Impact of intermittent volcanic forcing on ocean carbon uptake under climate overshoot

The ocean has absorbed between 20-35% of anthropogenic CO₂ emissions, acting as a major carbon sink despite its slower response times compared to the atmosphere and biosphere [1]. However, carbon uptake in the ocean is predicted to decrease in the future, particularly under scenarios that exceed global warming targets, resulting in the uptake rate being close to zero. Volcanic aerosol forcing introduces uncertainty into these projections by altering the Earth's radiation balance, which, in turn, affects ocean carbon fluxes by changing temperature and circulation patterns. Despite that, intermittent forcing is not considered in widely used CMIP or ScenarioMIP simulations.

Here, we leverage Earth system model simulations to explore the impacts of intermittent versus baseline volcanic forcing on the ocean carbon fluxes under a temperature overshoot scenario. We hypothesize that irregular forcing will amplify variability in ocean carbon uptake and we expect stronger responses in ocean basins such as the Atlantic due to AMOC sensitivity and downstream effects of eruptions. Two ensembles, generated with the Max Planck Institute Earth system model (MPI-ESM), are compared [2]. One ensemble is forced with semi-stochastic irregular volcanic events and another with a recurring, median intensity event. We analyze key variables, such as ocean carbon uptake, vertical temperature profiles, Atlantic Meridional Overturning Circulation (AMOC), and thermocline depth, to assess the variability and response timescales under intermittent forcing. To find responses on temporal and spatial scales, we quantify the response and recovery times of the ocean and determine where the strongest responses occur spatially to determine which regions are most or least affected. Our study aims to improve the understanding of the sensitivity of ocean carbon uptake to intermittent forcing and its implications for future projections of the carbon cycle.

[1] S. Khatiwala, F. Primeau, and T. Hall. “Reconstruction of the history of anthropogenic CO2 concentrations in the ocean”. In: Nature 462, pp. 346–349. 2009.
[2] T. Mauritsen et al. “Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and Its Response to Increasing CO2”. In: Journal of Advances in Modeling Earth Systems 11.4, pp. 998–1038. 2019.