Atlantic Heat Engine May Have Slowed 14%, Satellite Study Suggests

The Atlantic circulation that carries heat north has weakened by about 14% over 30 years, according to a new satellite-based estimate with implications for climate, coastal seas and marine life. ^[1][2]

The research, by Michael Bell, Richard Allan and colleagues, uses changes in the heat escaping from the North Atlantic to infer changes in the water moving through it. Satellites supply part of the evidence, alongside atmospheric reconstructions and climate models. They do not directly measure the current's flow. ^[1][2]

The system, known as the Atlantic Meridional Overturning Circulation, or AMOC, moves warm water north and colder water south at depth. Its reach makes a sustained slowdown important far beyond the ocean itself. NOAA says continued weakening could raise sea levels along the US East Coast and shift rainfall in South Africa, potentially causing drought. ^[3]

The findings are in a preprint under journal review. The estimated decline of 14.2% carries an uncertainty almost as large as the estimate itself, at plus or minus 13.9%. The study estimates weakening, not the timing or likelihood of collapse. ^[2]

Neil Fraser of the Scottish Association for Marine Science, who was not involved in the work, said the result was among the strongest evidence yet that the circulation has weakened over the past 40 years. ^[1]

Following the heat

The researchers' starting point was the heat lost by the Atlantic between 26.5 degrees and 70 degrees north, a stretch running from the subtropics into northern waters. They estimate that this heat loss fell by about 18% between 1985 and 2023, with a 95% confidence interval of plus or minus 10%. ^[2]

That heat exchange is part of the circulation's machinery. Warm, salty surface water travels north, cools and sinks, then returns south deep beneath the surface. The AMOC is a large system of connected currents, with the Gulf Stream forming one part. It transports nutrients as well as heat. ^[1][3]

To reconstruct the heat exchange, the team combined satellite measurements of Earth's energy imbalance - the difference between energy arriving and leaving - with models and atmospheric reanalyses. Those reanalyses bring observations together with models to reconstruct past atmospheric conditions. Together, the records let the researchers infer how much heat was passing from ocean to air. ^[1][2]

The next step supplied the estimate of circulation strength. In climate-change simulations, the researchers examined how trends in North Atlantic heat loss corresponded to trends in the AMOC. They used that relationship to infer the change in circulation at 26.5 degrees north. ^[2]

Michael Mayer of the University of Vienna, a member of the team, said the resulting estimate of net heat exchange between air and sea was demonstrably better than other estimates and better suited to inferring AMOC changes. ^[1]

The authors also examined simulations of a pre-industrial climate. They said changes in ocean heat loss as large as those found over 40 years were rare in those runs, arguing that the observed change was likely a consequence of forced climate change. ^[2]

A longer view than the buoys

The attraction of the method is its reach into the past. Ocean-buoy pressure measurements used to track the circulation date from 2004. Those measurements correspond to a decline of about 10% since then, but the circulation varies from year to year and can weaken for decades, making a long-term trend difficult to establish from a short record. ^[1]

Other approaches have also produced estimates of weakening. An earlier reconstruction based on North Atlantic sea-surface temperatures put the decline at 15% since the 1950s. A separate recent study using ocean reanalyses estimated a 15% decline over 30 years. ^[1]

Those numbers cover different periods and come from different methods. Their value lies in testing the circulation from several directions. Surface temperatures, for example, can change because winds shift or clouds shade the ocean, as well as because ocean circulation changes. ^[1]

Stefan Rahmstorf of the University of Potsdam said heat exchange was not necessarily a better indicator than directly measured sea-surface temperatures. He nevertheless argued that the two lines of evidence together increased concern about a possible tipping point. ^[1]

The useful advance is an additional way to monitor a system whose direct measurement record is short. Agreement between methods strengthens the case for a slowdown, while each method gives researchers another result to test.

What changes on shore

The consequences would extend beyond how much warmth reaches the North Atlantic. NOAA identifies higher sea levels along the US East Coast and changes to South African rainfall among the possible effects of continued slowing. ^[3]

Marine ecosystems give the problem another dimension. NOAA Fisheries' account of a 2018 reconstruction described changes in the distribution of fish and other species, and in their prey, amid warming in the Northeast US Shelf and Gulf of Maine. That reconstruction estimated about 15% weakening since the 1950s.

It provides regional context, rather than a prediction of what the new estimate means for any particular fishery. ^[4]

The next task is to establish whether heat-loss monitoring can become a dependable warning tool. Bell said regularly updated satellite measurements could eventually make the approach useful for early warning if the method withstands scrutiny. ^[1]

The UK is investing £81 million in research to build an AMOC-collapse early-warning system by 2030, although this study is separate from that initiative. For Bell and colleagues, the immediate tests are journal peer review and further observations of the ocean they are trying to measure. ^[1][2]