Melting Ice Caps May Not Shut Down Ocean Current

Most simulations of our climate's future may be overly sensitive to Arctic ice melt as a cause of abrupt changes in ocean circulation, according to new research led by scientists at the University of Wisconsin-Madison. Phys.Org reports: Climate scientists count the Atlantic Meridional Overturning Circulation (or AMOC) among the biggest tipping points on the way to a planetary climate disaster. The Atlantic Ocean current acts like a conveyor belt carrying warm tropical surface water north and cooler, heavier deeper water south. [...] In a study published [...] in the journal Nature Climate Change, He and Oregon State University paleoclimatologist Peter Clark describe a new model simulation that matches the warmth of the last 10,000 years. And they did it by doing away with the trigger most scientists believe stalls or shuts down the AMOC. Warming temperatures on Earth's surface cause sea ice in the Arctic Ocean and the Greenland Ice Sheet to melt, releasing fresh water into the ocean. Scientists widely believed that the freshwater influx disrupts the density differences in the North Atlantic that make the AMOC's north-bound water sink and turn back south. "The problem," says He, "is with the geological climate data." Though the climate record shows an abundance of freshwater that came from the final melting of the ice sheets over North America and Europe, the AMOC barely changed. So, He removed the assumption of a freshwater deluge from his model. "Without the freshwater coming in making the AMOC slow down in the model, we get a simulation with much better, lasting agreement with the temperature data from the climate record," He says. "The important result is that the AMOC appears to be less sensitive to freshwater forcing than has long been thought, according to both the data and model." [...] The widespread consequences of a drastic weakening of the AMOC include rapid sea-level rise on the eastern coast of North America, cooling over Europe that could disrupt agriculture, a parched Amazon rainforest and disruption of Asian monsoons. The new modeling study anticipates a much smaller reduction in AMOC strength, but that doesn't rule out abrupt change.

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