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

Abstract The terrestrial carbon sink slows the accumulation of carbon dioxide (CO 2) in the atmosphere by absorbing roughly 30% of anthropogenic CO 2 emissions, but varies greatly from year to year. The resulting variations in the atmospheric CO 2 growth rate (CGR) have been related to tropical temperature and water availability. The apparent sensitivity of CGR to tropical temperature ({{{{{ }}}}}{{{{₂₆ₑ}}}}^{{{{{T}}}}} 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 {{{{{ }}}}}{{{{₂₆ₑ}}}}^{{{{{T}}}}} 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 (r 2 = 0. 75, p γ CGR T.