Is the Amazon near a Tipping Point?

Abstract:

The long-term ecological balance of the Amazon tropical rainforest is progressively being affected by the simultaneous and synergistic effect of three anthropogenic drivers of change: (i) Global warming that not only has led to a 1.5C increase in temperatures across the basin, but also drives more frequent severe droughts(e.g., the droughts of 2005 and 2010 were among the most extreme droughts observed in about 120 years); (ii) Regional deforestation increases surface temperature over deforested areas by 1 C to 3 C and also reduces water recycling by the vegetation; and (iii) The combination of increased surface temperatures, more extreme droughts and continued use of manmade fires is turning the previously fire-resilient forest into a biome more vulnerable to fires.

That has led to a science-based concern that the Amazon forest may be not far from a tipping point of savannization, which is the conversion of large portions of the forest into tropical savannas—that is the biome with approximately of 70% grasslands and adapted to long dry seasons ranging from 4 to 6 months and to wildfires. If these three anthropogenic drivers continue to run unaltered in the next decades, more than 50% of the remaining forest may be transformed into dry savannas. One key driver of such massive transformation is regional deforestation. It has been estimated that if deforestation rates over the entire basin exceed 20% to 25% of the forest area, then an irreversible process of savannization might be initiated.

More than being a theoretical projection based on climate model simulations of these anthropogenic drivers, the great concern comes from observations of the increase in dry season length by three to four weeks over the central-southern Amazon since 1980. The length of the dry season is the most critical element differentiating tropical forest biome from tropical savanna biome. If the dry season length exceeds four months, the forest will be gradually converted into a dry savanna. That process may release emissions of CO₂ of about 200 to 300 gigatonnes, since the tropical savanna stores much less carbon in comparison to the tropical rainforest. The observed delay of the onset of the rainy season and consequent raise in dry season length is even higher in heavily deforested areas in southern Amazon, in comparison to more pristine areas.