Forests sequester carbon, and fires return it to the atmosphere. Huge fires may account for as much as 40 percent of the anthropogenic contribution of CO2 to Earth's atmosphere. Early humans used fire in their agriculture. When specific human populations collapsed, the absence of these fires resulted in reforestation and the removal of CO2 from the atmosphere.

Fire is an important factor in climate, whether it is stopping or starting. Study of charcoal deposits has shown that before 1500 c.e., indigenous populations in the Amazon Basin used fire to clear forests and maintain extensive agricultural enterprises. After European plagues killed 90 percent of the Amazon population, the rain forest grew back, sequestering 10 gigatons of carbon. This may have lowered atmospheric carbon dioxide (CO2) concentration by 2 parts per million by volume, contributing to the development of the Little Ice Age. In 1997-1998, forest fires in Indonesia are thought to have added between 0.81 and 2.57 gigatons of carbon to the atmosphere, out of the total 6.0 gigatons.

Wildfires are natural. Many habitats exist because wildfires occur and would disappear in the absence of fires. Fires occur when there is fuel, oxygen, and a source of ignition. Lightning and oxygen will always exist in the wild, so the principal variable is the fuel load. Nature limits the supply of fuel with frequent small fires, which burn through an area, removing accumulated deadwood and forest litter. Long periods of fire suppression interfere with this process, permitting large quantities of fuel to accumulate. The subsequent fires are then larger and more intense.

CO2 is sequestered in the wood of the forest, removing it from the atmosphere and preventing it from affecting the Earth's climate. As the natural succession results in shrubs replacing grasses and being replaced in turn by slightly larger trees, which are replaced by even larger trees, and so on, the amount of CO2 being stored in the forest increases. When the forest reaches its climax state, the species making up the forest will stabilize. Eventually, new growth will only replace older, dying growth, and no additional CO2 will be removed from the atmosphere. How rapidly this all occurs will vary with the climate, soil, and other factors, but it is generally thought to take fifty to one hundred years.

Early humans may have observed beavers. These creatures prefer foods such as aspens, which grow fairly early in the succession. If they encounter hickories or oaks, they will frequently girdle them, chewing away a strip of bark a few centimeters wide, all the way around the tree. The trees will die, standing where they are, eventually falling over, or being burned in a lightning-caused fire, and the area in which they were standing will revert to meadow to begin the process of succession again.

If early humans wanted to create a meadow, they could emulate this behavior of beavers, using blades to girdle the trees and, after the trees had died, starting fires to speed things along. This technique is called "slash and burn," and it is still a major cause of deforestation. It causes the CO2 that was sequestered in a forest to move into the atmosphere, where it contributes to the greenhouse effect. Some 3.5 to 4.5 million square kilometers of forest are burned in fires every year. Over 99 percent of these fires are set intentionally by humans, usually for agricultural purposes. Most are in Africa.

If agriculturalists abandon their fields, the opposite effect, reforestation, will occur. In the case of the Americas, scientists estimate that 35-90 million people died shortly after the European plagues were introduced. If each of these farmers had been keeping one hectare clear, and after regrowth to rain forest each hectare of land supported an additional 100 metric tons of carbon, the increase in the amount of carbon sequestered would have been on the order of 3.5-9 gigatons.

Climate change will alter precipitation patterns in time and space. Some areas will become more fire prone, others less. The makeup of forests will change along with the climate, and eventually the plants growing in an area will be those that are best suited for that environment. Some will require fires and will thrive in areas that burn frequently. Sound fire management practices should permit this. As changes occur, however, those places that are drying up will have more combustible fuel available, and wildfires can be expected until this fuel is consumed. Thinning, controlled burns, and other means of reducing this fuel should be used to avoid uncontrollable conflagrations.

For the foreseeable future, fire will be used as an agricultural tool. If done on a small scale, this is unlikely to contribute substantially more CO2 to the atmosphere than the oxidation performed by microbes. However, it should not be permitted on a large scale, particularly as part of lumbering operations, as under such circumstance the contribution of burning to atmospheric CO2 concentrations may be large and long lasting.

Bibliography:

1) Nevle, Richard J., and Dennis K. Bird. "Effects of Syn-pandemic Fire Reduction and Reforestation in the Tropical Americas on Atmospheric CO2 During European Conquest." Palaeogeography, Palaeoclimatology, Palaeoecology 264, nos. 1/2 (2008): 25-38.

2) Omi, Philip N. Forest Fires: A Reference Handbook. Santa Barbara, Calif.: ABC-CLIO, 2005.

3) Page, Susan E., et al. "The Amount of Carbon Released from Peat and Forest Fires in Indonesia During 1997." Nature 420, no. 6911 (2002): 61-65.

4) Pyne, Stephen J. Vestal Fire: An Environmental History, Told Through Fire, of Europe and Europe's Encounter with the World. Seattle: University of Washington Press, 1997.

5) Ruddiman, William F. Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton, N.J.: Princeton University Press, 2005.

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