Degradation of Amazon Forest and the Threat of a Runaway Greenhous

Projected increases in temperature and in the frequency and severity of droughts imply substantial tree mortality in Amazonian forest. With higher temperatures, plants need more water to survive, and many trees would not resist these combined stresses. Higher temperatures also lead to more forest fires, which set in motion a vicious cycle that leads to still more fires and destroys the forest. Invasion of forest by lianas and bamboos is also favored. These phenomena are already spreading in Amazonia. Dead trees from previous droughts, previous fires, or from logging damage make Amazonian forest fires both more likely and more damaging when they occur. With respect to average global temperatures, 2˚C above the global pre-industrial mean is probably beyond the temperature "tipping point" for Amazonian forest.

A dramatic surge of deforestation has occurred under the Jair Bolsonaro presidential administration in Brazil that took office in January 2019. The eastern half of Brazilian Amazonia is already heavily deforested, while the western half is largely intact. The eastern part has probably already passed a "tipping point" in terms of deforestation. The western part is under threat from planned reconstruction of an abandoned highway (BR-319) linking central Amazonia to the "arc of deforestation" in the southern part of the region, together with plans for roads that would connect to this highway and give deforesters entry to much of the huge block of intact forest between the highway and the border with Peru. President Bolsonaro promised to complete the BR-319 highway when he visited Manaus in July 2019.

The Amazon forest contains a large carbon stock both in the vegetation and in the soil. This can be released when forest is converted to non-forest vegetation, either deliberately or through radical degradation. Biomass loss and soil warming can release much of the carbon in parts of the region that remain forested. Globally, if non-deliberate emissions (such as those from Amazon forest degradation) exceed human society's deliberate emissions, then even eliminating deliberate emissions completely would be insufficient to avert a "runaway greenhouse" ending in a "hothouse earth." The potential role of Amazonia is a central part of this threat, but it is also one with the greatest possibility of being reduced by policy change.