FY 2022

Quantifying the role of seasonality in the marine carbon cycle feedback: An ESM2M case study

Fassbender, A.J., S.S. Schlunegger, K.B. Rodgers, and J.P. Dunne

Global Biogeochem. Cycles, 36(6), e2021GB007018, doi: 10.1029/2021GB007018, View online (open access) (2022)

Observations and climate models indicate that changes in the seasonal amplitude of sea surface carbon dioxide partial pressure (A-pCO₂) are underway and driven primarily by anthropogenic carbon (C_ant) accumulation in the ocean. This occurs because pCO₂ is more responsive to seasonal changes in physics (including warming) and biology in an ocean that contains more C_ant. A-pCO₂ changes have the potential to alter annual ocean carbon uptake and contribute to the overall marine carbon cycle feedback. Using the GFDL ESM2M Large Ensemble and a novel analysis framework, we quantify the influence of C_ant accumulation on pCO₂ seasonal cycles and sea-air CO₂ fluxes. Specifically, we reconstruct alternative evolutions of the contemporary ocean state in which the sensitivity of pCO₂ to seasonal thermal and biophysical variation is fixed at preindustrial levels, however the background, mean-state pCO₂ fully responds to anthropogenic forcing. We find near-global A-pCO₂ increases of >100% by 2100, under RCP8.5 forcing, with rising C_ant accounting for ∼100% of thermal and ∼50% of nonthermal pCO₂ component amplitude changes. The other ∼50% of nonthermal pCO₂ component changes are attributed to modeled changes in ocean physics and biology caused by climate change. C_ant-induced A-pCO₂ changes cause an 8.1 ± 0.4% (ensemble mean ± 1σ) increase in ocean carbon uptake by 2100. The is because greater wintertime wind speeds enhance the impact of wintertime pCO₂ changes, which work to increase the ocean carbon sink. Thus, the seasonal ocean carbon cycle feedback works in opposition to the larger, mean-state feedback that reduces ocean carbon uptake by ∼60%.

Plain Language Summary. Using simulations of an Earth System Model, we isolate different factors contributing to future changes in the surface ocean carbon dioxide partial pressure (pCO₂). We examine how the seasonal cycle of pCO₂, and the associated sea-air exchange of CO₂, responds to changes in the ocean's temperature, circulation, biology, and chemistry. We find that the pCO₂ seasonal cycle is significantly amplified across the global ocean (by ∼100% on average). This occurs because pCO₂ is more responsive to seasonal changes in temperature as well as biological and physical (biophysical) processes in a future ocean that contains more anthropogenic carbon (C_ant). The increased temperature sensitivity is almost exclusively due to added C_ant. The increased biophysical sensitivity is equally due to added C_ant and changes in ocean physics and biology caused by climate change. Seasonal wind speed variation systematically enhances the impact of altered pCO₂ seasonal cycles during wintertime, causing an 8% increase in the ocean carbon sink strength by the year 2100. Within the evaluated model, this indicates that the seasonal ocean carbon cycle feedback works in opposition to the larger, mean-state ocean carbon cycle feedback, which may cause up to a ∼60% reduction in the ocean sink by the year 2100.