The Amazon rainforest is often described as a natural bastion against climate change, but following decades of human activity, including deforestation and damaging changes in land use, a new first-of-its-kind study argues that the rainforest may now be warming the planet more than it cools it.
Much of the debate around the health of the Amazon rainforest and climate change has previously revolved around the carbon cycle: trees suck up atmospheric carbon dioxide, store carbon in their biomass, then release carbon when they rot or are burned. This carbon is released back into the atmosphere in the form of carbon dioxide, a greenhouse gas that traps heat in our atmosphere and contributes to climate change.
While some parts of the Amazon now produce more carbon than they absorb, primarily due to deforestation and changes in land use, the rainforest as a whole absorbs more carbon than it releases – for now, at least.
However, the overall picture is starting to appear to be much more complex than previously understood. In the study, published in Frontiers in Forests and Global Change, researchers have looked at all the greenhouse gases that affect the Amazon, such as methane and nitrous oxide, and worked out their impact on the global changes. Once these other factors are taken into account, it appears that the net warming effects created in the Amazon Basin now outweigh the cooling effects it provides.
Among the many factors looked at in the new study, the researchers highlight the role of methane (CH4), an extremely potent greenhouse gas that has a strong warming effect on Earth’s atmosphere. Like carbon, it’s also part of a natural cycle, with soils and sediments both pumping out and sucking in methane. Around 3.5 percent of global methane emissions come from the Amazon's trees. The new study predicts that increased flooding and dam-building are triggering an increased release of methane. Likewise, cattle ranching, one of the main drivers of deforestation in the basin, also pumps out a huge amount of methane.
There is a similar situation with nitrous oxide, another greenhouse gas, which is both sequestered and produced by soils. The new research, however, argues that as the land becomes more degraded due to deforestation and climate change, soils are now emitting more nitrous oxide than they sequester. On top of this, the researchers also consider how deforestation and the burning of land are worsening climate change by releasing black soot into the atmosphere, which helps to absorb sunlight and increase warming.
While the plight of the Amazon rainforest has become increasingly clear in recent years, it looks like the problem is much deeper and more complex than previously understood by many scientists.
“CO2 is not a lone actor. When you consider the whole cast of other characters, the outlook in the Amazon is that the impacts of human activities will be worse than we realize,” Patrick Megonigal, associate director of research at the Smithsonian Environmental Research Center, who was not directly involved with the study, told National Geographic.