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Publication Date
7 March 2022

Tropical Extreme Droughts Drive Long-Term Increase in Atmospheric CO2 Growth Rate Variability

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Science

This analysis combines atmospheric observations, multiple global vegetation models, and machine learning products to analyze the cause of the sensitivity change. We link the observed long-term changes in the variability of atmospheric CO2 growth rate (STDCGR) to tropic extreme droughts, highlighting the influence of tropical droughts on modulating the climate sensitivities of the global carbon cycle. The historical changes in STDCGR were dominated by extreme drought-affected areas in tropical Africa and Asia, and semi-arid ecosystems.

Impact

We provide a novel insight into the terrestrial carbon-water relation over long-term timescales. Our results show that terrestrial biosphere models misrepresent the long-term impact of severe droughts in the Amazon, calling into question their utility for diagnosing the climate sensitivities of the global carbon cycle.

Summary

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 (?TCGRγCGRT) has changed markedly over the past six decades, however, the drivers of the observation to date remain 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 ?TCGRγCGRT emerged due to the long-term changes in the magnitude of CGR variability (i.e., indicated by one standard deviation of CGR; STDCGR). We found a strong relationship between STDCGR and the tropical vegetated area 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 STDCGR. The historical changes in STDCGR 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.

Point of Contact
Trevor F. Keenan
Institution(s)
Lawrence Berkeley National Laboratory
University of California - Berkeley
Funding Program Area(s)
Publication