Abstract
Abstract. Observations from the Orbiting Carbon Observatory 2 (OCO-2) satellite have been used to estimate CO2 fluxes in many regions of the globe and provide new insight into the global carbon cycle. The objective of this study is to infer the relationships between patterns in OCO-2 observations and environmental drivers (e.g., temperature, precipitation) and therefore inform a process understanding of carbon fluxes using OCO-2. We use a multiple regression and inverse model, and the regression coefficients quantify the relationships between observations from OCO-2 and environmental driver datasets within individual years for 2015–2018 and within seven global biomes. We subsequently compare these inferences to the relationships estimated from 15 terrestrial biosphere models (TBMs) that participated in the TRENDY model inter-comparison. Using OCO-2, we are able to quantify only a limited number of relationships between patterns in atmospheric CO2 observations and patterns in environmental driver datasets (i.e., 10 out of the 42 relationships examined). We further find that the ensemble of TBMs exhibits a large spread in the relationships with these key environmental driver datasets. The largest uncertainty in the models is in the relationship with precipitation, particularly in the tropics, with smaller uncertainties for temperature and photosynthetically active radiation (PAR). Using observations from OCO-2, we find that precipitation is associated with increased CO2 uptake in all tropical biomes, a result that agrees with half of the TBMs. By contrast, the relationships that we infer from OCO-2 for temperature and PAR are similar to the ensemble mean of the TBMs, though the results differ from many individual TBMs. These results point to the limitations of current space-based observations for inferring environmental relationships but also indicate the potential to help inform key relationships that are very uncertain in state-of-the-art TBMs.
Highlights
Over the past decade, the field of space-based CO2 monitoring has undergone a rapid evolution
Using observations from Orbiting Carbon Observatory 2 (OCO-2), we find that precipitation is associated with increased CO2 uptake in all tropical biomes, a result that agrees with half of the terrestrial biosphere models (TBMs)
We use model selection based on the Bayesian information criterion (BIC) to determine a set of environmental driver datasets to include in the analysis using OCO-2 observations and using the TBMs
Summary
The field of space-based CO2 monitoring has undergone a rapid evolution. The sheer number of CO2-observing satellites has greatly increased, including GOSAT/GOSAT-2 (Kuze et al, 2009; Nakajima et al, 2012), TanSat (Yang et al, 2018), and OCO-2/OCO-3 (Crisp, 2015; Eldering et al, 2019). This expanding observing system provides atmospheric CO2 observations broadly across the globe, making it possible to estimate the distribution and magnitude of CO2 fluxes in many regions that have sparse in situ surface atmospheric CO2 monitoring (e.g., the tropics and the Southern Hemisphere). Miller et al (2018) evaluated the extent to which OCO-2 retrievals can detect patterns in biospheric CO2 fluxes and found that an early version of the OCO-2 retrievals (version 7) is only equipped to provide accurate flux constraints across very large continental or hemispheric regions; by contrast, in a follow-up paper, Miller and Michalak (2020) revisited satellite capabilities in light of recently improved OCO-2 retrievals, and the authors argued that new OCO-2 retrievals can be used to constrain CO2 fluxes for more detailed regions (i.e., for seven global biomes)
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