Tropical extreme droughts drive long-term increase in atmospheric CO2 growth rate variability.

The terrestrial carbon sink slows the accumulation of carbon dioxide (CO₂) in the atmosphere by absorbing roughly 30% of anthropogenic CO₂ emissions, but varies greatly from year to year. The resulting variations in the atmospheric CO₂ growth rate (CGR) have been related to tropical temperature and water availability. The apparent sensitivity of CGR to tropical temperature ( γ CGR T ) has changed markedly over the past six decades, however, the drivers of the observation to date remains unidentified. Here, we use atmospheric observations, multiple global vegetation models and machine learning products to analyze the cause of the sensitivity change. We found that a threefold increase in γ CGR T emerged due to the long-term changes in the magnitude of CGR variability (i.e., indicated by one standard deviation of CGR; STD_CGR), which increased 34.7% from 1960-1979 to 1985-2004 and subsequently decreased 14.4% in 1997-2016. We found a close relationship (r² = 0.75, p < 0.01) between STD_CGR and the tropical vegetated area (23°S – 23°N) affected by extreme droughts, which influenced 6-9% of the tropical vegetated surface. A 1% increase in the tropical area affected by extreme droughts led to about 0.14 Pg C yr⁻¹ increase in STD_CGR. The historical changes in STD_CGR were dominated by extreme drought-affected areas in tropical Africa and Asia, and semi-arid ecosystems. The outsized influence of extreme droughts over a small fraction of vegetated surface amplified the interannual variability in CGR and explained the observed long-term dynamics of γ CGR T . The apparent temperature sensitivity of atmospheric CO₂ growth rate has increased markedly over the past six decades, however, the increase remains unexplained. Here we show that tropical extreme droughts amplified the interannual variability in atmospheric CO₂ growth rate and drove the sensitivity change. [ABSTRACT FROM AUTHOR]