Preprints
https://doi.org/10.5194/acp-2021-16
https://doi.org/10.5194/acp-2021-16

  22 Jan 2021

22 Jan 2021

Review status: this preprint is currently under review for the journal ACP.

Was Australia a sink or source of CO2 in 2015? Data assimilation using OCO-2 satellite measurements

Yohanna Villalobos1,2, Peter J. Rayner1,2,3, Jeremy D. Silver1,4, Steven Thomas1, Vanessa Haverd5, Jürgen Knauer5, Zoë M. Loh6, Nicholas M. Deutscher7, David W. T. Griffith7, and David F. Pollard8 Yohanna Villalobos et al.
  • 1School of Earth Sciences, University of Melbourne, Australia
  • 2ARC Centre of Excellence for Climate Extremes, Sydney, Australia
  • 3Climate & Energy College, University of Melbourne, Australia
  • 4School of Mathematics and Statistics, University of Melbourne, Australia
  • 5CSIRO Oceans and Atmosphere, Canberra, 2601, Australia
  • 6CSIRO Oceans and Atmosphere, Aspendale, Victoria 3195, Australia
  • 7School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia
  • 8National Institute of Water and Atmospheric Research Ltd (NIWA), Lauder, New Zealand

Abstract. In this study, we present the assimilation of data from the Orbiting Carbon Observatory-2 (OCO-2) to estimate the Australian CO2 surface fluxes for the year 2015. We used a regional-scale atmospheric transport-dispersion model and a four-dimensional variational assimilation scheme. Our results suggest that Australia was a carbon sink of −0.3 ± 0.09 PgC y−1 compared to the prior estimate 0.09 ± 0.17 PgC y−1 (excluding fossil fuel emissions). Most of the uptake occurred over northern savannas, the Mediterranean ecotype in southern Australia and the sparsely vegetated ecotype in central Australia. Our results suggest that the majority of the carbon uptake over Mediterranean was associated with positive EVI anomalies (relative to 2000–2014). However, the stronger posterior carbon uptake estimated over savanna and sparsely vegetated ecosystem was due primarily to underestimation of the gross primary productivity by the land surface model (CABLE-BIOS3 model). To evaluate the accuracy of our posterior flux estimates, we compare our posterior CO2 concentration simulations against the column- averaged carbon retrievals from the Total Carbon Column Observing Network (TCCON) and ground-based in-situ monitoring sites located around our Australia domain. In general, the performance of our posterior concentration compared well with TCCON observations, except when TCCON concentrations were dominated by ocean fluxes which were tightly constrained to their prior values. Comparisons with in-situ measurements also show encouraging results though with similar difficulties for coastal stations. For stations located far from the coast, the comparison with in situ data was more variable, suggesting difficulties to match the column-integrated and surface data by the inversion, most likely linked to model vertical transport.

Yohanna Villalobos et al.

Status: open (until 23 May 2021)

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Yohanna Villalobos et al.

Yohanna Villalobos et al.

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Short summary
Semi-arid ecosystems such as Australia are evolving and might play an essential role in the future of climate change. We use carbon dioxide concentrations derived from OCO-2 satellite instrument and a regional transport model to understand if Australia was a carbon sink or source of CO2 in 2015. Our research's main findings suggest that Australia acted as a carbon sink of about -0.3 +- 0.09 petagram of carbon in 2015, driven primarily by sparsely vegetated, savanna, and Mediterranean ecosystems.
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