed by evidence from the North Atlantic region where most of them lived and where the historical record was best known. Especially notable was the mild weather that had encouraged medieval Vikings to establish colonies in Greenland — colonies that endured for centuries, only to perish from starvation in the Little Ice Age. Eddy warned that in our own times, "when we have observed the Sun most intensively, its behavior may have been unusually regular and benign."(36)
Decades later, after painstaking studies developed much fuller series of data covering the entire globe, these data showed a complex variety of periods of warmth and periods of cold. The so-called "Medieval Warm Period" when Iceland and Greenland were settled was a group of regional variations, significant but not as universal and extreme as the steep temperature rise felt around the world since the 1980s. The "Little Ice Age" was much clearer, but it was more a collection of regional cooling spells at different times than a coherent global phenomenon, not everywhere as obvious as around the North Atlantic. As one pair of experts remarked in 2004, "If the development of paleoclimatology had taken place in the tropical Pacific, Africa. or Latin America, the paleoclimatic community would almost certainly have adopted other terminology." Instead of a Little Ice Age and Medieval Warm Period, scientists of the 1970s might have talked, for example, about great periods of drought. Still, Eddy's central point would stand: regional climates were more susceptible to perturbing influences, including small changes on the Sun, than most scientists had imagined.)(36a)
Eddy worked hard to "sell" his findings. At a 1975 workshop where he first presented his full argument, his colleagues tentatively accepted that solar variability might be responsible for climate changes over periods of a few hundreds or thousands of years. (37) Eddy pressed on to turn up more evidence connecting temperature variations with carbon-14, which he took to measure solar activity. "In every case when long-term solar activity falls," he claimed, "mid-latitude glaciers advance and climate cools." (38)
Already while Eddy's sunspot figures were in press, other scientists began to explore how far his idea might account for climate changes. Adding solar variability to the sporadic cooling caused by dust from volcanic eruptions did seem to roughly track temperature trends over the entire last millennium. (39) Peering closer at the more accurate global temperatures measured since the late 19th century, a group of computer modelers got a decent match using only the record of volcanic eruptions plus greenhouse warming from increasing carbon dioxide &mdash but they improved the match noticeably when they added in a record of solar variations. All this proved nothing, but gave more reason to devote effort to the question. (40)
Meanwhile Stuiver and others confirmed the connection between solar activity and carbon-14, and this became a standard tool in later solar-climate studies. (41) An example was a study that reported a match between carbon-14 variations and a whole set of "little ice ages" (indicated by advances of glaciers) that had come at random over the last ten thousand years. (42) Other studies, however, failed to find such correlations. As a 1985 reviewer commented, "this is a controversial topic. the evidence relating solar activity and carbon-14 variations to surface temperatures is equivocal, an intriguing but unproven possibility."(43)
Scientists continued to report new phenomena at the border of detectability. In particular, Ronald Gilliland (another NCAR scientist) followed Eddy's example in analyzing a variety of old records and tentatively announced slight periodic variations in the Sun's diameter. They matched not only the 11-year sunspot cycle but also the 80-year cycle that had long hovered at the edge of proof. Adding these solar cycles on top of greenhouse warming and volcanic eruptions, Gilliland too found a convincing match to the temperature record of the past century. He calculated that the solar cycles were currently acting opposite to the rise in carbon dioxide, so as to give the world an equable climate until about the year 2000. This might lead to complacency about greenhouse warming, he feared, which "could be shattered" when the relentlessly increasing carbon dioxide added onto a solar upturn. Most of his colleagues awaited more solid proof of the changes in diameter and the long-term cycle (and they continue to await it). (44)
More Sun-Climate Connections (1980s - 1990s)
How could changes in the number of sunspots affect climate? The most direct influence would come if the change meant a rise or fall in the total energy the Sun radiated upon the Earth, the so-called "solar constant." The development of highly accurate radiometers in the 1970s raised hopes that variations well below one percent could be detected at last. But few trusted any of the measurements from the ground or even from stratospheric balloons. Rockets launched above the atmosphere provided brief observations that seemed to show variation over time, but it was hard to rule out instrumentation errors. Nor were many convinced by Peter Foukal when he applied modern statistical methods to Abbot's huge body of old data, and turned up a faint connection between sunspots and the amount of solar energy reaching the Earth. Even if that were accepted, was it because the Sun emitted less energy? Or was it because ultraviolet radiation from solar storms somehow changed the upper atmosphere, which in turn somehow influenced climate, and thus affected how much sunlight Abbot had seen at the surface? (45)
To try to settle the question, NASA included an instrument for measuring the solar constant on a satellite launched in 1980. The amazingly precise device was the work of a team at the Jet Propulsion Laboratory led by Richard C. Willson. Soon after the satellite's launch, they reported distinct if tiny variations whenever groups of sunspots passed across the face of the Sun. Essential confirmation came from an instrument that John Hickey and colleagues had previously managed to insert in the Nimbus-7 satellite, a spacecraft built to monitor weather rather than the Sun.(46) Both instruments proved stable and reliable. In 1988, as a new solar cycle got underway, both groups reported that total solar radiation did vary slightly with the sunspot cycle. (47)
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Sunspot numbers, compiled by European observatories. The roughly 11-year cycle has variable intensity, peaking in the 1780s, 1850s and 1960s. The solar magnetic field, ultraviolet radiation, and other features that may affect climate are found to rise and fall along with the sunspot number. Courtesy NASA

Critics of the report pointed out that the new finding sounded like the weary old story of sunspot work: if you inspected enough parameters, you were bound to turn up a correlation. As it happened, already by 2000 the correlation of climate with cycle length began to break down. Moreover, a reanalysis published in 2004 revealed that from the outset the only pattern had been a "pattern of strange errors" in the key study's data. Little more could be said without further decades of observations &mdash and a theory to explain why there should be any connection at all between the sunspot cycle and weather. The situation remained as an expert had described it a century earlier: "from the data now in our possession, men of great ability and laborious industry draw opposite conclusions." (51)

Satellite measurements pinned down precisely how solar brightness varied with the number of sunspots. Over a sunspot cycle the energy radiated varied by barely one part in a thousand measuring such tiny wiggles was a triumph of instrumentation. (48) A single decade of data was too short to support any definite conclusions about long-term climate change, but it was hard to see how such a slight variation could matter much. (49) Since the 1970s, rough calculations on general grounds had indicated that it should take a bigger variation, perhaps half a percent, to make a serious direct impact on global temperature. However, if the output could vary a tenth of a percent or so over a single sunspot cycle, it was plausible to imagine that larger, longer-lasting changes could have come during the Maunder Minimum and other major solar variations. That could have worked a real influence on climate.
Some researchers carried on with the old quest for shorter-term connections. Sunspots and other measures plainly showed that the Sun had grown more active since the 19th century. Was that not linked somehow to the temperature rise in the same decades? People persevered in the old effort to winkle out correlations between sunspots and weather patterns. For example, according to a 1991 study, Northern Hemisphere temperatures over the past 130 years correlated surprisingly well with the length of the sunspot cycle (which varied between 10 and 12 years). This finding was highlighted the following year in a widely publicized report issued by a conservative group. The report maintained that the 20th-century temperature rise might be entirely due to increased solar activity. The main point they wanted to make was less scientific than political: "the scientific evidence does not support a policy of carbon dioxide restrictions with its severely negative impact on the U.S. economy." (50)
The most straightforward correlation, if it could be found, would connect climate with the Sun's total output of energy. Hopes of finding evidence for this grew stronger when two astronomers reported in 1990 that certain stars that closely resembled the Sun showed substantial variations in total output. Perhaps the Sun, too, could vary more than we had seen in the decade or so of precise measurements? In fact, studies a decade later showed that the varying stars were not so much like the Sun after all. Still, it remained possible that the Sun's total luminosity had climbed enough since the 19th century to make a serious impact on climate — if anyone could come up with an explanation for why the climate should be highly sensitive to such changes.(51a)
A more promising approach pursued the possibility of connections between climate shifts and the slow changes in the Sun's magnetic activity that could be deduced from carbon-14 measurements. A few studies that looked beyond the 11-year sunspot cycle to long-term variations turned up indications, as one group announced, of "a more significant role for solar variability in climate change. than has previously been supposed."(52) In 1997 a pair of scientists drew attention to a possible explanation for the link. Scanning a huge bank of observations compiled by an international satellite project, they reported that global cloudiness increased slightly at times when the influx of cosmic rays was greater. Later studies and reanalysis of the data found severe errors, and the authors themselves shifted from claiming an effect on high-level clouds to claiming an effect on low-level clouds. But the study did serve to stimulate new thinking.
The proposed mechanism roughly resembled the speculation that Dickinson had offered, with little confidence, back in 1975. It began with the fact that in periods of low solar activity, the Sun's shrunken magnetic field failed to divert cosmic rays from the Earth. When the cosmic rays hit the Earth's atmosphere, they not only produced carbon-14, but also sprays of electrically charged molecules. Perhaps this electrification promoted the condensation of water droplets on aerosol particles? If so, there was indeed a mechanism to produce extra cloudiness. A later study of British weather confirmed that at least regionally there was "a small yet statistically significant effect of cosmic rays on daily cloudiness." (53)
Other studies meanwhile revived the old idea that increased ultraviolet radiation in times of higher solar activity might affect climate by altering stratospheric ozone. While total radiation from the Sun was nearly constant, instruments in rockets and satellites found the energy in the ultraviolet varying by several percent over a sunspot cycle. Plugging these changes into elaborate computer models suggested that even tiny variations could make a difference, by interfering in the teetering feedback cycles that linked stratospheric chemistry and particles with lower-level winds and ocean surfaces. By the end of the 1990s, many experts thought it was possible that changes in the stratosphere might affect surface weather after all. Meanwhile others speculated about mechanisms through which the powerful electric circuit that circles the planet, and which varies in response to solar activity, might influence cloudiness. (54)
While the physics of how solar activity could affect clouds remained obscure, it was now undeniable that possible mechanisms could exist. And while the data were noisy, a growing variety of evidence, some of it going back thousands of years, showed credible correlations between solar activity and one or another feature of the climate. Whatever the exact form solar influences took, most scientists were coming to accept that the climate system was so unsteady that many kinds of minor external change could trigger a shift. It might not be necessary to invoke exotic cosmic ray mechanisms, for the system might be sensitive even to the tiny variations in the Sun's total output of energy, the solar constant. The balance of scientific opinion tilted. Many experts now thought there was indeed a solar-climate connection. (55)
The Sun vs. Greenhouse Gases (2000s) TOP OF PAGE
When a 1999 study reported evidence that the Sun's magnetic field had strengthened greatly since the 1880s, it brought still more attention to the key question: was increased solar activity the main cause of the rise of average global temperature over that period? As the 21st century began, most experts thought it likely that the Sun had driven at least part of the previous century's warming. Most convincingly, the warming from the 1880s to the 1940s had come when solar activity had definitely been rising, while the carbon dioxide buildup had not yet been large enough to matter much. A cooling during the 1950s and 1960s followed by the resumption of warming also correlated loosely with changes in solar activity. How far the solar changes had influenced climate, however, remained speculative. The temporary cooling had probably been at least partly related to an increase in smoke from smoggy haze, dust from farmlands, volcanic eruptions, and other aerosols. It was also possible that the climate system had just swung randomly on its own. One senior solar physicist insisted, "We will have to know a lot more about the Sun and the terrestrial atmosphere before we can understand the nature of the contemporary changes in climate." (56*)
By the early 21st century, however, evidence of connections between solar activity and weather was strengthening. Extremely accurate satellite measurements spanning most of the globe revealed a distinct correlation between sea-surface temperatures and the eleven-year solar cycle. The effect was tiny, not even a tenth of a degree Celsius, but it was undeniable. Similarly weak but clear effects were detected in the atmosphere near the surface and, somewhat stronger, in the thin upper atmosphere. (56a) The practical significance of these effects was minor &mdash after all, if the sunspot cycle had a truly powerful effect on weather, somebody would have proved it much earlier. The new findings, however, did pose an important challenge to computer modelers. A climate model could no longer be considered entirely satisfactory unless it could reproduce these faint, but theoretically significant, decade-scale cycles.
Rough limits could now be set on the extent of the Sun's influence. For average sunspot activity decreased after 1980, and on the whole, solar activity had not increased during the half-century since 1950. As for cosmic rays, they had been measured since the 1950s and likewise showed no long-term trend. The continuing satellite measurements of the solar constant found it cycling within narrow limits, scarcely one part in a thousand. Yet the global temperature rise that had resumed in the 1970s was accelerating at a record-breaking pace, chalking up a total of 0.8°C of warming since the late 19th century. It seemed impossible to explain that using the Sun alone, without invoking greenhouse gases. "Over the past 20 years," a group reviewing the data reported in 2007, "all the trends in the Sun that could have had an influence on the Earth's climate have been in the opposite direction to that required to explain the observed rise in global mean temperatures." It was a stroke of good luck that the rise of solar activity since the 19th century halted in the 1960s. For if solar activity had continued to rise, global temperatures might have climbed slightly faster &mdash but scientists would have had a much harder job identifying greenhouse gases as the main cause of the global warming.
The most advanced computer modeling groups did manage to reproduce the faint influence of the sunspot cycle on climate. Their calculations showed that since the 1970s that influence had been overtaken by the rising effects of greenhouse gases. The modelers got a good match to maps of the climate changes observed over the past century, but only if they included the effects of the gases, and not if they tried to attribute it all to the Sun. For example, if they put in only an increase of solar activity, the results showed a warmer stratosphere. Adding in the greenhouse effect made for stratospheric cooling (since the gases trapped heat closer to the surface). And cooling was what the observations showed.(57*)
What about global Sun-climate correlations farther back through time? Paleontologists' studies of isotopes stemming from cosmic rays continued to show a rough connection with the Medieval and Little Ice Age climate anomalies. And an especially neat study of deposits in a cave in China found a solid correlation between weather and solar activity spanning the past two millennia. However, the correlation had broken down after 1960, just when greenhouse gases began to kick in &mdash evidently overwhelming weaker influences. Painstaking studies simply failed to find any significant correlation between cosmic rays and cloudiness. The consensus of most scientists, arduously hammered out in a series of international workshops, flatly rejected the argument that the soaring temperatures since the 1960s could be dismissed as a consequence of changes on the Sun. In 2004 when a group of scientists published evidence that the solar activity of the 20th century had been unusually high, they nevertheless concluded that "even under the extreme assumption that the Sun was responsible for all the global warming prior to 1970, at most 30% of the strong warming since then can be of solar origin."(57a)
When Foukal reviewed the question in 2006, he found no decisive evidence that the Sun had played the central role in any climate change, not even the Little Ice Age. The cold spells of the early modern centuries, experts were beginning to realize, could be at least partly explained by other influences. For one, a spate of sky-darkening volcanic eruptions that had triggered a period of increased sea ice which reflected sunlight from the North Atlantic region. Even more striking was evidence that the CO 2 level in the atmosphere had dipped during those centuries &mdash perhaps because so much farmland had reverted to carbon-absorbing forest as a result of the depopulation caused by the Black Death in Eurasia and the great die-off of native Americans with the arrival of European conquerors and their diseases. The greenhouse effect, even back then, looked like the dominant influence on global climate.
Still, many experts thought it likely that the Maunder Minimum of solar activity could have had something to do with the early modern climate anomalies, contributing perhaps a couple of tenths of a degree of cooling. One theory, for example, held that the changes in ozone (less ultraviolet=less ozone=less warming in the stratosphere) would have had a particularly strong effect on the Northern Hemisphere jet stream. This particularly affected the weather in Europe, the classic location of Little Ice Age cold spells: perhaps low solar activity did make for colder winters there. Whatever the mechanism, a group convened in 2012 concluded that solar ultraviolet variations had mainly regional effects and could "contribute very little to global temperature variations."(57b*)
A few scientists persevered in arguing that much smaller solar changes (which they thought they detected in the satellite record) had driven the extraordinary warming since the 1970s. But even among these outlying groups, leaders admitted that in the future, "solar forcing could be significant, but not dominant." Nevertheless the argument that solar activity was the true cause of global warming continued to circulate. It was one example of the indestructible "zombie" theories that plagued discussions. As it happened, solar activity sank to historic lows after 2005. Some prominent figures among the opposition to regulating greenhouse gases publicly predicted rapid global cooling. When temperatures climbed to a new record in 2014 (and a higher record in 2015, higher still in 2016, etc.) while solar activity remained unusually low, only the ignorant or disingenuous could persist in denying that greenhouse gases were the only plausible cause.(58*)
By the 2010s the study of "solar-terrestrial relations" (as scientists called the topic) had settled down to teasing out the numerous complex and subtle ways solar activity might possibly influence specific weather patterns. Such research required, first, assembling and standardizing vast collections of weather data, and second, adapting one or another of the elaborate supercomputer models of the atmosphere to test hypotheses for complex mechanisms like ozone interactions. The research was pursued vigorously as part of the perpetual enterprise of improving short-term weather predictions, but it was scarcely relevant to climate change.(59)
The import of the claim that solar variations influenced climate was now reversed. Critics had used the claim to oppose regulation of greenhouse gases. But what if the planet really was at least a bit sensitive to almost imperceptible changes in the total radiation arriving from the Sun? The planet would surely react no less strongly to changes in the interference by greenhouse gases with the radiation after it entered the atmosphere. Some of the scientists who reported evidence of past connections between the Sun and climate changes warned explicitly that their data did not show that the current global warming was natural — it only showed the extreme sensitivity of the climate system to small perturbations.
Back in 1994 a U.S. National Academy of Sciences panel had estimated that if solar radiation were to weaken as much as it had during the 17th-century Maunder Minimum, the entire effect would be offset by another two decades of accumulation of greenhouse gases. A 2010 study reported that with the growing rate of emissions, by the late 21st century a Maunder-Minimum solar effect would be offset in a single decade. As one expert explained, the Little Ice Age "was a mere 'blip' compared with expected future climatic change."(60)

For more on temperature changes over the past millennium or so, see the conclusion and figure captions in the essay on The Modern Temperature Trend.

1. This essay is partly based, by permission, on an essay by Theodore S. Feldman, "Solar Variability and Climate Change," rewritten and expanded by Spencer Weart. For additional material, see Feldman's site. BACK

3. Notably, for variations related to the evolution of the Sun and stars, Dubois (1895) for sunspot cycles Czerney (1881) . BACK

6. Douglass (1936) Webb (2002), chapter 3 Webb (1986).Fritts (1962) pioneered accurate use of tree rings Fritts (1976) notes the skepticism (page v) and shows how it was overcome. Climate periods of 11-12 years as well as longer cycles also appeared in annual layers of clay laid down in lake beds (varves), Bradley (1929) for references and summary, see Brooks (1950a). BACK

7. Abbot and Fowle (1913) similarly A. Ångström, using Abbot's data, said the solar constant varied with sunspot number, although decades later he retracted. Ångström (1922) Ångström (1970) historical studies are Hufbauer (1991), p. 86 DeVorkin (1990). BACK Angstrom

11. J. Eddy, interview by Weart, April 1999, AIP, online here, p. 6. BACK

13. Simpson (1934) Simpson (1939-40). Simpson cited A. Penck, who argued that the entire world had cooled and only solar changes could explain this. BACK

14. Simpson (1939-40), p. 210. Solar models were also put into doubt by the "faint early Sun paradox" (or "faint young Sun. ") Astrophysicists calculated that in its youth Earth should not have received enough sunlight to prevent it from freezing over. See the essay on "Venus and Mars" here. Zirin et al. (1976), p. 379, also p. 381 (neutrinos). BACK

15. Öpik (1958) "flickering" (due to uncertain convective changes): Öpik (1965), p. 289. BACK

16. E.g., Brooks (1949), ch. 1 Shapley (1953) Wexler (1956), quote p. 494, adding that turbidity (from volcanoes) was equally important. BACK

17. Willett (1949), pp. 34, 41, 50 see Lamb (1997), p. 193 the earlier hypothesis (not cited by Willett) came from Haurwitz (1946) (admitting it was "vague"), whose work inspired Wexler (1956), p. 485 (citing glacier papers) subsequently Wexler (1960) dismissed the idea. BACK

18. A possible connection between cosmic rays and clouds was already established at the end of the 19th century by the inventor of the cloud chamber, Wilson (1899) it was admittedly "speculation" that ionization in the upper troposphere affected storminess. Ney (1959) the ideas found some favor with, e.g., Roberts (1967), pp. 33-34. BACK

22. Johnsen et al. (1970) similarly, Dansgaard et al. (1971), same quote p. 44 the period they reported was precisely 78 years, and Schove (1955) had reported a 78-year variation between long and short sunspot cycles as well as a possible 200-year period in addition, not noted by the glaciologists, a roughly 80-year modulation in the amplitude of the sunspot cycle was reported by Gleissberg (1966) weather correlations with the 80-year cycle were reported in 1962 by B.L. Dzerdzeevski as cited by Lamb (1977), p. 702. BACK

26. Roberts and Olson (1975) (admitting that "A mere coincidence in timing. will not, of course, constitute proof of a physical relationship") Mock and W.D. Hibler (1976) (a "pervasive" but only "quasi-periodic" 20-year cycle) Mitchell et al. (1979) (tree-ring data analysis "strongly supports earlier evidence of a 22 yr drought rhythm. in the U.S. in some manner controlled by long-term solar variability. " ). BACK

29. Suess (1968), p. 146 in the best review of sunspot history available to Suess at this time, D.J. Schove took no notice of any anomaly such as the early-modern minimum, although it is visible in his data. Schove (1955) a tentative longer-term correlation of climate (glacier advances) with C-14 was shown by Denton and Karlén (1973), who suggest that "climatic fluctuations, because of their close correlation with short-term C14 variations, were caused by varying solar activity," p. 202 for the Little Ice Age, see Fagan (2000) Lamb (1995), ch. 12. BACK

32. Dickinson (1975) a similar speculation, connecting cosmic rays with storminess, was offered by Tinsley et al. (1989). Tinsley's work was stimulated by a correlation reported by Wilcox et al. (1973), which attracted some attention but grew weaker as the next decade of data accumulated. Another weather-Sun correlation was laid out in Herman and Goldberg (1978), which met strong resistance including attempts to suppress publication, according to Herman (2003), ch. 18. BACK

33. Maunder (1890) attributes the discovery to Spörer some authors now refer to a 17th-century Maunder Minimum and a 15th-century Spörer Minimum. Eddy chose "Maunder" to make a phrase that would be memorable: Eddy, interview by Weart, op.cit., p. 11. For history and references, see Eddy (1976) examples of neglect of Maunder: he was cited, but only for other work, in Abetti (1957) Kuiper (1953) Menzel (1949) the 17th-century paucity of sunspots was noted without any reference by Willett (1949), p. 35. On Eddy and sunspot cycles in general see Henderson (2018). BACK

34. The first published statement was an abstract for the March 1975 meeting of the American Astronomical Society Eddy (1975a) and next at a Solar Output Workshop in Boulder, Colo., Eddy (1975b) the famous publication was Eddy (1976), "defeat" p. 1200 "felt certain," Eddy (1977a), pp. 80-81. See Eddy, interview by Weart, op. cit. BACK

36a. Jones and Mann (2004), p. 20, see p. 7 and passim Neukom et al. (2019) PAGES 2k Consortium (2019). See also this note on the "hockey stick" graph in the essay on the modern temperature trend. BACK

37. Workshop: Zirin et al. (1976). White (1977), see Mitchell p. 21, Hays p. 89 note also the earlier, more doubting response of Mitchell (1976), p. 491. "Salesman": Eddy, interview by Weart, op. cit., p. 14. BACK

38. Eddy (1977b), quote p. 173 for more extensive speculations and reflections, see Eddy (1977a). BACK

40. Hansen et al. (1981), using what was admittedly a "highly conjectural" (p. 93) measure of variability by D.V. Hoyt. BACK

44. Gilliland (1981), reporting 11- and 76-year variations in solar size Gilliland (1982a) Gilliland (1982b), quote p. 128. BACK

45. Hufbauer (1991), pp. 278-80 for example, a 1978 workshop concluded that changes in stratospheric ozone due to ultraviolet radiation might influence climate McCormac and Seliga (1979), pp. 18, 20. BACK

52. "More significant" (an "admittedly crude" analysis): Cliver et al. (1998), p. 1035. BACK

54. Stratosphere and ultraviolet: Haigh (1994) Rind and Balachandran (1995) Haigh (1996) McCormack et al. (1997) Shindell et al. (1999) Labitzke and van Loon (1999) for discussion, see Wallace and Thompson (2002) more recently, White (2006). Global electric circuit: Tinsley (1996) Tinsley (2000). More recent work by Tinsley and others is reviewed by Ram et al. (2009). Arnold (2002) is an example of the complexity of the arguments and provides historical references. Two reviews are Bard and Frank (2006), p. 5 Kirkby (2007). BACK

55. For example, correlations of cosmic rays (as an indicator of solar activity) with Asian monsoons, Neff et al. (2001) Wang et al. (2005) and with North Atlantic Ocean events, Bond et al. (2001). BACK

56. Lockwood et al. (1999) reviewing various claims, including some based on observations of variations in supposedly Sun-like stars, three experts concluded in 2004 that "Any relationship" between long-term solar variations and climate "must remain speculative," Foukal et al. (2004). Know a lot more: Parker (1999) cf. criticism of Parker by Hoffert et al. (1999) BACK

57. Tett et al. (1999) stratosphere: IPCC (2001a), p. 709. Benestad (2005) reports that ". comparison with the monthly sunspot number, cosmic galactic rays and 10.7 cm absolute radio flux since 1950 gives no indication of a systematic trend in the level of solar activity that can explain the most recent global warming." Similarly see Wang et al. (2005). "Over the past 20 years": Lockwood and Fröhlich (2007) another review: Bard and Frank (2006), on model sensitivity see p. 7. BACK

How close would the sun have to get to Earth for there to be severe consequences? - Astronomy

Earth's close call with a solar super storm
Posted: 6 APRIL 2014

On 23 July 2012 the Sun launched a magnetic attack that only narrowly missed Earth by nine days -- a relatively close and terrifying call.

The magnetic storm sent by the Sun came from a succession of coronal mass ejections that propelled disastrous amounts of magnetised plasma through Earth's orbit. Had this solar super storm hit Earth, it would have produced the most severe geomagnetic storm since the nineteenth century.

The storm would have disrupted electrical power grids, radio communications, satellite navigation and spacecraft operations and, as estimated by the Space Study Board of the United States, the impact of such a storm on technological grounds alone could have had a worldwide cost of a trillion dollars, with a recovery time up to a decade. The severity of this storm would have been so enormous that the Internet, phone lines and all other forms of modern communication would be gone, knocking us back several years. Computers would have crashed, with some places in total darkness through the loss of power.

This cannot be said for all countries however, as it all comes down to how much a country depends on technology, says Professor Ying Liu of China's State Key Laboratory of Space Weather, who led research into the solar eruptions.

"For example, a developing country that does not depend much on technology may be affected less than those that depend heavily on such technology," he says. "On the other hand, if the technological systems are better prepared for such storms, the country may be affected less."

Based on the research that Liu and his colleagues have conducted on this particular magnetic storm, they conclude that the Sun hurled the magnetic cloud through the solar wind at a speed of more than 2,000 kilometres per second, which is around four times the speed of a normal magnetic cloud. Had Earth been in the line of fire that day, it would have felt the full impact of the storm. The resulting major power blackouts would have meant a lot of related repairs, the loss of frozen food storages and many of the world's electrical power grids, while trans-oceanic cables could have also been affected.

An image of the Sun emitting a coronal mass ejection (right) on 23 July. Image: ESA/NASA (SOHO).

Space agencies would have had to replace lost satellites and monitor the existing ones as the upper atmosphere would become so inflated by heating that it would have lead to faster satellite orbital decay in several cases.

However, as Liu's co-researcher, Janet Luhmann of the University of California, Berkeley explains, knowing when these storms are going to occur is hard to predict.

"Most solar storms, like regular weather, only give some warning and different types of space weather have different levels of predictability," she says. "For example if a big, complex sunspot group is on the centre of the Sun's disc or to the west of it, we watch it very closely for any major flares or coronal mass ejections. If these are observed, one expects high-energy particles to arrive in Earth's vicinity within a few tens of minutes and then a few days later a big geomagnetic storm is possible."

Fortunately for us, the chances of another storm similar to the one in 2012 happening again so soon are very slim, although not impossible, according to Luhmann.

"We get fairly big storms about once in every 11-year solar cycle in a specific location but this type of 'perfect storm' is fortunately a lot rarer. However, like major earthquakes, typhoons and tornadoes, there is always a chance of one at any time."