Atlantic Ocean’s Heat Valve Weakening

Nature Geoscience Study by Christo Buizert

The vast network of ocean currents responsible for regulating global climate operates with a mechanism far more complex than previously understood. New scientific models reveal that the Atlantic Meridional Overturning Circulation, commonly known as the AMOC, behaves less like a simple loop moving water from place to place and more like a massive thermal valve governing whether the planet absorbs or releases energy. This system of ocean currents redistributes heat from tropical regions up to cooler places, and cools warmer waters down too, which is important for sensitive marine life like corals. Nutrients that ocean species rely on also get pushed around on this giant, mostly invisible conveyor belt beneath the waves. Right now, the AMOC is weakening due to human-caused climate change, which could have huge repercussions, changing temperature and rainfall patterns humans rely on to grow food.

As lead author Christo Buizert writes in a research briefing, researchers want to understand when and how a critical threshold beyond which AMOC weakening becomes self-reinforcing and irreversible. That was the focus of a truly international team’s study for Nature Geoscience that included Buizert. Buizert was encouraged to submit this work to the journal after a chat over cheese fondue at a meeting in Switzerland, proving that giving scientists delicious free food is always a wise investment. The AMOC is a circuit of ocean currents that carries warm, salty surface water northwards from the tropics up into the North Atlantic, where it cools, sinks, and flows back southwards at depth, continuing the cycle in a circulating system. Over the past few years, a considerable number of studies have suggested that the AMOC is slowing down and weakening as a result of warming temperatures. Freshwater from the thawing Greenland ice sheet is diluting the salty water in the Atlantic that normally sinks and drives the current back south, which in theory could put the brakes on the giant circulatory system and eventually stop heat from being carried back to higher latitudes.

Reimagining the AMOC as an Ocean Heat Valve

For decades, researchers viewed the AMOC as a giant conveyor belt carrying warm, salty water north from the tropics into the North Atlantic, where it cools, sinks, and loops back south at depth. Looking to the past might hold some answers: During the Pleistocene ice ages, the AMOC repeatedly switched between strong and weak states, and these shifts corresponded with abrupt climate events known as Dansgaard–Oeschger, or D–O, events, recorded particularly clearly in Greenland ice cores. The conventional explanation for these D–O events was what the researchers call a thermal bipolar seesaw, a model that seemed to explain how the AMOC-driven heat transport caused warming at one pole and cooling at the other pole.

Taking a new perspective, an international team of scientists came up with another explanation by using several computer climate models to track how heat moves around in the oceans and atmosphere, synthesizing two decades of observational and theoretical advances in understanding the crucial parts played by ocean heat content, sea ice, and the planetary radiation balance, which is the difference between incoming solar radiation and outgoing radiation emitted by the Earth, according to Buizert’s statement. Instead of thinking of the AMOC primarily as a conveyor belt shuttling heat from one hemisphere to another, researchers propose thinking of it more like an ocean heat valve. Their findings suggest that the system actually controls Earth’s overall energy budget by regulating heat escape.

Atlantic Ocean’s ‘Heat Valve’: A Threat to Global Temperature Control

The AMOC works like a heat valve that controls the energy budget of the planet, said Christo Buizert, a paleoclimatologist at Oregon State University and lead author of the study.

Oregon State University College of Earth, Ocean, and Atmospheric Sciences

How the Global Heat Valve Operates Under Ice Age Conditions

Researchers have long known that the AMOC plays an important role in the circulation of heat by global ocean currents. While the AMOC has been strong over the last 11,700 years since the end of the last Ice Age, many climate models project future weakening of the AMOC due to human-driven climate change. Such AMOC weakening would exacerbate warming across the planet, said Buizert, an associate professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences. During past Ice Age periods between 11,700 years ago and 2.7 million years ago, the AMOC underwent a series of abrupt changes known as Dansgaard-Oeschger events, which are probably the best example of climate tipping points—the critical thresholds that, when crossed, lead to sudden and potentially irreversible climate change.

When the AMOC is strong, more heat is able to escape the ocean and radiate into space. When it is weak, that heat stays trapped in the ocean and builds up because the AMOC acts like a vent that could either let heat escape through the surface or keep it plugged up in the lower depths. The key finding was that it’s primarily the surface layer of the North Atlantic that cools down when the AMOC weakens, while temperatures in the rest of the ocean interior actually increase as warmth builds up. The old theory was that a weak AMOC would transfer heat from the northern hemisphere to the southern hemisphere in a process called the thermal bipolar seesaw, but this latest study suggests this isn’t the case. During past Ice Age periods of AMOC weakening, the resulting global heat gain was equivalent to roughly 25 ppm of CO₂—about a decade of current human emissions—while only a thin North Atlantic surface layer cooled.

Implications for Modern Climate Change and Future Stability

Intergovernmental Panel on Climate Change Projections

Some have even suggested the entire AMOC could be on course for total collapse, although the Intergovernmental Panel on Climate Change suggests this tipping point is unlikely to happen before 2100. A weakening AMOC would potentially bring extreme cold weather to the parts of the North Atlantic that rely on the currents to receive heat, such as Northeast North America, Northwestern Europe, and Greenland. This idea is a key premise of the 2004 blockbuster The Day After Tomorrow, in which the Northern Hemisphere is suddenly plunged into a devastating ice age nearly overnight because of an upset to Atlantic current systems. However, a new study suggests the impact of a warming planet on the AMOC is more complex than this simple picture puts forward.

The team concluded that while future warming projects future weakening of the AMOC under human-driven warming, which would exacerbate global heat gain, the study suggests the circulation may be more stable in a warmer world, potentially reducing the risk of irreversible collapse, though further research is needed. The most direct impact of AMOC weakening is cooling in the North Atlantic and the regions surrounding it, including Greenland. But as Buizert noted, when zooming out and looking at the entire planet, the total amount of heat actually increases.