Abstract
Year-to-year variability in CO2 fluxes can yield insight into climate-carbon cycle relationships, a fundamental yet uncertain aspect of the terrestrial carbon cycle. In this study, we use global observations from NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite for years 2015–2019 and a geostatistical inverse model to evaluate 5 years of interannual variability (IAV) in CO2 fluxes and its relationships with environmental drivers. OCO-2 launched in late 2014, and we specifically evaluate IAV during the time period when OCO-2 observations are available. We then compare inferences from OCO-2 with state-of-the-art process-based models (terrestrial biosphere model, TBMs). Results from OCO-2 suggest that the tropical grasslands biome (including grasslands, savanna, and agricultural lands within the tropics) makes contributions to global IAV during the 5 year study period that are comparable to tropical forests, a result that differs from a majority of TBMs. Furthermore, existing studies disagree on the environmental variables that drive IAV during this time period, and the analysis using OCO-2 suggests that both temperature and precipitation make comparable contributions. TBMs, by contrast, tend to estimate larger IAV during this time and usually estimate larger relative contributions from the extra-tropics. With that said, TBMs show little consensus on both the magnitude and the contributions of different regions to IAV. We further find that TBMs show a wide range of responses on the relationships of CO2 fluxes with annual anomalies in temperature and precipitation, and these relationships across most of the TBMs have a larger magnitude than inferred from OCO-2. Overall, the findings of this study highlight large uncertainties in process-based estimates of IAV during recent years and provide an avenue for evaluating these processes against inferences from OCO-2.
Highlights
Interannual variability (IAV) in CO2 fluxes is of critical importance in understanding the global carbon cycle
The ensemble mean of these TBMs across different biomes and the globe are generally within the uncertainty bounds of interannual variability (IAV) estimated using Orbiting Carbon Observatory-2 (OCO-2), but numerous individual TBMs still fall outside the uncertainty bounds; this result implies that there is an opportunity to evaluate estimates of IAV within individual TBMs using current satellite observations of CO2, at least during the time period for which satellite observations are available
Using OCO-2, we find large variations in the carbon cycle associated with the 2015–2016 El Niño and subsequent recovery; these perturbations dominate global IAV during 2015–2019 and account for the very large contribution of the tropics to IAV in the OCO-2 estimate (figures 1(a) and 2)
Summary
Interannual variability (IAV) in CO2 fluxes is of critical importance in understanding the global carbon cycle. In situ observations of atmospheric CO2 have been extensively used to estimate global- and regional-scale IAV In situ measurement sites are unevenly distributed across the globe; inverse models using in situ observations are arguably not sensitive to CO2 fluxes in undersampled regions like the tropics, where IAV is the largest (e.g. Baker et al 2006, Peylin et al 2013, Piao et al 2020). Existing studies have leveraged GOSAT observations to estimate IAV (e.g. Guerlet et al 2013, Byrne et al 2019, 2020, Liu et al 2021). In a recent study, Byrne et al (2019) argued that current GOSAT observations can only be used to constrain IAV across continent-sized regions or larger spatial scales
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