Time varying changes and uncertainties in the CMIP6 ocean carbon sink from global to regional to local scale

Abstract. As a major sink for anthropogenic carbon, the oceans slow the increase of carbon dioxide in the atmosphere and regulate climate change. Future changes in the ocean carbon sink, and its uncertainty at a global and regional scale, are key to understanding the future evolution of the climate. Here, we conduct a multimodel analysis of the changes and uncertainties in the historical and future ocean carbon sink using output data from the latest phase of the Coupled Model Intercomparison Project: CMIP6, and observations. We show that the ocean carbon sink is concentrated in highly active regions – 70 percent of the total sink occurs in less than 40 percent of the global ocean. High pattern correlations between the historical and projected future carbon sink indicate that future uptake will largely continue to occur in historically important regions. We conduct a detailed breakdown of the sources of uncertainty in the future carbon sink by region. Scenario uncertainty dominates at the global scale, followed by model uncertainty, and then internal variability. We demonstrate how the importance of internal variability increases moving to smaller spatial scales and go on to show how the breakdown between scenario, model, and internal variability changes between different ocean basins, governed by different processes. Moreover, we show that internal variability changes with time based on the scenario. As with the mean sink, we show that uncertainty in the future ocean carbon sink is also concentrated in the known regions of historical uptake. The resulting patterns in the signal-to-noise ratio have strong implications for observational detectability and time of emergence, which varies both in space and with scenario. Our results suggest that to detect human influence on the ocean carbon sink as early as possible and to efficiently reduce uncertainty in future carbon uptake, modeling and observational efforts should be focused in the known regions of high historical uptake, including the Northwest Atlantic and the Southern Ocean.