Atlantic Current Weakening Confirmed by Satellites

Satellite measurements show the Atlantic current system that carries warmth toward Europe has been moving less heat northward, giving researchers some of the strongest evidence yet that Atlantic Current Weakening is underway. The system, known as the Atlantic Meridional Overturning Circulation, includes the Gulf Stream and helps keep Europe's climate mild, so a sustained slowdown would reach far beyond ocean science.

The circulation works like a loop. Warm, salty water travels from the Gulf of Mexico toward the north Atlantic, where it cools and sinks, then returns south along the sea floor. As that water cools, it releases heat into the air above it. According to a team led by Richard Allan at the University of Reading, satellite data combined with atmospheric records and models shows that the heat escaping from the north Atlantic has fallen by about eighteen percent over four decades. That drop in released heat is the fingerprint of Atlantic Current Weakening, the researchers say.

Because the flow varies from year to year, the researchers averaged the first decade of data and compared it with the decades that followed. That comparison points to a decline of about fourteen percent in the circulation over three decades, the researchers report. The study is posted as a preprint and is still going through peer review, so its numbers may be revised before formal publication.

What the satellites measured

Earlier attempts to track the circulation leaned on ocean buoys and sea-surface temperatures. Buoys spread across the Atlantic have recorded shifts in water pressure consistent with a decline of roughly ten percent over the past two decades, but the flow can ebb for years at a time, which makes a short record hard to read as a trend. Sea-surface temperatures pointed the same way: a cold patch in the north Atlantic, the only part of the world ocean that has cooled since the middle of the last century, led a 2018 analysis to estimate a weakening of about fifteen percent. Both records feed into the Atlantic Current Weakening picture.

The new work takes a different measurement. It combines satellite observations of Earth's energy imbalance with atmospheric reanalyses and models to work out how much less heat the north Atlantic has been giving up to the sky. Michael Mayer, a team member at the University of Vienna, said the resulting estimate of heat moving between sea and air is of demonstrably better quality than other estimates, making it more suitable for inferring changes in the circulation. Richard Allan said the method, if it holds up to scrutiny, could someday provide a new early warning method for detecting Atlantic Current Weakening. Britain is spending eighty-one million pounds on research toward an early warning system for a collapse of the circulation by the end of the decade, although this study is not part of that effort.

Not every researcher reads the new measurement the same way. Stefan Rahmstorf at the University of Potsdam argues that heat flux is not necessarily a better indicator than sea-surface temperatures, which can be measured directly. He added, though, that the two lines of evidence build on each other and raise concern that the circulation could be approaching a tipping point. Another recent study that examined ocean records rather than atmospheric ones reached a similar result, finding a decline of similar size over three decades. As Rahmstorf put it, the strongest evidence is the combination of several different lines of evidence, all pointing to a slowing circulation. That convergence is why the Atlantic Current Weakening evidence carries weight beyond any single study.

Why a slower current matters

Climate models suggest the surge of fresh water from the melting Greenland ice sheet will dilute the salty water that drives the circulation and slow its sinking. Some research has warned that Atlantic Current Weakening could end in a collapse within decades, a shift that would bring far colder winters to Europe along with stronger storms and widespread drought. A separate study from the Danish Meteorological Institute, published in the journal Science Advances, found no sign of an irreversible tipping point in its model and suggested the circulation could regain strength if the climate cools, although the recovery could take centuries and the Greenland ice sheet itself could not be restored once melted. A disrupted circulation could still unsettle rainfall patterns around the world and push coastal sea levels higher, the researchers said.

For now, the satellite result adds a fresh line of evidence while the debate continues. The preprint's authors note that satellite records can be updated regularly, which means the same method can be rerun as new data arrives. Ocean monitoring from the RAPID programme, which tracks the circulation directly, will give researchers another check on whether Atlantic Current Weakening continues. The test will be whether the long decline holds as the record grows.

Sources for this article include New Scientist's report on the satellite analysis and the preprint by Bell and colleagues, posted on Research Square.