Climate change may be causing a slowing of the Gulf Stream and broader-based Atlantic Meridional Overturning Circulation (AMOC), a development that could cause higher sea levels off Florida and the southeastern US Atlantic coast, as well as result in much colder temperatures in Western Europe, according to recently released research results.
Rapid melting of Arctic ice sheets and glaciers is sending more cold, freshwater into the northern Atlantic off Greenland, thereby slowing the Gulf Stream's and AMOC, according to the research report. The Gulf Stream and AMOC transports warm, tropical ocean waters from the tropical southwestern Atlantic to Greenland and the Arctic, where they sink and mix with cold, deep Arctic Ocean waters. The current then carries on around to Western Europe, reducing surface temperatures that result in what would otherwise be much colder winters across the region. The mixing and overturning also supports a rich diversity of marine species
The AMOC has reached its weakest point in 1,600 years, having lost some 15 percent of its strength since the middle of the 20th century, according to a science news report. Scientists disagree on whether climate change or natural cycles account for the slowdown. But a consensus has emerged that climate change will lead to a slower Gulf Stream system in the future, as melting ice sheets in Greenland disrupt the system with discharges of cold fresh water, the news report says.
Mounting evidence of a a Gulf Stream, AMOC slowdown and its prospective effects
A slowdown in the Gulf Stream and AMOC would result in higher sea levels in southeast Florida, something that is already happening and has led southeastern Florida cities, such as Miami Beach, to invest in many millions of dollars to carry out emergency measures, such as raising street levels. It would also mean that a lot of the heat that would have been transported by ocean waters to the Arctic and onward around to Western Europe would remain in the atmosphere and result in warmer land temperatures in Florida and the US East Coast, according to the news report.
"If you slow down the sinking of water in the North Atlantic, that means you have a pileup of waters along the eastern seaboard of the United States and the Gulf of Mexico," said Brenda Ekwurzel, director of climate science for the Union of Concerned Scientists, an environmental group.
"That means that you have increased regional sea level rise just from that ocean circulation change. So that's not good for New York City, Norfolk or along Florida."
"Your cooling mechanism to get that water to the north is slowing down," she said. "This slowing down of your natural air conditioning, by getting that hot water from the Gulf Stream flowing northward, means that you have that hotter water sticking around and not getting out of your region as fast.
Recent research results
Geological climate evidence has given scientists cause to believe that a slowdown in the AMOC caused a mini Ice Age in Europe known as the Younger Dryas around 11,000 years ago. Widely considered the father of modern climate science, Columbia University Lamont-Doherty Earth Observatory's Wallace S. Broecker first hypothesized and found evidence indicating that an increase in ice melt in the Arctic was causing a slowing of the Gulf Stream and AMOC decades ago. More recent research backs up his claims.
Lamont-Doherty Professor Marco Tedesco and colleagues in 2016 suggested that a narrowing of the temperature difference between polar and temperate regions was pulling the atmospheric Jet Stream northwards, causing warm, moist air to accumulate over Greenland. Temperatures in the Arctic are warming twice as fast as the global mean, according to climate research scientists. The Columbia Lamont-Doherty researchers found that the resulting above average temperatures in Greenland, as well as increasing rates of snow and ice melt across Siberia, has been increasing the amount of freshwater streaming into the North Atlantic and Arctic Oceans.
“The melting in Greenland is within the mean, but it’s still above the average of what was happening 20 years ago…The snow melt from Siberia has also been melting sooner, there’s been more fresh water from Greenland, there’s more fresh water from sea ice in the Arctic Ocean and more fresh water from North Canada which has been melting at an increasing rate. All these factors are pointing in the direction of increasing the freshwater discharge in the North Atlantic section of the Arctic. It’s very likely going to have an impact," Tedesco was quoted in a Columbia University research news report.
The ocean absorbs more heat from the atmosphere and stores it in waters as much as 1,500 meters (~1 mi.) deep when the AMOC is moving faster current moves faster, according to a duo of climate scientists reported in April this year. Less heat is stored when the AMOC slows down, which results in higher land surface temperatures. The researchers say that that a large decline in AMOC flow since 2004 will mean less heat is absorbed in Atlantic and Arctic Ocean waters and remains in the atmosphere, which lead to higher temperatures globally, as opposed to the cooling effect it's expected to have in Europe.
Contrary to previous beliefs, the research duo concluded that "the AMOC minimum [from 1975-1988] "was a period of rapid surface warming. More generally, in the presence of greenhouse-gas heating, the AMOC’s dominant role changed from transporting surface heat northwards, warming Europe and North America, to storing heat in the deeper Atlantic, buffering surface warming for the planet as a whole.
"During an accelerating phase from the mid-1990s to the early 2000s, the AMOC stored about half of excess heat globally, contributing to the global-warming slowdown. By contrast, since mooring observations began in 2004, the AMOC and oceanic heat uptake have weakened."