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What would happen to a cold planet that has a large amount of flammable gas, like Neptune, if we throw some fire into it? Will it burn, or would the flame be extinguished due to the cold?
The outer parts of Neptune are mostly hydrogen and helium. There are small amounts of other gases such as methane, ammonia and water vapour. However, there is no oxygen at all.
If you took some of Neptune's outer layer back to earth and mixed it with our air, it could burn. Even very cold hydrogen can burn (it soon heats up!) This couldn't happen on Neptune, because a fire needs both fuel and oxygen to burn.
It is very unlikely that any planet would have large amounts of both fuel and oxygen in its atmosphere. Oxygen is very reactive and will react with any flammable gases to produce (mostly) water and carbon dioxide. Oxygen is not stable in a hydrogen atmosphere over a period of millions of years. If we find oxygen in a planet's atmosphere, we can be fairly sure that something on the planet is making it.
The sky on fire: The cold flames of the Perseus Molecular Cloud
Some of the largest single structures in the galaxy are huge dark nebulae, clouds of gas and dust that are extremely cold in human terms and emit almost no visible light. These are called Giant Molecular Clouds — they're cold enough that atoms can stick together to form more complex molecules — and are so utterly dark that we tend to see them in silhouette against the star-filled Milky Way.
. to our eyes, that is. We see only a narrow band of the entire electromagnetic spectrum. But others parts exist, like radio waves, X-rays, ultraviolet. We've built telescopes and equipped them with detectors to see these other kinds of light many are in space because we need them to be above our atmosphere, which absorbs those kinds of light, making it opaque to them.
More Bad Astronomy
The Spitzer Space Telescope is one of them. Operated for 16 years before budgetary restraints shut it down, it still orbits the Sun on an "Earth-trailing" path that lets it slowly drift away from our home planet. Designed to see in the infrared, it observed the Universe from nearby asteroids to galaxies at the edge of the observable Universe billion so of light years away.
It also saw many Giant Molecular Clouds. While these are black to our eyes, they're warm enough (though still very chilly) that they glow in the infrared. And when Spitzer saw them, it really saw them:
Spitzer Space Telescope image of a part of the enormous Perseus Molecular Cloud, a nearby star-forming factory. Credit: NASA/JPL-Caltech
That piece of magnificence is the Perseus Molecular Cloud, located very roughly 1000 light years away in the constellation of (can you guess?) Perseus. This image shows a huge patch of sky over 5° across — ten full Moons could fit across it.
The Spitzer image of the Perseus Molecular Cloud superposed on a much area of the sky in visible light, with constellations marked. The nebula to its upper left is the California Nebula, and below it is the Pleiades star cluster. Credit: NASA/JPL-Caltech
This shot was taken early on in the mission after 2009 the liquid helium cryogenic coolant ran out, and Spitzer lost a couple of its longest wavelength channels. But when this image was taken it could still see at a wavelength of 24 microns, over 30 times longer than the reddest light the human eye can see. At this wavelength, the view is dominated by dust at about -150° C (-240° F) dust is composed of tiny grains of rocky material or long molecules of carbon, created when stars die, blowing the material into space. The color in the image isn't real it's just been converted to color we can see, so the orange looks like flame, but it's a cold flame.
The Spitzer image of the Perseus Molecular Cloud with several key features labeled. Credit: NASA/JPL-Caltech and Phil Plait
There are several features in the cloud visible in this image. The bright knot on the left is IC 348, a very young cluster of stars. Molecular clouds can be rife with star formation regions where the local density is high can collapse under their own gravity to form stars. The most famous example of this is the Orion Nebula, which is on the near side of the enormous Orion Molecular Complex. IC 348 is probably only a couple of million years old, and most of the stars are low mass like the Sun. If it had high-mass stars in it they would light it up like a Christmas tree, and this would be as spectacular as the Orion Nebula. Instead, it's mostly hidden from our view, enshrouded in thick dust.
Just to the right of it is a larger circular ring called Barnard 3. I've written about that before in the center of it is a massive star called HD 278942, which is blasting out a wind of subatomic particles that snowplow up material, creating that ring.
In infrared light seen by Spitzer Space Telescope, NGC 1333 shows copious young stars scattered throughout its gas. Credit: NASA/JPL-Caltech/R. A. Gutermuth (Harvard-Smithsonian CfA)
On the right hand side of the image is another clutch of stars called NGC 1333. I've written extensively about this cluster as well (here and here). It's loaded with young, hot stars which are blasting out radiation and gas, carving up and sculpting the gas and dust in the molecular cloud around them in bizarre and beautiful shapes. You can see a handful of other small clusters below NGC 1333 in the bigger image, too.
The Perseus Molecular Cloud (center) in context this image from the AKARI FIS all sky far infrared survey is a whopping 36° across. Credit: Aladin / ISIS / JAXA
Molecular clouds like this can claim the lion's share of star formation in our galaxy, some birthing thousands upon thousands of stars. Mind you, this Spitzer image is only a piece of a much bigger complex of gas and dust you're seeing maybe a 100 light year strip of a sprawling 500 light-year long cloud. These clouds are enormous and they're everywhere in the galaxy.
I often wonder if the Sun was born in a cloud like this. There are also much smaller clouds but they form stars at much lower rates, so it seems pretty likely something like Perseus is where we got our start. It happened 4.6 billion years ago, and that nebula is likely long gone by now, or possibly on the other side of the galaxy anyway, so we may never know.
But studying objects like these is literally trying to understand where we come from. It's one of the things that makes me love astronomy so much we want to know our origins, and how we came to be.
And on top of that the objects we study are just so flipping gorgeous. That is something I will never tire of.
During timeperiods with more oxygen in the atmosphere, did fires burn faster/hotter?
Yes. And during periods with lower oxygen levels, fires burned more slowly or not at all. Some natural fuels will burn at high oxygen concentrations but not low. This article examines these relationships. Wildfires may actually act to stabilize atmospheric oxygen levels. If the concentration increases, fires will burn faster and consume the excess. If the concentration decreases, fires slow down and consume less oxygen, allowing the concentration to rise again. Check out this excellent paper(PDF) to learn more about this and other relationships between fire and climate, ecology, evolution, etc.
So it's just like the piggies/trees in the Ender sequels?
You seem to know what you are talking about. If I have a fire with the same kind and amount of wood. So, it's actually two fires same wood same amount of wood. One burns in cold temperature, I'll say 20 degrees. Not super cold, bug cold to us. Then I have a second fire in like 75 degrees. Does the cold affect how the fire burns?3
Wildfires may actually act to stabilize atmospheric oxygen levels. If the concentration increases, fires will burn faster and consume the excess
If this is true, then why did California just have its largest wildfire in hundreds of years?
C02 emissions are the highest they’ve been in 1000s of years. Wouldn’t our wildfire have been smaller rather than larger?
Absolutely - we actually have a ton of fossil evidence for fires during the Carboniferous era, when both biomass and atmospheric oxygen content were high. If you look at the coal deposits from that time, there's a high percentage of charcoal in it. Some of the evidence actually suggests that wildfires may have been a regular feature of Carboniferous forests.
Meanwhile at the end of the Permian, we had a decrease in atmospheric oxygen which contributed to the "coal gap" along with the decreased biomass from the end-Permian extinction.
How exactly was the carboniferous period named - was it these fires and high oxygen levels directly leaving a lot of carbon material around, or a coincidence that they occurred at the same time as other processes? (I seem to remember it being because of coal and other deposits being left from the time, and was wondering whether I'm correct and whether there was a link.)
So does this mean with more CO2 being put into the atmosphere by increase in wildfires and fossil fuel burning, we are significantly lowering the amount of O2 in the atmosphere? ANd if so, maybe this is the earth's way of slowing down burning (of wildfires cause human caused fossil fuel burning will continue)? And if so, how much increase in CO2 and decrease in O2, has happend to date, or by say 2100?
I thought to make charcoal you try to exclude oxygen by burning it in a mound of earth or airtight kiln
As has been said, the answer is yes. An interesting corollary I haven’t seen mentioned yet, though, is that Earth is the only planetary body we know of on which fires are actually possible. Fire is a redox reaction, in which a reduced substrate (wood in the case of a forest fire) is oxidized by the oxygen in the atmosphere. The wild part is that only on Earth do reduced substrates and an oxidized atmosphere occur together, and it’s only because of life that they do here. Photosynthesis is responsible for nearly 100% of the oxygen that exists on Earth, and thus cyanobacteria, plants, and algae are the reason fire can occur on our planet.
Other bodies can be either highly reduced or highly oxidized, but not both. Saturn’s moon Titan, for instance, is covered in oceans of liquid methane. This would be a dangerous occurrence on Earth, but there’s no oxidant available on Titan for the methane to react with, so it will never burn. Mars on the other hand has an oxidized surface, but no reduced substrates that could burn even exposed to oxygen.
The fact that fire can only occur on Earth because life has produced enough oxygen to change the composition of our entire atmosphere means that wildfires are a very good candidate biosignature if we can detect them on other worlds. We just don’t know of any way for them to occur on lifeless planets.
Yes. NASA experimented with 100% oxygen in space capsules, as to have to ship up less nitrogen, since sending things in orbit is extremely pricy. They changed out of this amongst other reasons because a fire started in Apollo 1 which burnt out the entire cabin and overpressurised it within half a minute.
It's a bit more complicated than that.
The reactivity of Oxygen isn't about it's percentage out of what gas is there. It's about it's Partial Pressure. That is the pressure exerted by just the oxygen in a gas mixture. We find this by multiplying the total pressure of the gas by the the percent that is oxygen. So in our atmosphere the partial pressure of oxygen is found by taking 14.7 psi (atmospheric pressure) * 21% (percent of atmosphere that is oxygen) to get 3 psi.
In flight the capsule was designed to contain a pure oxygen atmosphere at 5 psi. Higher than on earth, but not dramaticaly so. This was done because it allowed for the capsule to be lighter since it only had to hold in 5 psi when in space, not 14.7 psi, and there is less mass in the atmosphere and storage tanks of the capsule. 5 psi of pure oxygen does not present a significant fire risk. It is pretty similar to what you get on the ground in our atmosphere.
The danger arose from what you had to do to get a pure oxygen environment in the capsule.
To push the air out of the capsule and make it a pure oxygen environment they pressurized it to greater than ambient pressure. This was done to 16.7 psi. More than 5 times the normal partial pressure of oxygen.
This is incredibly dangerous. All kinds of things that are normally non-flammable normally will basically explode at these levels of oxygen. Most notably in this case, Velcro.
They had not had an incident using these same procedures for the Mercury and Gemini missions. So they believed it to be safe.
The test that was performed for the Apollo 1 that day involved pressurizing the capsule with pure oxygen using the same procedures as on launch.
If they had found a way to transition to pure oxygen at 5 psi in flight that did not require starting at high pressure pure oxygen, there would be no increase in the risk of a fire.
Could 'flammable ice' be the key to discovering alien life?
In studying what's known as "flammable ice," researchers have discovered that microscopic bubbles within the strange material contain life. These findings could inform the quest to identify extraterrestrial life.
Flammable ice, also known as methane hydrate, is created when methane gas is trapped within ice's molecular structure. Sheets of this frozen gas and ice contain microscopic bubbles of oil and water. In a new study, scientists studying "flammable ice" in the Sea of Japan found microscopic, living creatures within these tiny bubbles.
The researchers in this study came upon this discovery in a unique manner. While melting hydrate to study the methane gas it contains, Glen T. Snyder, a researcher at Meiji University and lead author of the new study, noticed a powder with little, microscopic spheroids in it that contained tiny spheres with dark centers in them. The finding was so strange that Snyder gathered a team together to probe further.
"In combination with the other evidence collected by my colleagues, my results showed that even under near-freezing temperatures, at extremely high pressures, with only heavy oil and saltwater for food-sources, life was flourishing and leaving its mark" inside these little bubbles in the "flammable ice," Stephen Bowden of the University of Aberdeen's School of Geosciences in Scotland, a co-author on this study, said in a statement.
To come to this conclusion, Bowden used analytic techniques developed at the University of Aberdeen that are specially designed for small sample sizes. Using these techniques, Bowden was able to show that the oil in this unique material was degrading in the tiny environments of the bubbles within the flammable ice.
So how does this work inform the search for extraterrestrial life? "The methane in 'methane hydrate' is known to form as microbes degrade organic matter on the seafloor. But what we never expected to find was microbes continuing to grow and produce these spheroids, all of the time while isolated in tiny cold dark pockets of saltwater and oil," Snyder said in the statement. "It certainly gives a positive spin to cold dark places, and opens up a tantalizing clue as to the existence of life on other planets."
"It certainly changes how I think about things," Bowden added, as he thought about what this discovery could mean for the search for life on cold exoplanets. "Providing they have ice and a little heat, all those frigid cold planets at the edge of every planetary system could host tiny microhabitats with microbes building their own 'death stars' and making their own tiny little atmospheres and ecosystems, just as we discovered here."
These findings were detailed in a paper published Feb. 5 in the journal Scientific Reports.
This particular ivy was planted in the mid-1900s because it is a fast-growing ground cover that can quickly cover bare spots on hills, and pretty much everywhere else. Its invasiveness—it attaches firmly to surfaces like walls, fences, and arbors—has made it despised by some and considered a deep-rooted weed, spreading, and hard to get rid of.
Lighting Farts on Fire: The Blue Flame
Human flatus may contain hydrogen gas and/or methane, which are flammable. If sufficient amounts of these gases are present, it's possible to light the fart on fire. Keep in mind, not all farts are flammable. Although flatus has great YouTube fame for producing a blue flame, it turns out only about half of people have the archaea (bacteria) in their bodies that are necessary to produce methane. If you don't make methane, you may still be able to ignite your farts (a dangerous practice!), but the flame will be yellow or possibly orange rather than blue.
Smoking and Oxygen Therapy
If a person on oxygen smokes a cigarette, they won't explode or even burst into flame. Smoking around oxygen is not particularly dangerous, as least as far as fire is concerned. However, there are good reasons to avoid smoking if you or someone nearby is on oxygen therapy:
- Smoking produces smoke, carbon monoxide, and other chemicals, which reduce oxygen availability and irritate the respiratory system. If someone is on oxygen therapy, smoking is counterproductive and harmful to their health.
- If burning ash falls from a cigarette and starts to smolder, the extra oxygen will foster a flame. Depending on where the ash falls, there may be enough fuel to start a significant fire. The oxygen would make the situation that much worse.
- An ignition source is needed to light a cigarette. Oxygen could cause the flame of a lighter to flare or a lit match to burst into an unexpectedly large flame, leading to a burn on the person. Or it could cause them to drop a burning object onto a potentially flammable surface. Oxygen flare-up fires do occur in emergency rooms, so the risk is present, although somewhat reduced in a home setting.
- If oxygen therapy is conducted in a hospital, smoking is prohibited for several reasons. Aside from the negative health effects on the smoker, secondhand smoke is produced and can be inhaled by others. Plus the residue from smoking remains even after the cigarette is extinguished, making the room unhealthy for patients who come in afterward.
- In a medical setting, there may be other gases (e.g., anesthesia) or materials present which could be ignited by a spark or a cigarette. The extra oxygen makes this risk especially dangerous since the combination of spark, fuel, and oxygen could lead to a serious fire or explosion.
Key Takeaways: Oxygen and Flammability
- Oxygen does not burn. It is not flammable, but it is an oxidizer.
- Oxygen feeds fire, so it's dangerous to use around something that is burning because it will help the fire burn much more quickly.
- Patients on oxygen therapy who are smokers are not going to burst into flame or explode if they smoke. However, the risk of a fire or accident is greatly increased. And smoking negates some of the benefits of using oxygen.
Finney ticks off three primary ingredients for a fire. First is combustion, which is the chemical reaction of a fuel with oxygen that results in glowing, flaming, and heat release. Then there’s a transfer of energy, as by radiation (the release of heat) or convection (the movement of hot gases or fluids). The third ingredient is the ignition of new fuels that are encountered by the released energy. “Those each have open questions associated with them that are basic in nature,” says Finney.
Recent studies at Missoula have started to crack open some of fire’s physics mysteries. One of the biggest, in recent years, is the role of convection. Since fire research began in earnest more than 70 years ago, scientists have largely assumed that radiation was the most important factor in spreading a fire. The idea was that combustion produced radiant energy, which heated and ignited new fuel. “Nobody had ever studied flame structure before,” says Finney. And why would they have? If radiation – a clean and simple phenomenon – suffices as an explanation, then why bother with convection?
But the assumption that radiation would suffice hit a snag when Finney and other researchers began reporting on experiments showing that radiation is not sufficient. Small particles of fuel failed to ignite when exposed to radiation at levels equivalent to those from a forest fire, which in turn led researchers to look at whether convection played a significant role (2015 PNAS 112 9833). Detailed experiments using high-speed cameras revealed structures lurking within the fire. Imaging revealed vortex pairs rotating in opposite directions, forcing flames into patterns of upward-pointing peaks and lower troughs. These vortices looked familiar – in fluid dynamics, they’re known as “Taylor–Görtler vortex pairs” and they arise when a turbulent fluid encounters a concave boundary. The vortices help explain the bright streaks that are often observed in fires (figure 1).
Finney and his team also found that the vortices could explain a phenomenon that’s been observed since the 1960s, in which powerful bursts of flame sometimes surge out of a leading edge of a fire and engulf the surrounding environment. In small fires, that surge may be only a few inches in large crown fires, it may produce flame bursts many tens of metres long. Such bursts can be deadly, especially to the brave firefighters trying to control a wild blaze. Their study suggests that convection, rather than radiation, is the secret ingredient (or just one of them) to pushing a fire forward, but they’ll need to run more experiments at larger scales to see if that conclusion holds.
Fires start when a flammable or a combustible material, in combination with a sufficient quantity of an oxidizer such as oxygen gas or another oxygen-rich compound (though non-oxygen oxidizers exist), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron. Fire cannot exist without all of these elements in place and in the right proportions. For example, a flammable liquid will start burning only if the fuel and oxygen are in the right proportions. Some fuel-oxygen mixes may require a catalyst, a substance that is not consumed, when added, in any chemical reaction during combustion, but which enables the reactants to combust more readily.
Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion and may propagate, provided there is a continuous supply of an oxidizer and fuel.
If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection, which removes combustion products and brings a supply of oxygen to the fire. Without gravity, a fire rapidly surrounds itself with its own combustion products and non-oxidizing gases from the air, which exclude oxygen and extinguish the fire. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight.   This does not apply if oxygen is supplied to the fire by some process other than thermal convection.
Fire can be extinguished by removing any one of the elements of the fire tetrahedron. Consider a natural gas flame, such as from a stove-top burner. The fire can be extinguished by any of the following:
- turning off the gas supply, which removes the fuel source
- covering the flame completely, which smothers the flame as the combustion both uses the available oxidizer (the oxygen in the air) and displaces it from the area around the flame with CO2
- application of water, which removes heat from the fire faster than the fire can produce it (similarly, blowing hard on a flame will displace the heat of the currently burning gas from its fuel source, to the same end), or
- application of a retardant chemical such as Halon to the flame, which retards the chemical reaction itself until the rate of combustion is too slow to maintain the chain reaction.
In contrast, fire is intensified by increasing the overall rate of combustion. Methods to do this include balancing the input of fuel and oxidizer to stoichiometric proportions, increasing fuel and oxidizer input in this balanced mix, increasing the ambient temperature so the fire's own heat is better able to sustain combustion, or providing a catalyst, a non-reactant medium in which the fuel and oxidizer can more readily react.
A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum of which depends on the chemical composition of the burning material and intermediate reaction products. In many cases, such as the burning of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of "fire". This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Other possible combinations producing flames, amongst many, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide. Hydrogen and hydrazine/UDMH flames are similarly pale blue, while burning boron and its compounds, evaluated in mid-20th century as a high energy fuel for jet and rocket engines, emits intense green flame, leading to its informal nickname of "Green Dragon".
The glow of a flame is complex. Black-body radiation is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon emission by de-excited atoms and molecules in the gases. Much of the radiation is emitted in the visible and infrared bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra. The dominant color in a flame changes with temperature. The photo of the forest fire in Canada is an excellent example of this variation. Near the ground, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.
The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In micro gravity or zero gravity,  such as an environment in outer space, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although it may go out if not moved steadily, as the CO2 from combustion does not disperse as readily in micro gravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is sufficiently evenly distributed that soot is not formed and complete combustion occurs.  Experiments by NASA reveal that diffusion flames in micro gravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth, because of a series of mechanisms that behave differently in micro gravity when compared to normal gravity conditions.  These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.
In combustion engines, various steps are taken to eliminate a flame. The method depends mainly on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel.
Typical adiabatic temperatures
The adiabatic flame temperature of a given fuel and oxidizer pair is that at which the gases achieve stable combustion.
- –dicyanoacetylene 4,990 °C (9,000 °F) –acetylene 3,480 °C (6,300 °F) 2,800 °C (5,100 °F) –acetylene 2,534 °C (4,600 °F) (air–MAPP gas) 2,200 °C (4,000 °F) (air–natural gas) 1,300 to 1,600 °C (2,400 to 2,900 °F)  (air–paraffin) 1,000 °C (1,800 °F)
Every natural ecosystem has its own fire regime, and the organisms in those ecosystems are adapted to or dependent upon that fire regime. Fire creates a mosaic of different habitat patches, each at a different stage of succession.  Different species of plants, animals, and microbes specialize in exploiting a particular stage, and by creating these different types of patches, fire allows a greater number of species to exist within a landscape.
Fire science is a branch of physical science which includes fire behavior, dynamics, and combustion. Applications of fire science include fire protection, fire investigation, and wildfire management.
The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago ,  permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire.  Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago , by fossils of charcoalified plants.   Apart from a controversial gap in the Late Devonian, charcoal is present ever since.  The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.  Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago  this kindling provided tinder which allowed for the more rapid spread of fire.  These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire. 
The ability to control fire was a dramatic change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook food, simultaneously increasing the variety and availability of nutrients and reducing disease by killing organisms in the food.  The heat produced would also help people stay warm in cold weather, enabling them to live in cooler climates. Fire also kept nocturnal predators at bay. Evidence of cooked food is found from 1 million years ago ,  although fire was probably not used in a controlled fashion until 400,000 years ago.  There is some evidence that fire may have been used in a controlled fashion about 1 million years ago.   Evidence becomes widespread around 50 to 100 thousand years ago, suggesting regular use from this time interestingly, resistance to air pollution started to evolve in human populations at a similar point in time.  The use of fire became progressively more sophisticated, with it being used to create charcoal and to control wildlife from 'tens of thousands' of years ago. 
Fire has also been used for centuries as a method of torture and execution, as evidenced by death by burning as well as torture devices such as the iron boot, which could be filled with water, oil, or even lead and then heated over an open fire to the agony of the wearer.
By the Neolithic Revolution, [ citation needed ] during the introduction of grain-based agriculture, people all over the world used fire as a tool in landscape management. These fires were typically controlled burns or "cool fires", [ citation needed ] as opposed to uncontrolled "hot fires", which damage the soil. Hot fires destroy plants and animals, and endanger communities. This is especially a problem in the forests of today where traditional burning is prevented in order to encourage the growth of timber crops. Cool fires are generally conducted in the spring and autumn. They clear undergrowth, burning up biomass that could trigger a hot fire should it get too dense. They provide a greater variety of environments, which encourages game and plant diversity. For humans, they make dense, impassable forests traversable. Another human use for fire in regards to landscape management is its use to clear land for agriculture. Slash-and-burn agriculture is still common across much of tropical Africa, Asia and South America. "For small farmers, it is a convenient way to clear overgrown areas and release nutrients from standing vegetation back into the soil", said Miguel Pinedo-Vasquez, an ecologist at the Earth Institute’s Center for Environmental Research and Conservation.  However this useful strategy is also problematic. Growing population, fragmentation of forests and warming climate are making the earth's surface more prone to ever-larger escaped fires. These harm ecosystems and human infrastructure, cause health problems, and send up spirals of carbon and soot that may encourage even more warming of the atmosphere – and thus feed back into more fires. Globally today, as much as 5 million square kilometres – an area more than half the size of the United States – burns in a given year. 
There are numerous modern applications of fire. In its broadest sense, fire is used by nearly every human being on earth in a controlled setting every day. Users of internal combustion vehicles employ fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity.
The use of fire in warfare has a long history. Fire was the basis of all early thermal weapons. Homer detailed the use of fire by Greek soldiers who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. In the First World War, the first modern flamethrowers were used by infantry, and were successfully mounted on armoured vehicles in the Second World War. In the latter war, incendiary bombs were used by Axis and Allies alike, notably on Tokyo, Rotterdam, London, Hamburg and, notoriously, at Dresden in the latter two cases firestorms were deliberately caused in which a ring of fire surrounding each city [ citation needed ] was drawn inward by an updraft caused by a central cluster of fires. The United States Army Air Force also extensively used incendiaries against Japanese targets in the latter months of the war, devastating entire cities constructed primarily of wood and paper houses. The use of napalm was employed in July 1944, towards the end of the Second World War  although its use did not gain public attention until the Vietnam War.  Molotov cocktails were also used.
Researchers Analyze the Evolving Human Relationship with Fire
Humanity's relationship to fire –– including wildfires, burning of fossil fuels, controlled burns, and human-caused fire –– is the focus of a report by an international team of scientists. The team was organized by UC Santa Barbara's National Center for Ecological Analysis and Synthesis (NCEAS).
Fire, both friend and foe, is a controversial force in the world. The team of 18 researchers analyzed the history and possible future of our ever-changing relationship with fire in an article published today in the Journal of Biogeography. The article is titled, "The Human Dimension of Fire Regimes on Earth."
"The value of this study is that it presents a critical assessment of the diversity of human uses of fire, from tamed landscape fire, to agricultural fire, to industrial fire," said Jennifer K. Balch, postdoctoral associate at NCEAS and second author on the paper. "Human use and misuse of fire has been so prevalent in our evolutionary history, and the evolution of cultures, that we've forgotten how dominant a force fire really is."
The research team noted that wildfires are often viewed as major disasters, and there is concern that climate change will increase their incidence. However, it is difficult to consider the true impact of past or future wildfires without understanding their place in natural and human history, about which much is unknown.
The researchers offer a historical framework to help other researchers, as well as managers, to develop a context for considering the relationships humans have with fire. This framework is key to planning for future fire risk and understanding the role of fire in natural ecosystems, according to David M. J. S. Bowman, lead author and professor at the School of Plant Science, University of Tasmania.
"There are often needless debates about whether or not fire has any place in flammable landscapes," Bowman said. "These debates are not helpful because of the intertwined relationships among humans, landscapes, and fire throughout human history, which blur any distinction between natural and human-set fires."
The researchers' analysis recognizes four fire phases:
Natural fires that occur without human influence.
Tame fire used by hunter-gatherers to manage landscapes for game and wild food production.
Agricultural fire used to clear land, grow food, and burn fallow.
Industrial fire to power modern societies that have switched from using living to fossilized plants as the primary fuel.
All these phases still occur today. The researchers explain that this remarkable diversity of human uses of fire, albeit imperfectly controlled, has powered all cultures. However, the problem is that the excessive combustion of fossil fuels is driving climate change. "Our fossil-fuel-dependent economy is yet another extension of our dependence on combustion," Balch said. "We have effectively put fire in a box." The result of massive dependence on this one use of fire may ultimately overwhelm human capacities to control landscape fire, given more extreme fire weather and more production of fuels, according to the researchers.
Considering Earth's fire history before human influence also offers great insights into the flammable planet we have inherited, according to the team. "Unraveling the nature of fire before any human influence is an important element of the current debate," said co-author Andrew C. Scott, professor in the Department of Earth Sciences, Royal Holloway, University of London. "Some only see fires in terms of human causation and impact. Understanding the ways that humans have and are altering natural wildfire systems has profound political and economic significance."
The research highlights the fact that understanding the relative influences of climate, human ignition sources, and cultural practices in particular environments is critical to the development of sustainable fire management to protect human health, property, ecosystems, and diminish greenhouse gas pollution. "Fire is such a defining feature of humans, and we are the only animals that use fire," Bowman said. "We could have been called Homo igniteus as much as Homo sapiens."
With future climate change all of us may have to confront wildfire –– even if we do not do so now –– so understanding human's relationship with fire will be important for all of us, according to Balch. "Companion with changing climate, human ignitions are also changing therefore, it is imperative that we better understand the human relationship to fire," she said.
NCEAS is funded by the National Science Foundation.
† Top photo: View of Canada's 2003 Okanagan Mountain Park Fire from McCulloch Road, with Harvest Golf Club in the foreground.
Credit: Wenda Pickles/ Library and Archives Canada
†† Middle photo: Smoke plume from a deforestation fire in the Amazon's expanding agricultural frontier.
Credit: Jennifer K. Balch, 2006