Abstract
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 ({{{{{{rm{gamma }}}}}}}_{{{{{{rm{CGR}}}}}}}^{{{{{{rm{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 {{{{{{rm{gamma }}}}}}}_{{{{{{rm{CGR}}}}}}}^{{{{{{rm{T}}}}}}} emerged due to the long-term changes in the magnitude of CGR variability (i.e., indicated by one standard deviation of CGR; STDCGR), 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 STDCGR 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 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 of {{{{{{rm{gamma }}}}}}}_{{{{{{rm{CGR}}}}}}}^{{{{{{rm{T}}}}}}}.
Highlights
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
Following model selection based on minimizing predictor collinearity, which can cause artificial temporal changes in the derived coefficients, we quantified γTCGR and γWCGR based on a multivariate linear regression of ΔCGR on ΔMAT, ΔMAP and ΔRAD. γTCGR was significant (p < 0.05) in every 20year window (Fig. 1a), increasing threefold between 1960 and 1999 (1960−1979: 1.83 ± 0.45 PgC yr−1 K−1; 1980−1999: 5.49 ± 0.53 PgC yr−1 K−1), consistent with previous reports[8,10], and decreasing by 33.6% in the most recent two decades (1997−2016: 3.64 ± 0.53 PgC yr−1 K−1) (Fig. 1a)
Considering the Dynamic Global Vegetation Models (DGVMs) and FLUXCOM products we examined were forced by a similar climate dataset (i.e., Climate Research Unit (CRU) and CRU-NCEP, see Methods) that we used to obtain γTCGR and γWCGR, the disagreement between the observed and modeled climate sensitivities can only be attributed to the difference between the observed variance of CO2 growth rate (CGR) and the modeled variance of net ecosystem exchange (NEE)
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. Extreme water deficits can further induce changes in land-atmosphere CO2 exchange over longer time scales, through lagged responses and legacy effects of terrestrial ecosystems (i.e. mortality[25–27], fire[28], recovery[29,30] and deadwood decomposition[31]). This hierarchy of water-related processes can modulate CGR and manifest as the changes in γTCGR, especially under extreme drought conditions. We further examined the relationship between STDCGR and tropical droughts, using several key indicators of tropical water availability, an ensemble of dynamic global vegetation models[34], and the FLUXCOM machine learning products[21] based on observations from the global FLUXNET network (see Methods)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
