Rapid Global Warming Could Trigger the Collapse of the Atlantic Ocean Current

The Atlantic Meridional Overturning Circulation (AMOC), a major system of ocean currents that helps regulate Earth’s climate, may withstand extremely high levels of gradual warming. However, a new study suggests that rapid global warming could cause the circulation to collapse at far lower temperatures than previously expected.

Climate models indicate that the AMOC can adapt when global temperatures rise slowly. In these scenarios, increasing evaporation and changes in Arctic sea ice help preserve the circulation even after warming exceeds 9 degrees Fahrenheit (5 degrees Celsius) above pre-industrial levels.

But the pace of climate change appears to be just as important as the final temperature. When greenhouse gas emissions remain high and temperatures rise quickly, the AMOC may reach a tipping point after only about 3.6 F (2 C) of global warming, researchers found.

“Under slow warming, the entire ocean, from the surface to the deepest layers, has time to gradually reorganize and adapt to changing conditions,” study co-author Henk Dijkstra, a professor of physical oceanography at Utrecht University in the Netherlands, said in a statement. “As global warming continues, the oceans simply cannot keep up.”

The study examined how the rate of climate change affects the stability of the AMOC, a vast network of currents that transports warm water toward the North Atlantic and helps distribute heat across the Northern Hemisphere. The Gulf Stream is one of the best-known parts of this broader circulation system.

If the AMOC weakens substantially, some regions could experience major climate disruptions. Previous research has linked a slowing circulation to colder conditions in parts of Europe, faster sea-level rise along the U.S. East Coast and changes in rainfall that could increase drought risk near the equator.

Earlier research suggested that the AMOC is already at its weakest point in more than 1,000 years and could eventually shut down if global warming reaches a threshold of approximately 7.2 F (4 C) above pre-industrial levels. A previous study estimated this potential threshold. The new research suggests, however, that the risk cannot be explained by a single temperature number.

The AMOC depends on a process known as deep-water formation. In the North Atlantic, cold, salty surface water is denser than the water below it and sinks, helping drive the circulation. Global warming is disrupting this process by heating the ocean surface, while melting Arctic ice adds fresh water and reduces the salinity of the North Atlantic.

Dijkstra and his colleagues used a series of climate-model experiments to investigate how different rates of atmospheric carbon dioxide, or CO₂, increase affect the AMOC. In the slowest experiment, CO₂ concentrations rose by 0.5 parts per million per year. In the faster experiments, concentrations increased by 2.5 ppm and 5 ppm per year.

The slowest scenario represented a much slower increase than the current rate of atmospheric CO₂ growth, which is approximately 2.4 to 2.5 ppm per year.

“We intentionally considered a much slower scenario than the situation we are currently experiencing,” study co-author Rike Berner, a physical oceanographer at Utrecht University, said in a statement. “This allowed us to isolate the effects of the rate of warming, regardless of how much warmer the planet ultimately became.”

The results, published August 13 in Nature Climate Change, indicate that the speed of global warming may determine whether the AMOC survives. In the slow CO₂ increase experiment, the circulation remained strong until the climate warmed by approximately 9.9 F (5.5 C), after which it collapsed. In the faster-warming experiment, the AMOC collapsed at roughly 3.6 F (2 C) of warming.

The researchers identified two processes that allow the AMOC to adapt to slow warming. First, gradually increasing temperatures may boost evaporation over the North Atlantic. This removes fresh water and concentrates salt at the surface, making the water denser and more likely to sink. Second, after large amounts of Arctic ice have already melted, a warmer world may produce less additional meltwater entering the North Atlantic.

These changes can help stabilize the circulation when freshwater inputs remain limited. However, the study suggests that the ocean needs time to adjust. Rapid warming may overwhelm these natural feedbacks before they can protect the AMOC from collapse.

The findings could have important implications for climate policy. Many climate targets and risk assessments focus on the amount of warming reached, but the researchers argue that policymakers must also consider how quickly temperatures rise.

The study also raises concerns about temporary temperature overshoots. Some climate strategies allow global temperatures to exceed a target temporarily before falling again through emissions reductions and carbon removal. However, using technologies such as carbon capture to lower temperatures later may not reverse an AMOC tipping point once it has been crossed, the researchers said.

“The tipping point that causes AMOC collapse may have already been triggered during such a temporary overshoot,” lead author René van Westen, a physical oceanographer at Utrecht University, said in a statement. “Once that happens, you can’t easily go back.”

The study does not establish a definitive carbon dioxide limit or prove exactly when the AMOC will collapse. Instead, it highlights the importance of slowing the rate of global warming while scientists continue to refine estimates of the circulation’s tipping points.

Jenny Mecking, a research scientist at the United Kingdom’s National Oceanography Centre who was not involved in the research, told Live Science by email that the findings reinforce the need to consider the rate of CO₂ increase when assessing the Earth system’s response to climate change.

“While this study alone is not sufficient to establish limits on carbon dioxide concentrations, it emphasizes the importance of considering the rate at which CO₂ rises when investigating the Earth system’s response to climate change,” Mecking said.