The Carbon Cycle of Southeast Australia During 2019–2020: Drought, Fires, and Subsequent Recovery

B Byrne,K Miyazaki,N B Jones,V A Velazco,K W Bowman,J Joiner,J Liu,A J Norton,D F Pollard,D W T Griffith,C Paton‐Walsh,N M Deutscher,M Lee,Y Yin

doi:10.1029/2021av000469

Abstract

Abstract2019 was the hottest and driest year on record for southeast Australia leading to bushfires of unprecedented extent. Ecosystem carbon losses due to drought and fire are believed to have been substantial, but have not been well quantified. Here, we utilize space‐based measurements of trace gases (TROPOspheric Monitoring Instrument XCO, Orbiting Carbon Observatory 2 ) and up‐scaled GPP (FluxSat GPP) to quantify the carbon cycle anomalies resulting from drought and fire in southeast Australia during the 2019–2020 growing season. We find that biomass burning released 113–236 TgC of CO2 while drought and fire‐induced anomalies in net ecosystem exchange reduced growing season carbon uptake by an additional 19–52 TgC of CO2. These carbon losses were concentrated during the spring and early summer, when hot‐dry conditions were most severe. A shift to cooler conditions with above average rainfall during February is found to result in a partial recovery and greening in unburned ecosystems, but not in fire‐impacted areas. The net 2019–2020 carbon loss substantially exceeded interannual variations in net uptake over 2010–2019 estimated from top‐down constraints (∼5σ anomaly), and exceeded Australia's annual fossil fuel emissions (∼104 TgC year−1). Top‐down constraints show that the regional carbon budget is strongly regulated by climate variability, and suggest that cool‐wet conditions are required for a rapid recovery of carbon stocks. This has implications for the regional carbon budget as more frequent climate‐change‐driven heat and drought events may increase the frequency of fire events and the recovery time of ecosystems, threatening the carbon stocks of the region.

Highlights

Extreme drought and heat events can result in single-year carbon losses equal to many years of carbon sequestration (Bastos et al, 2014; Ciais et al, 2005)
We utilize space-based measurements of trace gases (TROPOspheric Monitoring Instrument XCO, Orbiting Carbon ObservaAtAory 2 XCO2 ) and up-scaled gross primary productivity (GPP) (FluxSat GPP) to quantify the carbon cycle anomalies resulting from drought and fire in southeast Australia during the 2019–2020 growing season
Top-down constraints show that the regional carbon budget is strongly regulated by climate variability, and suggest that cool-wet conditions are required for a rapid recovery of carbon stocks

Summary

Introduction

Extreme drought and heat events can result in single-year carbon losses equal to many years of carbon sequestration (Bastos et al, 2014; Ciais et al, 2005). Hot-dry conditions can directly suppress both gross primary productivity (GPP) and ecosystem respiration (TER), with greater suppression of GPP leading to carbon loss (Reichstein et al, 2007; Sippel et al, 2018). These conditions dry fuels, increase litterfall, and elevate levels of tree mortality, all of which may trigger additional carbon losses by means of wildfire Impacted ecosystems often experience legacy effects that can impact the carbon cycling for years after the extreme events have passed (Batllori et al, 2020; Frank et al, 2015; Lindenmayer et al, 2021)

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