System of currents in the Northern Atlantic Ocean estimated to tip mid century

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Data analysis suggests that the AMOC, an ocean current system responsible for the relatively mild climate in Western Europe and a cooling of the tropics, will likely shift to a less active state within decades

The North Atlantic Overturning circulation (AMOC) is approaching a tipping point. The likelihood of reaching this point of no return is increasing and is estimated to happen in the mid-21st century. This is the conclusion of an analysis of the fingerprint of the AMOC in sea surface temperature data from the North Atlantic region. The result contrasts the latest IPCC assessment, which states that such a transition is unlikely in this century. The study, published in Nature Communications by Peter Ditlevsen, The Niels Bohr Institute, and Susanne Ditlevsen, Institute of Mathematical Sciences, the University of Copenhagen, Denmark, is part of the European Horizon 2020 TiPES-project.

“The shutdown of the AMOC could have significant consequences on global climate patterns, such as temperatures and precipitation. Though a cooling in Northern Europe seems less dramatic in a warming world, where heatwaves hit with higher frequency. The shutdown would potentially contribute to an increase in heating in the tropics, where livelihood is strongly impacted by rising temperatures. This prediction adds to the urgency for immediate reductions in greenhouse gas emissions.” says Peter Ditlevsen.

A podcast with Peter Ditlevsen on the findings is available here.

Tipped in the past

The Atlantic Meridional Overturning Circulation (AMOC) is a part of the global ocean current system largely responsible for the redistribution of heat from the tropics to the Northern regions of the Atlantic Ocean, in particular Western Europe. The AMOC vertically overturns surface seawater to deep water in the Northern Atlantic Ocean. The overturning is crucial for the redistribution of heat from the Equatorial region towards the Arctic and thus for the Atlantic region’s relatively mild climate.

The redistribution of heat by the AMOC is significant. It has been in its current stable state since the ice age. However, the AMOC is known to have changed abruptly in the past. Paleo data from the Greenland ice sheet, marine sediments, and other sediments document how the AMOC more than 20 times shifted within decades between its strong, active state and a weak, less active state during the latest ice age.

In addition, climate theory predicts such shifts will happen when the AMOC ocean system transits through a tipping point. A tipping of the AMOC into a weak state will lead to a relatively sudden cooling of Western Europe and the North Atlantic region, as well as impact the global heat distribution.

It has been shown that the AMOC is currently showing signs of approaching such a tipping point, being at its closest to an abrupt transition to its weak state for the last 1000 years. It has not been known, however, how close the AMOC is to its tipping point.

Tipping around mid-century

Now, the study ”Warning of a forthcoming collapse of the Atlantic meridional overturning circulation” predicts the most likely time for the abrupt slowing of the ocean system and its transport of heat to the Northern Hemisphere is between 2021 and 2079 with a 95% probability. Most likely, the abrupt transition will occur mid-century around the year 2057.

The prediction is based on observations of Early Warning Signals for tipping points, which are typical signs of a physical system that has lost its stability.

An early warning signal for the AMOC has been reported before, but now advanced statistical methods have been developed to actually predict the timing of a forthcoming tipping point. The new theoretical insight was applied to Atlantic sea surface temperature data from 1870-2020. The historical surface temperatures are considered ”fingerprints” reflecting the strength of the overturning of surface water to deep seawater in the Arctic and sub-Arctic regions.