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Abstract

Possible changes in Atlantic meridional overturning circulation (AMOC) provide a key source of uncertainty regarding future climate change. Maps of temperature trends over the twentieth century show a conspicuous region of cooling in the northern Atlantic, as well as excessive warming along the North American coast. Both combined are a ‘fingerprint’ characteristic of an AMOC slowdown by about 15%. Proxy data studies suggest this slowing of the AMOC is unique in the past millennium or more. Continued melting of the Greenland Ice Sheet in the coming decades could contribute to further weakening of the AMOC, with important consequences for the ocean ecosystem, the weather in the wider North Atlantic region, regional sea levels and the ocean’s ability to absorb carbon dioxide. Moreover, the AMOC has a known tipping point. The question of how close we already are to this tipping point is becoming increasingly urgent. Rahmstorf, S., et al. (2015). "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation." Nature Climate Change 5: 475-480. Caesar, L., et al. (2018). "Observed fingerprint of a weakening Atlantic Ocean overturning circulation." Nature 556: 191-196. Caesar, L., et al. (2021). "Current Atlantic Meridional Overturning Circulation weakest in last millennium." Nature Geoscience 14: 118–120. Boers, N. (2021). "Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation." Nature Climate Change 11: 680-688.