Assimilation of the column-averaged CO2 concentrations from the Orbiting Carbon Observatory-2 (OCO-2) satellite data to improve our knowledge of Australian carbon flux estimates

Abstract

© 2021 Yohanna Lesly Villalobos CortesExisting estimates of carbon fluxes for Australia primarily rely on process-based terrestrial ecosystem model simulations. Even though they are built to consider important ecosystem processes that control the exchange of CO2 between the land surface and the atmosphere, such as the connection between carbon uptake and water use by plants, their carbon flux exchange estimates are highly uncertain. Improving carbon flux estimates from global ecosystem models and its uncertainties is essential for advancing our understanding of the Earth system and carbon cycle-climate feedback. This dissertation contributes to solving this challenge through atmospheric data assimilation, also known as inverse modelling. The main structure of this thesis consists of three studies. The first study involved running a series of simulation experiments (OSSEs) to assess the potential of the Orbiting Carbon Observatory-2 (OCO-2) satellite retrievals to reduce the uncertainties in CO2 fluxes over Australia for 2015. In this study, we assumed that most of the uncertainties in the Australian carbon fluxes were driven by the net primary productivity estimated by the CABLE land surface model (Australian land biosphere model). After performing OSSEs, we found that Australian carbon flux uncertainties can be reduced by up to 90 percent at a grid-point resolution over productive ecosystems. Given that the first study showed promising results about the potential of OCO-2 data to constrain fluxes around Australia, the second study focused on the quantification of CO2 sources and sinks over the continent. The main results of this study suggest that Australia acted as a carbon sink of -0.41 +- 0.08 PgC/y compared to the prior estimate 0.09 +- 0.2 PgC/y (excluding fossil fuel emissions). Analysis of the seasonal variation of the posterior CO2 fluxes aggregated by bioclimatic regions shows that the savannas in northern Australia and the sparsely vegetated ecosystem in central Australia were the primary drivers of stronger carbon uptake in 2015. Examination of the enhanced vegetation index (EVI) indicates that the primary reason for the stronger posterior carbon uptake (relative to the prior) registered over the savanna ecosystem was due to an increase of vegetation productivity (positive EVI anomalies) caused by an anomalous increase of rainfall in summer period. Additionally, we found that a slight increase of carbon over areas with sparse vegetation (the largest ecosystem by area in Australia), was also driven by a slight increase in land productivity and had a substantial impact on the Australian carbon budget for 2015. Underestimation of the gross primary productivity flux simulated by the CABLE model over the savanna and sparsely vegetated ecosystem was also a contribution of why OCO-2 lead to a stronger carbon estimate in 2015. The final study was built upon the second study and focused on understanding the Australian carbon flux variability from 2015-2019 and evaluating how Australian semi-arid ecosystems responded to changes in rainfall and temperature anomalies. This study suggests the 2015 carbon sink's size over Australia increased in 2016 due to increased vegetation productivity in this period. Australia's 2016 carbon uptake contributed almost all the long-term mean terrestrial sink estimated for 2015-2019 (-0.33 +- 0.09 PgC/y). The ecosystems that most contributed to this carbon sink were savanna and sparsely vegetated regions driven by a higher than expected greenness in vegetation (expressed by positive EVI anomalies) strongly influenced by positive rainfall anomalies and negative air temperature anomalies