Rainforests are aptly named. They not only need rain, they induce it, allowing them to thrive. So when we chop them down, less rain falls. A comparison of models of the impact on deforestation in the Amazon suggests we are getting close to the tipping point where the Amazon basin may lose the rain it needs to sustain its ecology.
The vast canopies of rainforest trees transpire astonishing amounts of water. As a result, much more water vapor is released per unit area of rainforest than a similarly sized lake, let alone a prairie. This vapor soon comes down as rain. A more radical spin on this idea suggests that the winds rainforests produce push this rain inland, referred to as a “biotic pump”, which allows forests to colonize the center of continents.
The consequence is that when rainforest is felled for agriculture, or drowned for hydroelectricity, less rain falls, threatening the longevity of surviving forests nearby. Knowing this, however, is very different from predicting the size of effect when a specific area is lost. In Geophysical Research Letters Dr. Dominick Spracklen and Dr. Luis Garcia-Carreras of the University of Leeds combined data from all the peer reviewed simulations published in the last 40 years to quantify the problem for the Amazon, which makes up 40 percent of the world's tropical rainforest.
Despite widespread deforestation since the 1970s, by 2010 the Amazon basin's rainfall had been reduced by just 1.8 ± 0.3 percent. Even this may hide much dangerous decreases in specific areas, but the real danger occurs if clearing restarts at the rate seen early this century, under which 47 percent would be gone by 2050.
“We estimate that business-as-usual deforestation (based on deforestation rates prior to 2004) would lead to an 8.1 ± 1.4 percent reduction in annual mean Amazon basin rainfall by 2050,” the authors wrote.
By 2050, the pair predict, even average years could have less basin-wide rain than drought years under a preservation scenario. Dry seasons risk catastrophic fires like those now occurring in Indonesia. The potential consequences are disastrous, not only for the rainforests but for what is displacing them. Agriculture on land that was once rainforest relies on good rainfalls, Spracklen pointed out, as do dams.
The good news is that after the destruction of 2.7 million hectares (6.7 million acres) of rain forest in 2004, Brazil introduced protection programs which reduced clearance by three quarters in six years. Spracklen described this in a statement as, “one of the big environmental success stories of the past decade.” However, he added, "But I think at the moment we're at a kind of cusp, where there's continued pressure within Brazil to relax some of the forest laws."
Moreover, deforestation has moved to the countries that cover smaller portions of the Amazon basin, highlighting the importance of Peru's decision this month to protect a major section of its territory along the Brazilian border.