Quantifying the Role of Seasonality in the Marine Carbon Cycle Feedback: An ESM2M Case Study
Andrea J. Fassbender, Sarah Schlunegger, Keith B. Rodgers, John P. Dunne
Abstract
Abstract Observations and climate models indicate that changes in the seasonal amplitude of sea surface carbon dioxide partial pressure (A‐ p CO 2 ) are underway and driven primarily by anthropogenic carbon (C ant ) accumulation in the ocean. This occurs because p CO 2 is more responsive to seasonal changes in physics (including warming) and biology in an ocean that contains more C ant . A‐ p CO 2 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 p CO 2 seasonal cycles and sea‐air CO 2 fluxes. Specifically, we reconstruct alternative evolutions of the contemporary ocean state in which the sensitivity of p CO 2 to seasonal thermal and biophysical variation is fixed at preindustrial levels, however the background, mean‐state p CO 2 fully responds to anthropogenic forcing. We find near‐global A‐ p CO 2 increases of >100% by 2100, under RCP8.5 forcing, with rising C ant accounting for ∼100% of thermal and ∼50% of nonthermal p CO 2 component amplitude changes. The other ∼50% of nonthermal p CO 2 component changes are attributed to modeled changes in ocean physics and biology caused by climate change. C ant ‐induced A‐ p CO 2 changes cause an 8.1 ± 0.4% (ensemble mean ± 1σ) increase in ocean carbon uptake by 2100. This is because greater wintertime wind speeds enhance the impact of wintertime p CO 2 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%.