Ocean system that moves heat gets closer to collapse, which could cause weather chaos, study says

BY SETH BORENSTEIN
Updated 4:39 PM EST, February 9, 2024

An abrupt shutdown of Atlantic Ocean currents that could put large parts of Europe in a deep freeze is looking a bit more likely and closer than before as a new complex computer simulation finds a “cliff-like” tipping point looming in the future.

A long-worried nightmare scenario, triggered by Greenland’s ice sheet melting from global warming, still is at least decades away if not longer, but maybe not the centuries that it once seemed, a new study in Friday’s Science Advances finds. The study, the first to use complex simulations and include multiple factors, uses a key measurement to track the strength of vital overall ocean circulation, which is slowing.

A collapse of the current — called the Atlantic Meridional Overturning Circulation or AMOC — would change weather worldwide because it means a shutdown of one of key the climate and ocean forces of the planet. It would plunge northwestern European temperatures by 9 to 27 degrees (5 to 15 degrees Celsius) over the decades, extend Arctic ice much farther south, turn up the heat even more in the Southern Hemisphere, change global rainfall patterns and disrupt the Amazon, the study said. Other scientists said it would be a catastrophe that could cause worldwide food and water shortages.

“We are moving closer (to the collapse), but we we’re not sure how much closer,” said study lead author Rene van Westen, a climate scientist and oceanographer at Utrecht University in the Netherlands. “We are heading towards a tipping point.”

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Physics-based early warning signal shows that AMOC is on tipping course
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Abstract
One of the most prominent climate tipping elements is the Atlantic meridional overturning circulation (AMOC), which can potentially collapse because of the input of fresh water in the North Atlantic. Although AMOC collapses have been induced in complex global climate models by strong freshwater forcing, the processes of an AMOC tipping event have so far not been investigated. Here, we show results of the first tipping event in the Community Earth System Model, including the large climate impacts of the collapse. Using these results, we develop a physics-based and observable early warning signal of AMOC tipping: the minimum of the AMOC-induced freshwater transport at the southern boundary of the Atlantic. Reanalysis products indicate that the present-day AMOC is on route to tipping. The early warning signal is a useful alternative to classical statistical ones, which, when applied to our simulated tipping event, turn out to be sensitive to the analyzed time interval before tipping.

INTRODUCTION
The Atlantic meridional overturning circulation (AMOC) effectively transports heat and salt through the global ocean (1) and strongly modulates regional and global climate. Continuous section measurements of the AMOC, available since 2004 at 26°N from the RAPID-MOCHA array (2), have shown that the AMOC strength has decreased by a few Sverdrups (1 Sv = 106 m3 s−1) from 2004 to 2012, and thereafter, it has strengthened (3) again. Longer timescale variability of the AMOC strength, estimated by using sea surface temperature (SST) time series based on “fingerprint” patterns (4), indicates that the AMOC weakened by 3 ± 1 Sv since about 1950. From proxy records, it has been suggested that the AMOC is currently in its weakest state in over a millennium (5).

The AMOC has been labeled as one of the tipping elements in the climate system (6, 7), indicating that it may undergo a relatively rapid change under a slowly developing forcing. The AMOC is particularly sensitive to the ocean’s freshwater forcing, either through the surface freshwater flux (e.g., precipitation) or by input of fresh water due to river runoff or ice melt (e.g., from the Greenland Ice Sheet). Although no AMOC tipping has been found in historical observations, there is much evidence from proxy records that abrupt AMOC changes have occurred in the geological past during the so-called Dansgaard-Oeschger events (8–10).

Classical early warning indicators, such as the increase in the variance and/or the (lag-1) autocorrelation, when applied to SST-based time series, suggest that the present-day AMOC approaches a tipping point before the end of this century (11, 12). Apart from the fact that the SST-based AMOC fingerprints may not represent the AMOC behavior adequately, many (statistical) assumptions are required to estimate the approaching AMOC tipping point (12–15). Hence, there is strong need for a more physics-based, observable, and reliable early warning indicator that characterizes the AMOC tipping point.

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