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Abrupt Atlantic Ocean Changes May Have Been Natural

Climate change may not have been to blame for an abrupt recent slowdown of a sweeping Atlantic Ocean current, a change that delivered an intense pulse of ocean warming and sea level rise through the Gulf of Maine and elsewhere along the East Coast.

Modeling-based analysis by British scientists, published Monday in Nature Geoscience, concluded that the decline in the Atlantic meridional overturning circulation (AMOC) from 2004 to 2014 was “part of decadal variability of the North Atlantic,” representing a recovery following a previous strengthening of its currents.

Atlantic Ocean currents influence the weather and sea levels.
Credit: John Spade/Flickr

Still, leading scientists warn that greenhouse gas pollution appears to be causing the circulation pattern to slow down, and that it will continue to do so with far-reaching implications for weather and for flood-prone cities and farms around the world.

The findings were based on analysis of data from Met Office weather forecasting models. They were generally welcomed by other scientists, who said they point to the powerful role that the whims of nature can play on the ocean cycle — even as greenhouse gas pollution causes ice sheets to melt, which prior research has shown is causing the circulation to slow overall.

“The decline we’ve seen is much larger than we would have expected just from increasing greenhouse gases over the last decade,” said Tom Delworth, a climate scientist with the National Oceanic and Atmospheric Administration and Princeton University. He wasn't involved with the study.

The AMOC washes warm equatorial waters toward the Arctic and shuffles cold Arctic waters southward, helping to regulate ocean and air temperatures.

Greenhouse gas pollution has caused air temperatures to rise nearly 2°F globally since the Industrial Revolution, warming oceans and causing seas to rise worldwide now by an average of more than an inch per decade. The Atlantic circulation slowdown caused impacts to be especially pronounced along the East Coast, worsening floods and devastating some fisheries.

The slowdown of the Atlantic circulation was particularly pronounced around 2009 and 2010, which research has shown contributed to a frenetic Atlantic hurricane season and to unusually cold winters in parts of North America and Europe. More recently, it may have intensified the January blizzard that briefly shut down New York and other East Coast cities.

As ocean currents go, the AMOC also plays an outsized cultural role in framing conversations about global warming. The effects of its slowdown were fictionalized in the 2004 science fiction movie, “The Day After Tomorrow.” Two years later, links between climate change and the ocean current were explained in the climate change documentary, “An Inconvenient Truth.”

The new paper didn’t attempt to determine the root cause of the cycles that periodically cause the Atlantic circulation to strengthen and weaken. Like other scientists, Laura Jackson, a Met Office scientist who led the research, said “the most likely explanation” was “natural variability.”

“Our results suggest that there hasn’t been a persistent weakening of the AMOC over the last 20 years,” Jackson said. “More years of observations would be required to identify an ongoing trend. This does not change the view that it is very likely that the AMOC will weaken over the 21st century.”

A January blizzard in New York may have been intensified by changes in the Atlantic Ocean's circulation patterns.
Credit: Stacey Bramhall/Flickr

Although the findings were well received overall by other scientists, the suggestion that the recent slowdown represented a “recovery” following a previous speedup triggered some consternation. That’s because it could be seen as diminishing the long-term role of climate change in affecting the circulation.

Michael Mann, a Penn State meteorology professor who contributed to research last year showing the AMOC is weaker now than in the last 1,000 years, said the conclusions of the new paper were “consistent with our findings.”

But Mann described the use of the word “recovery” in the title and abstract of the paper as a “gaffe” that “detracts considerably from what would otherwise be a useful contribution to the peer-reviewed literature.”

Chris Roberts, a Met Office scientist who contributed to the new paper, defended the work by pointing out that the team’s focus was limited to addressing changes that have been observed over the last couple of decades — rather than over recent centuries or into the future.

“Our results do not exclude a longer-term weakening,” Roberts said.

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