UNIVERSITY PARK, Pa. -- One of Earth’s most important heat transport systems, ocean currents that carry warm water to the Northern Hemisphere, has weakened significantly during the past several decades, according to evidence found by a team of international climate scientists.
Michael Mann, Distinguished Professor of Meteorology at Penn State, is co-author of a new study, which found evidence of a slowdown in Atlantic overturning, or the Gulf Stream system, which pumps warm ocean water from the tropics and Southern Hemisphere north and carries cold water south.
In recent decades, Atlantic overturning has become as weak as it’s been in the last century, or even millennium, and further weakening could impact marine ecosystems and sea levels and weather systems in the U.S. and Europe, according to the study published March 23 in Nature Climate Change.
“Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both,” said Mann, who is also an associate in Penn State’s Earth and Environmental Systems Institute and director of the Earth System Science Center. “This is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”
The melting of the Greenland ice sheet, caused by man-made global warming, is a possible major contributor and could cause a further slowdown of Atlantic overturning if temperatures continue to rise, according to the study.
Atlantic overturning is driven by differences in the density of warm ocean water from the south and cold water from the north. The warm water is lighter and flows north, while the cold water is denser and sinks to deeper ocean layers and flows south. Fresh water released from the melting ice sheet dilutes the saline ocean water and makes it less dense, less likely to sink to the deep and be carried south.
The potential cooling above the Northern Atlantic would only slightly reduce the continued warming of the continents, but a large change in Atlantic Ocean circulation, even if gradual, could have major negative effects, the team wrote in the study.
“If the slowdown of the Atlantic overturning continues, the impacts might be substantial,” said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research and lead author of the study. “Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”