The Orbiting Carbon Observatory-2 (OCO-2) informs process-based estimates of inter-annual variability in the global carbon cycle

Abstract:

The Orbiting Carbon Observatory-2 (OCO-2) offers an enormous opportunity to improve estimates of carbon fluxes across the globe. These observations can be used to estimate the magnitude of fluxes from different regions and seasons, thereby informing scientific understanding of the global carbon cycle. An even greater challenge is to use these observations in a way that can not only inform flux totals but also the environmental processes in process-based terrestrial biosphere models (TBMs). In this talk, we use an inverse modeling framework to quantify inter-annual variability (IAV) in the global carbon cycle and the relationships between CO₂ fluxes and environmental processes like temperature and precipitation. We further compare the relationships that we infer from OCO-2 against those in an ensemble of TBMs. State-of-the-art TBMs do not agree on the magnitude of IAV in different biomes of the globe, but most TBMs tend to estimate larger IAV compared to the estimate in this study based on OCO-2. Furthermore, TBMs disagree on the sensitivity of IAV to anomalies in temperature and precipitation, but we find that most TBMs are more sensitive to these anomalies across the tropics compared to results using OCO-2; TBMs with stronger sensitivity to these anomalies in environmental drivers also tend to estimate larger IAV over the tropics. In spite of these findings, we are not able to evaluate more complex environmental relationships using current OCO-2 observations. Overall, the results illustrate both the potential and limitations of using current space-based observations of CO₂ to inform a process-based estimates of CO₂ fluxes. Furthermore, our findings highlight large uncertainties in process-based estimates of IAV and provide a new avenue for evaluating TBMs using satellite observations of CO₂.