Ocean carbon cycle feedbacks in CMIP6 models: contributions from different basins

Anna Katavouta,Richard G Williams

doi:10.5194/bg-18-3189-2021

Anna Katavouta, Richard G Williams

Open Access

https://doi.org/10.5194/bg-18-3189-2021

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Abstract

Abstract. The ocean response to carbon emissions involves the combined effect of an increase in atmospheric CO2, acting to enhance the ocean carbon storage, and climate change, acting to decrease the ocean carbon storage. This ocean response can be characterised in terms of a carbon–concentration feedback and a carbon–climate feedback. The contribution from different ocean basins to these feedbacks on centennial timescales is explored using diagnostics of ocean carbonate chemistry, physical ventilation and biological processes in 11 CMIP6 Earth system models. To gain mechanistic insight, the dependence of these feedbacks on the Atlantic Meridional Overturning Circulation (AMOC) is also investigated in an idealised climate model and the CMIP6 models. For the carbon–concentration feedback, the Atlantic, Pacific and Southern oceans provide comparable contributions when estimated in terms of the volume-integrated carbon storage. This large contribution from the Atlantic Ocean relative to its size is due to strong local physical ventilation and an influx of carbon transported from the Southern Ocean. The Southern Ocean has large anthropogenic carbon uptake from the atmosphere, but its contribution to the carbon storage is relatively small due to large carbon transport to the other basins. For the carbon–climate feedback estimated in terms of carbon storage, the Atlantic and Arctic oceans provide the largest contributions relative to their size. In the Atlantic, this large contribution is primarily due to climate change acting to reduce the physical ventilation. In the Arctic, this large contribution is associated with a large warming per unit volume. The Southern Ocean provides a relatively small contribution to the carbon–climate feedback, due to competition between the climate effects of a decrease in solubility and physical ventilation and an increase in accumulation of regenerated carbon. The more poorly ventilated Indo-Pacific Ocean provides a small contribution to the carbon cycle feedbacks relative to its size. In the Atlantic Ocean, the carbon cycle feedbacks strongly depend on the AMOC strength and its weakening with warming. In the Arctic, there is a moderate correlation between the AMOC weakening and the carbon–climate feedback that is related to changes in carbonate chemistry. In the Pacific, Indian and Southern oceans, there is no clear correlation between the AMOC and the carbon cycle feedbacks, suggesting that other processes control the ocean ventilation and carbon storage there.

Highlights

Carbon emissions drive an Earth system response via direct changes in the biogeochemical carbon cycle and the physical climate
In the Atlantic Ocean, β∗ is strongly correlated with the preindustrial Atlantic Meridional Overturning Circulation (AMOC) in the Earth system models (r = 0.87), and this correlation is due to the contribution from βdis (Table 4 and Fig. 10a), which suggests that a stronger pre-industrial AMOC leads to a larger β∗ via a stronger physical transfer of anthropogenic carbon below the ocean surface there, which is similar to the behaviour of the idealised model (Fig. 7c)
Our study reveals the contribution of the Atlantic, Pacific, Southern, Indian and Arctic oceans to the ocean carbon cycle feedbacks in a set of CMIP6 Earth system models

Summary

Introduction

Carbon emissions drive an Earth system response via direct changes in the biogeochemical carbon cycle and the physical climate. The carbon cycle is further modified by changes in the physical climate, such as warming and an increase in ocean stratification leading to an amplification of the initial increase in atmospheric CO2. This response of the carbon cycle to changes in the physical climate is characterised by the carbon–climate feedback. A regional extension of the carbon cycle feedbacks has been used to explore their geographical distribution and the mechanisms that control the land and ocean carbon uptake and storage in difference regions (Yoshikawa et al, 2008; Boer and Arora, 2010; Tjiputra et al, 2010; Roy et al, 2011)

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