Tropical Ecosystems Live Carbon Losses in a Changing Climate

Tropical forests store about two-thirds of the global terrestrial biomass as live biomass. Global climate change is expected to release part of this large carbon pool into the atmosphere and trigger a positive feedback loop. Estimating the impacts of climate change on tropical live biomass is a difficult task because it involves multiple changing environmental conditions and biogeochemical processes. In this study, we estimate potential changes in equilibrium aboveground biomass (AGB) across the tropics using the empirical relationship between climate, biome distribution, and biomass. In our approach, we assume that tropical ecosystems biomass equilibrates with climate, and will continue to do so under future climate conditions. We made use of the relationship between historic (observed) climate conditions (precipitation and maximum climatological deficit) and biomass to quantify future potential carbon stocks under a new climate, assuming the climate-biomass equilibrium will be met. By the end of the century, changes in climate under the high-emissions (RCP8.5) scenario will likely cause changes in tropical biomes area and biomass. There is a general shift from wetter to drier climates. Humid biomes will lose the most area, while the semi-arid biomes gain the most. The overall increase in area of the driest regions is associated with a modest net decrease in the equilibrium AGB of the tropics (~2%) in the last 50 years. By the end of the century, the total tropical equilibrium AGB is reduced by 10% compared to 1950. However, in addition to this net change in total biome areas, there is considerable gross turnover inside any given biome. We estimate that almost 30% of the current tropical biomes will have a climate no longer compatible with the current biome and biomass. For some biomes, up to 80% of their original area could move into a different climate equilibrium. The speed at which vegetation responds to being out of equilibrium will determine how much carbon is emitted how soon in the coming decades.