Is the AMOC shutting down soon?

The Atlantic Meridional Overturning Circulation (AMOC) is the Atlantic component of the global ocean conveyor belt, which is a large-scale ocean circulation system that connects the Arctic, Atlantic, Indian, and Pacific Oceans via the Southern Ocean. Predominantly driven by deep convection in the high-latitude North Atlantic Ocean, the AMOC carries heat, salt, carbon, and other biogeochemical elements along its paths, redistributing them between hemispheres and across ocean basins, and thus is a crucial component of the global heat, salt, and carbon balances. For instance, the AMOC-related sinking in the high-latitude North Atlantic accounts for approximately 25% of the anthropogenic carbon inventory of the world’s oceans (e.g., Park et al., 2023). Therefore, the amount of anthropogenic carbon uptake by the ocean is closely related to the strength of the AMOC. In a new study published in Nature Communications, two Danish scientists Peter Ditlevsen & Susanne Ditlevsen used a highly idealized stochastic differential equation, with its parameters tuned with some observational data, to “estimate a collapse of the AMOC to occur around mid-century under the current scenario of future emissions“.

The physical basis for a potential collapse in the AMOC was first brought up by Henry Stommel (Stommel, 1961). Specifically, sufficiently strong freshwater forcing (e.g., ice sheet melting from Greenland’s glaciers due to increasing anthropogenic carbon) in the subpolar North Atlantic may decrease the AMOC, which in turn should decrease the salt transport from the subtropical North Atlantic to the sinking site (e.g., Labrador and Irminger Seas), further reducing the salinity in the subpolar North Atlantic. This positive feedback may lead to a complete shutdown of the AMOC. Stommel (1961) demonstrated this cascade mechanism using simple temperature and salinity equations and the equation of state (i.e., ocean density as a function of temperature and salinity). Although Stommel’s study supports the possibility of a complete shutdown of the AMOC in the future, the current state-of-the-art climate models do not agree with the new study. The majority of climate models that contributed to the Coupled Model Intercomparison Project Phase 6 (CMIP6) point to a weakening of the AMOC by 34 – 45% of its present-day strength by 2100, while none of those models show a complete shutdown of the AMOC (Weijer et al., 2020).

Some proxy-based studies suggested a long-term reduction in the AMOC since the earlier twentieth century (e.g., Rahmstorf et al., 2015; Caesar et al., 2018). However, several recent studies used long-term historical hydrographic observations (Worthington et al., 2021; Caínzos et al., 2022; Lee et al., 2023) and ocean reanalysis products (Jackson et al., 2021) to conclude that the AMOC changes during the past several decades are not externally forced by increasing greenhouse gases but largely modulated by long-term natural climate variability associated with the North Atlantic Oscillation (NAO).

In summary, the idealized stochastic differential equation-based study of Ditlevsen & Ditlevsen (2023) suggested “a collapse of the AMOC to occur around mid-century under the current scenario of future emissions“. Although there is a clear physical basis for a potential collapse in the AMOC, the current state-of-the-art climate models do not back up this conclusion. Additionally, there is no clear observational evidence to support a significant slowdown of the AMOC in the past several decades.

Image Credit: http://editors.eol.org/eoearth/wiki/File:OCP07_Fig-6.jpg

Ditlevsen, P., Ditlevsen, S. Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nat Commun 14, 4254. https://doi.org/10.1038/s41467-023-39810-w (2023)

Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. & Saba, V. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556, 191–196. (2018)

Caínzos, V. et al. Thirty years of GOSHIP and WOCE data: Atlantic Overturning of mass, heat, and freshwater transport. Geophys. Res. Lett. 49, e2021GL096527. (2022)

Jackson, L. C. et al. The evolution of the North Atlantic Meridional Overturning Circulation since 1980. Nat. Rev. Earth Environ. 3, 241–254. (2022)

Lee, S.-K., R. Lumpkin, F. Gomez, S. Yeager, H. Lopez, F. Tagklis, S. Dong, W. Aguiar, D. Kim, and M. Baringer. Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean. Communications Earth & Environment, 4:69. https://doi.org/10.1038/s43247-023-00727-3 (2023)

Park, I.-H., S.-W. Yeh, W. Cai, G. Wang, S.-K. Min, and S.-K. Lee. Present-day North Atlantic salinity constrains future warming of the Northern Hemisphere. Nature Climate Change. https://doi.org/10.1038/s41558-023-01728-y (2023)

Rahmstorf, S. et al. Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Clim. Change 5, 475–480. (2015)

Stommel, H. Thermohaline Convection with Two Stable Regimes of Flow. Tellus, 13: 224-230. https://doi.org/10.1111/j.2153-3490.1961.tb00079.x (1961)

Weijer, W., Cheng, W., Garuba, O. A., Hu, A. & Nadiga, B. T. CMIP6 models predict significant 21st century decline of the Atlantic Meridional Overturning Circulation. Geophys. Res. Lett. 47, e2019GL086075. (2020)

Worthington, E. L. et al. A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline. Ocean Sci. 17, 285–299. (2021)