Alternate histories: synthetic large ensembles of sea-air CO₂ flux

Holly C. Olivarez (1, 2) , Nicole S. Lovenduski (2, 1) , Riley X. Brady (2, 1) , Amanda R. Fay (3, 4) , Marion Gehlen (5) , Luke Gregor (6) , Peter Landschützer (7) , Galen A. Mckinley (3, 4) , Karen A. Mckinnon (8) , David R. Munro (9)

1 University of Colorado [Boulder]
2 Department of Atmospheric and Oceanic Sciences and Institute of Arctic and Alpine Research
3 Columbia University [New York]
4 LDEO - Lamont-Doherty Earth Observatory
5 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
6 IBP - Institute of Biogeochemistry and Pollutant Dynamics [ETH Zürich]
7 MPI-M - Max Planck Institute for Meteorology
8 Department of Statistics [Los Angeles]
9 NOAA - National Oceanic and Atmospheric Administration

We use a statistical emulation technique to construct synthetic ensembles of global and regional sea-air carbon dioxide (CO₂) flux from four observation-based products over 1985-2014. Much like ensembles of Earth system models that are constructed by perturbing their initial conditions, our synthetic ensemble members exhibit different phasing of internal variability and a common externally forced signal. Our synthetic ensembles illustrate an important role for internal variability in the temporal evolution of global and regional CO₂ flux and produce a wide range of possible trends over 1990-1999 and 2000-2009. We assume a specific externally forced signal and calculate the rank of the observed trends within the distribution of statistically modeled synthetic trends during these periods. Over the decade 1990-1999, three of four observation-based products exhibit small negative trends in globally integrated sea-air CO₂ flux (i.e., enhanced ocean CO₂ absorption with time) that are within one standard deviation of the mean in their respective synthetic ensembles. Over the decade 2000-2009, however, three products show large negative trends in globally integrated sea-air CO₂ flux that have a low rate of occurrence in their synthetic ensembles. The largest positive trends in global and Southern Ocean flux over 1990-1999 and the largest negative trends over 2000-2009 fall nearly two standard deviations away from the mean in their ensembles. Our approach provides a new perspective on the important role of internal variability in sea-air CO₂ flux trends, and furthers understanding of the role of internal and external processes in driving observed sea-air CO₂ flux variability.

Mots clés: global carbon cycle; air-sea CO<SUB>2</SUB> flux; ocean carbon uptake; large ensemble; Earth system modeling; decadal trends

Domaines: Planète et Univers [physics]

Holly C. Olivarez, Nicole S. Lovenduski, Riley X. Brady, Amanda R. Fay, Marion Gehlen, et al.. Alternate histories: synthetic large ensembles of sea-air CO₂ flux. Global Biogeochemical Cycles, 2022, 36 (6), pp.e2021GB007174. ⟨10.1029/2021GB007174⟩. ⟨insu-03721925⟩