Forest fires have become a wildcard in the global-warming game.

Biosphere-Atmosphere Experiment in Amazonia

With tributaries extending from the vast savannas to its north and south, the Amazon River runs almost 4,000 miles (1 mile equals 1.6 kilometers) across northern South America from the highland biomes in the foothills of the Andes Mountains to the Atlantic Ocean. It carries twenty percent of all river water discharged into Earth’s oceans—ten times the volume of the Mississippi River. If the Amazon River Basin were draped over the continental United States, it would cover more than three fourths of the country.

Escape From the Amazon

In this era of heightened concern about the relationship between the build up of atmospheric carbon dioxide and climate change, scientists are working to itemize all the ways carbon moves between the atmosphere and the elements of Earth’s surface, including life, water and soil. Forests are of particular interest in large part because many nations now manage the forests within their borders, deciding where and when to harvest trees and when to leave the forest alone. Now those decisions are influenced by the role forests play in the global carbon cycle. Forests’ ability to take in and sequester carbon during photosynthesis has ceased to be something we accept without thought; the biological services they provide have instead become a product with a market value to be traded between nations like radio parts or soybeans. Just as humans have turned to forests for fuel, food, and shelter for hundreds of thousands of years, we now look to them to help us compensate for the atmospheric excesses of our combustion-engine civilization.

Escaping Carbon

At first glance, the simplest explanation might appear to be deforestation. When forests are cut down or burned, the carbon stored in the forest biomass is released into the atmosphere. Combined with the other processes that carry carbon out of the rainforest ecosystem—decomposition, respiration, soil and sediment run off into the Atlantic—deforestation might be the big source of carbon scientists are seeking. But calculations suggest otherwise. In Brazil alone, deforestation is proceeding at a rate of about 20,000 square kilometers per year as the Amazon is cleared for farming and ranching (Houghton, et al., 2002), but these losses still do not appear to be large enough to offset the large carbon intake measured by the flux towers.

From Forest to Field

Before widespread human settlement began to encroach on the borders of South America’s Amazon forests, there was no such thing as an Amazon fire season. Now, fire may pose the biggest threat to the survival of the Amazon ecosystem.

Slash-and-burn agriculture converts forest to farm land, but that obvious destruction is only the beginning. Intentional fires get out of control and burn through the understory of nearby forests, killing, but not completely burning small trees, vines and shrubs. The dead and dying trees collapse, spilling firewood and kindling to the ground and ripping a great tear in the tent of the forest overhead. Logging has a similar effect. The intense tropical sun, previously deflected by the green canopy, heats the forest floor, pushing fire danger even higher. Smoke hangs over the forest and suppresses rainfall. In this damaged, fragmented landscape, the onset of the natural dry season becomes ominous. The El Niño-driven droughts that typically arrive a couple of times per decade become devastating.