Fires and Weather
Cutting forests can affect climate in ways that have nothing to do with greenhouse gases. Because forests draw water from the soil and emit it from their leaves by evaporation, a forest acts like a giant humidifier. This exhalation of water vapor also moves heat from the surface to high in the atmosphere where the water condenses. The net effect decreases surface temperatures. Cutting forest pulls the plug on this air conditioner and decreases the water vapor that might otherwise become raindrops.
Satellite view of the Amazon River in Brazil.
Clouds form over the thick tree canopy, but are sparse over the dryer marshland along the river.
As a result, clearing more than 30% of the rainforest could trigger a permanently drier Amazon with a self-reinforcing—or positive—feedback loop that converts more and more tropical rainforest to savanna. As more trees die, the faster everything else dries out, which kills more trees, and so on.
A recent study by the Federal University in Viçosa, Brazil suggests that if just another 3% of the forest in the Mato Grosso region is cleared, the forest may become a dry savannah permanently.
These changes can also affect weather patterns far downwind. Some studies have indicated that large-scale deforestation in the Amazon may increase global temperatures and significantly reduce rainfall in faraway places like Mexico and Texas.
What do you think?
The two images below are satellite views of smoke activity for 2005 (left) and 2006 (right). What do you think is the reason for the higher smoke activity in 2005 compared to 2006? (Choose all that apply.)
The best answers are probably a) and d).
It’s difficult to say for sure what affects the number of fires in a particular year. We do know that 2005 was a very strong drought, which was probably a big factor in the fires that year. The drop in 2006 was probably due to stronger laws enacted as a result of the fires in 2005.
A similar pattern also occurred between 2007 (left) and 2008 (right), but scientists aren’t sure what caused the drop in these years since weather doesn’t appear to have been a factor. Perhaps crop and timber market conditions or regulatory actions played a part.
