Inferring changes to the global carbon cycle with WOMBAT v2.0, a hierarchical flux-inversion framework

Abstract

The natural cycles of the surface-to-atmosphere fluxes of carbon dioxide (CO₂) and other important greenhouse gases are changing in response to human influences. These changes need to be quantified to understand climate change and its impacts, but this is difficult to do because natural fluxes occur over large spatial and temporal scales and cannot be directly observed. Flux inversion is a technique that estimates the spatiotemporal distribution of a gas’ fluxes using observations of the gas’ mole fraction and a chemical transport model. To infer trends in fluxes and identify phase shifts and amplitude changes in flux seasonal cycles, we construct a flux-inversion system that uses a novel spatially-varying time-series decomposition of the fluxes. We incorporate this decomposition into the Wollongong Methodology for Bayesian Assimilation of Trace-gases (WOMBAT, Zammit-Mangion et al., Geosci. Model Dev., 15, 2022), a Bayesian hierarchical flux-inversion framework that yields posterior distributions for all unknowns in the underlying model. We also extend WOMBAT to accommodate physical constraints on the fluxes and to take direct in situ and flask measurements of trace-gas mole fractions as observations. We apply the new method, which we call WOMBAT v2.0, to a mix of satellite observations of CO₂ mole fraction from the Orbiting Carbon Observatory-2 (OCO-2) satellite and direct measurements of CO₂ mole fraction from a variety of sources. We estimate the changes in the natural cycles of CO₂ fluxes that occurred from January 2015 to December 2020, and compare our posterior estimates to those from an alternative method based on a bottom-up understanding of the physical processes involved. We find substantial trends in the fluxes, including that tropical ecosystems trended from being a net source to a net sink of CO₂ over the study period. We also find that the amplitude of the global seasonal cycle of ecosystem CO₂ fluxes increased over the study period by 0.11 PgC/month (an increase of 8%) and that the seasonal cycle of ecosystem CO₂ fluxes in the northern temperate and northern boreal regions shifted earlier in the year by 0.4–0.7 and 0.4–0.9 days, respectively (2.5th to 97.5th posterior percentiles), consistent with expectations for the carbon cycle under a warming climate.