“The Atlantic Meridional Overturning really is one of our planet’s key circulation systems,” says Boers. “We already know from some computer simulations and from data from Earth’s past, so-called paleoclimate proxy records, that the AMOC can exhibit – in addition to the currently attained strong mode – an alternative, substantially weaker mode of operation. This bi-stability implies that abrupt transitions between the two circulation modes are, in principle, possible.”

It has been shown previously that, despite being in “strong mode,” the AMOC is currently at its weakest in over 1,000 years. However, so far scientists are unsure whether the observed weakening corresponds to a change in the mean circulation state, or whether it is associated with an actual loss of dynamical stability. “The difference is crucial,” says Boers, “because the loss of dynamical stability would imply that the AMOC has approached its critical threshold.” Once past this threshold, it’s possible we could see a substantial – and probably irreversible – transition to the weak circulation mode.

Unfortunately, there are no long-term observational data on the strength of the AMOC, but the AMOC leaves “fingerprints” in patterns of observational sea-surface temperature and salinity data from across the Atlantic Ocean basin that can be used to assess developments. “A detailed analysis of these fingerprints in eight independent indices now suggests that the weakening of the AMOC over the last century is indeed likely to be associated with a loss of stability,” says Boers. “The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse.”

A number of factors could be contributing to this phenomenon – factors that amplify the direct effect that the warming of the Atlantic Ocean has on its circulation. These include freshwater inflow from the melting of the Greenland ice sheet, melting sea-ice, increasing precipitation, and river run-off. Freshwater is lighter than saltwater and reduces the tendency of the water to sink from the surface to greater depths, which is one of the drivers of the overturning.

“I wouldn’t have expected that the excessive amounts of freshwater flowing into the ocean over the course of the last century would already produce such a response in the overturning circulation,” says Boers. “We urgently need to reconcile our models with the presented observational evidence to assess how far from or how close to its critical threshold the AMOC really is.” While the respective relevance of the different factors requires further investigation, one thing is clear: they are all linked to human-induced climate change.

Further Information and Interview Requests

Niklas Boers, Potsdam Institute for Climate Impact Research and Freie Universität Berlin, Tel.: +49 331 288 20768, Email: boers@pik-potsdam.de