#amoc

From the article:

"Earth, being 71% covered in water, is influenced by the ocean and its movements. In the Atlantic Ocean, a system of connected currents, called the Atlantic Meridional Overturning Circulation (AMOC), moves water throughout the world's oceans powered by a combination of winds and ocean density. It not only distributes the ocean's heat, moisture, and nutrients, but regulates the Earth's climate and weather.

As the climate is continuously changing and the atmosphere is warming, many scientists fear that fresh water from melting polar ice sheets could significantly disrupt—or collapse—the AMOC. While a decline of the AMOC would have grave consequences, a collapse would be truly catastrophic.

However, studies about the AMOC's long term future are uncertain. Instead of predicting the future, a team of scientists from Woods Hole Oceanographic Institution (WHOI) quantified the past to help inform where we could be going.

In a new paper published in Nature Communications, scientists found that the AMOC has not declined in the last 60 years.

Authors [...] say their results mean that the AMOC is currently more stable than expected.

"Our paper says that the Atlantic overturning has not declined yet," said Foukal, who conducted the research while at WHOI. "That doesn't say anything about its future, but it doesn't appear the anticipated changes have occurred yet."

Their findings contrast with previous work, notably a paper from 2018 cited in their study, which reported that the AMOC has declined over the last 70 years. This past work relied on sea surface temperature measurements to understand how the AMOC has changed, but "we've learned that sea surface temperature doesn't work as well as initially thought," said Terhaar, who began leading this study at WHOI as a postdoctoral scientist and completed the work in Bern.

To address the uncertainty, Terhaar and the team relied on new data from the Coupled Model Intercomparison Project (CMIP), climate-earth models produced by the World Climate Research Program. They used 24 different CMIP models and found that the most recently available surface temperature data did not accurately reconstruct the AMOC.

Going a step further, the researchers looked at a different measure: air-sea heat fluxes, which is the exchange of heat from the ocean to the atmosphere. When the AMOC is stronger, more heat is released from the ocean to the atmosphere over the North Atlantic...

The authors derived this AMOC proxy with the CMIP models, then applied it to observational data. The best data for surface heat fluxes over the North Atlantic come from reanalysis products that incorporate direct observations into a model, similar to the way weather forecasts work. The study authors focused on two reanalysis data sets that extend back to the late 1950s to reconstruct the AMOC.

"Based on the results, the AMOC is more stable than we thought," Vogt said. "This might mean that the AMOC isn't as close to a tipping point as previously suggested." ...

As with all proxy-based reconstructions, there are limitations and caveats. The authors point out that direct measurements of air-sea heat flux going back in time are sparse, and thus the reanalysis products contain significant uncertainty. However, despite these limitations, "a decline in AMOC over the last 60 years," Terhaar concludes, "seems very unlikely.""

-via Phys.org, January 15, 2025

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Excerpt from this story from The Guardian:

The collapse of a critical Atlantic current can no longer be considered a low-likelihood event, a study has concluded, making deep cuts to fossil fuel emissions even more urgent to avoid the catastrophic impact.

The Atlantic meridional overturning circulation (Amoc) is a major part of the global climate system. It brings sun-warmed tropical water to Europe and the Arctic, where it cools and sinks to form a deep return current. The Amoc was already known to be at its weakest in 1,600 years as a result of the climate crisis.

Climate models recently indicated that a collapse before 2100 was unlikely but the new analysis examined models that were run for longer, to 2300 and 2500. These show the tipping point that makes an Amoc shutdown inevitable is likely to be passed within a few decades, but that the collapse itself may not happen until 50 to 100 years later.

The research found that if carbon emissions continued to rise, 70% of the model runs led to collapse, while an intermediate level of emissions resulted in collapse in 37% of the models. Even in the case of low future emissions, an Amoc shutdown happened in 25% of the models.

Scientists have warned previously that Amoc collapse must be avoided “at all costs”. It would shift the tropical rainfall belt on which many millions of people rely to grow their food, plunge western Europe into extreme cold winters and summer droughts, and add 50cm [about 20 inches] to already rising sea levels.

The new results are “quite shocking, because I used to say that the chance of Amoc collapsing as a result of global warming was less than 10%”, said Prof Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany, who was part of the study team. “Now even in a low-emission scenario, sticking to the Paris agreement, it looks like it may be more like 25%.

“These numbers are not very certain, but we are talking about a matter of risk assessment where even a 10% chance of an Amoc collapse would be far too high. We found that the tipping point where the shutdown becomes inevitable is probably in the next 10 to 20 years or so. That is quite a shocking finding as well and why we have to act really fast in cutting down emissions.”

Scientists spotted warning signs of a tipping point in 2021 and know that the Amoc has collapsed in the Earth’s past. “Observations in the deep [far North Atlantic] already show a downward trend over the past five to 10 years, consistent with the models’ projections,” said Prof Sybren Drijfhout, at the Royal Netherlands Meteorological Institute, who was also part of the team.

“Even in some intermediate and low-emission scenarios, the Amoc slows drastically by 2100 and completely shuts off thereafter. That shows the shutdown risk is more serious than many people realise.”

A group of scientists warned Monday of the greatly underestimated risk of a collapse of ocean currents in the Atlantic which could have catastrophic consequences for the Nordic countries as the region's leaders gathered in Iceland. In an open letter addressed to the Nordic Council, which is meeting this week in Iceland's capital Reykjavik, the scientists said they wanted to bring attention "to the serious risk of a major ocean circulation change in the Atlantic."

A significant part of the earth's climate infrastructure is under threat: "A complete guide to the Atlantic Meridional Overturning Circulation (AMOC)"

(Plus- Andrew Sharrett)