An Atlantic Ocean current system weakened by global warming could recover over time if the climate cools, a study by the Danish Meteorological Institute showed on Wednesday.
The Atlantic Meridional Overturning Circulation (AMOC) brings warm water from the tropics northward toward the Arctic and the flow of warm water helps keep Europe's winters mild.
Published in the journal Science Advances, the study found no sign there was an irreversible tipping point for AMOC, as suggested by some previous studies, although the effects of the changes could last for centuries.
However, it also said that the Greenland ice sheet that contributes to weakening the AMOC as it pours cold freshwater into the ocean could not be restored once it had melted — even if the warming trend was reversed.
A disrupted AMOC could still destabilize global rainfall patterns and accelerate coastal sea-level rise.
The study said the AMOC could slow to about 6 million cubic metres of water per second from around 17 million at around 7 degrees Celsius (44.6 degrees Fahrenheit) of global warming.
"In our model, we do not see a tipping point leading to a collapse. The AMOC gradually weakens, but we find that it also immediately regains its strength if global warming is reversed," lead author Chuncheng Guo said.
However, he warned that even though recovery starts as soon as temperatures fall, it could take several centuries to finish, all while the effects of a weakened current would persist.
The 7 degrees Celsius (44.6 degrees Fahrenheit) rise used in the modeling used for scientific interest is far beyond the current projections for the warming trend.
The U.N.'s environmental body said in September that global temperature rises are set to exceed the key threshold of 1.5 degrees Celsius (2.7 degrees Fahrenheit) "within a few years." Governments committed to keeping temperature rises within 1.5 degrees Celsius to limit the impacts of extreme weather under the 2015 Paris Agreement.
The cooling rate required to reverse the AMOC also exceeds what is considered realistic with current technology, Guo added, pointing to a limitation of the single-model study. Further testing against other models was also needed, the study said.
Willem Huiskamp at the Potsdam Institute for Climate Impact Research, who was not involved in the study, said a more stable AMOC would be good news, but said he doubted the real-world current was as stable as some models suggested.
The researchers estimated a long-term sea-level rise of around 20 centimeters (7.9 inches) from the melting of the Greenland ice sheet following a 1.5 degrees Celsius rise, and urged faster action to rein in emissions.
"Instead of relying on the global cooling to happen, it's a much, much safer strategy to not have warming in the first place," Guo said.
