Refining the Global Picture: the Impact of Increased Resolution on CO₂ Atmospheric Inversions using OCO-2 XCO₂ retrievals

Abstract

The threat posed by the increasing concentration of carbon dioxide (CO₂) in the atmosphere motivates a detailed and precise estimation of CO₂ emissions and absorptions over the globe. This study refines the spatial resolution of the CAMS/LSCE inversion system, achieving a global resolution of 0.7° latitude and 1.4° longitude, or three times as many grid boxes as the current operational setup. In a two-year inversion assimilating the midday clear-sky retrievals of the column-average dry-air mole fraction of carbon dioxide (XCO₂) from NASA’s second Orbiting Carbon Observatory (OCO-2), the elevated resolution demonstrates an improvement in the representation of atmospheric CO₂, particularly at the synoptic time scale, as validated against independent surface measurements. Vertical profiles of the CO₂ concentration differ slightly above 22 km between resolutions compared to AirCore profiles, and highlight differences in the vertical distribution of CO₂ between resolutions. However, this disparity is not evident for XCO₂, as evaluated against independent reference ground-based observations. Global and regional estimates of natural fluxes for 2015-2016 are similar between the two resolutions, but with North America exhibiting a higher natural sink at high-resolution for 2016. Overall, both inversions seem to yield reasonable estimates of global and regional natural carbon fluxes. The increase in calculation time is less than the increase in the number of operations and in the volume of input data, revealing greater efficiency of the code executed on a Graphics Processing Unit. This allows us to make this higher resolution the new standard for the CAMS/LSCE system.