Evidence the ocean is losing less heat in the northern Atlantic and the AMOC has started to slowly weaken
This study provides robust evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened by approximately 14% over the last 30 years, a change driven by a significant reduction in ocean heat loss in the northern Atlantic that is attributed to forced climate change.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The Atlantic Ocean is home to a massive, slow-moving conveyor belt of water known as the Atlantic Meridional Overturning Circulation. This system acts as a global heat engine, pulling warm water from the tropics up toward the north and carrying cold, deep water back south. As this warm water travels north, it releases heat into the air, a process that keeps the climate of Europe significantly milder than it would be at such high latitudes. Scientists have long understood that if this circulation were to slow down or stop, the consequences for weather patterns and sea levels could be profound. However, measuring the strength of this underwater current is difficult because it fluctuates naturally from year to year, making it hard to tell if a temporary dip is just noise or the beginning of a long-term decline.
A new study led by researchers at the UK Met Office and other institutions offers a fresh way to look at this problem. Instead of trying to measure the current directly, which requires decades of continuous observation to separate signal from noise, the team looked at the heat the ocean loses to the atmosphere. The logic is straightforward: the strength of the ocean's northward flow is tightly linked to how much heat it sheds along the way. If the circulation weakens, less warm water reaches the north, and the ocean loses less heat to the air. By analyzing satellite data and atmospheric records from 1985 to 2023, the researchers found that the amount of heat lost by the northern Atlantic has dropped significantly. They calculated that the ocean in this region is now losing about 18 percent less heat than it did at the start of their study period. This reduction is so large that it is extremely rare to see such a change in computer simulations of the climate before human influence began, suggesting that the shift is a direct result of global warming.
Using the strong relationship between heat loss and current strength found in climate models, the researchers translated this drop in heat loss into an estimate for the speed of the ocean current. Their calculations suggest that the Atlantic Meridional Overturning Circulation has weakened by roughly 14 percent over the last 30 years. This finding aligns with, and helps explain, the slower decline observed in direct measurements taken by a network of moored sensors in the ocean since 2004. The study argues that looking at heat loss is a more reliable way to detect long-term trends than relying solely on the short, noisy record of direct measurements or on indirect clues like sea surface temperatures, which can be misleading. While the current has not collapsed, the evidence points to a steady, human-driven slowdown that has already begun, offering a clearer picture of how the planet's oceanic heart is responding to a warming world.
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