A Comprehensive Assessment of Anthropogenic and Natural Sources and Sinks of Australasia's Carbon Budget

Regional carbon budget assessments attribute and track changes in carbon sources and sinks and support the development and monitoring the efficacy of climate policies. We present a comprehensive assessment of the natural and anthropogenic carbon (C-CO2) fluxes for Australasia as a whole, as well as for Australia and New Zealand individually, for the period from 2010 to 2019, using two approaches: bottom-up methods that integrate flux estimates from land-surface models, data-driven models, and inventory estimates; and top-down atmospheric inversions based on satellite and in situ measurements. Our bottom-up decadal assessment suggests that Australasia's net carbon balance was close to carbon neutral (-0.4 +/- 77.0 TgC yr-1). However, substantial uncertainties remain in this estimate, primarily driven by the large spread between our regional terrestrial biosphere simulations and predictions from global ecosystem models. Within Australasia, Australia was a net source of 38.2 +/- 75.8 TgC yr-1, and New Zealand was a net CO2 sink of -38.6 +/- 13.4 TgC yr-1. The top-down approach using atmospheric CO2 inversions indicates that fluxes derived from the latest satellite retrievals are consistent within the range of uncertainties with Australia's bottom-up budget. For New Zealand, the best agreement was found with a national scale flux inversion estimate based on in situ measurements, which provide better constrained of fluxes than satellite flux inversions. This study marks an important step toward a more comprehensive understanding of the net CO2 balance in both countries, facilitating the improvement of carbon accounting approaches and strategies to reduce emissions. Human activities-including the extraction and use of fossil fuels (coal, oil, and natural gas), cement production, and land-use change (e.g., land clearing), release carbon dioxide (CO2) to the atmosphere, while biospheric processes such as the CO2 uptake by forests and revegetation remove CO2 from the atmosphere. In this study, we assess the balance of natural and human-driven sources and sinks of CO2 for Australia and New Zealand (referred to as the Australasia carbon budget) for 2010-2019. Our findings indicate that Australasia was close to carbon neutral, with large uncertainties, suggesting that the CO2 sinks from vegetation in this region largely offset the CO2 emissions from human activities. An independent assessment using the latest satellite observations and modeling shows consistent results for Australia. For New Zealand, a national system of ground observations and modeling agreed better with the bottom-up budget than satellite-derived flux estimates. We synthesize Australasia's carbon C-CO2 budget (together with Australia and New Zealand) based on bottom-up and top-down approachesAustralasia's bottom-up carbon budget suggests that this region was close to neutral (-0.4 +/- 77.0 TgC yr-1) from 2010 to 2019Australasia's annual CO2 balance fluctuates significantly, particularly in Australia, shifting from a strong carbon sink to a strong carbon source