levels in the atmosphere . CMIP5 and CMIP6 Earth System Models  project an increasing future tropical carbon sink, which is particularly strong in the scenarios with more pronounced increases in atmospheric CO2 concentration . However, major uncertainties regarding the ecophysiological processes governing carbon turnover and tree mortality under a changing climate , and the ecosystem- level responses of tropical forests to elevated atmospheric CO2  explain the contrast between observational data and modelling results . Observational data show that structurally intact old-growth tropical forests have been net sinks of atmospheric carbon in recent decades, but there is evidence that the capacity of such intact tropical forests to build up carbon stock may be limited as biomass peaked during the 1990s and has since weakened by 30% in the Amazon since the 1990s , mainly due to increased tree mortality and faster carbon turnover, and the African tropical forest sink following this trend since about 2010 . From a peak pan-tropical  forest sink of 1.26 Pg C yr-1 during the 1990s, it is projected to decline to an uptake of only 0.29 Pg C yr-1, reaching zero in the Amazon, during the 2030s . This decline will possibly be driven by the reduced rates of forest carbon uptake from the weakening global CO2 fertilisation effect mediated by limiting soil nutrient, and reduced water availability and higher temperatures during extreme droughts , reinforced by deforestation and forest degradation .
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report