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<\/figure>\n\n\n\nHowever, building upon previous work, He says researchers are revising their understanding of the relationship between AMOC and freshwater from melting polar ice.<\/p>\n\n\n\n
In the past, a stalled AMOC has accompanied abrupt climate events like the B\u00f8lling-Aller\u00f8d warming, a 14,500-year-old, sharp global temperature hike. He successfully reproduced that event using a climate model he conducted in 2009 while a UW\u2013Madison graduate student.<\/p>\n\n\n\n
\u201cThat was a success, reproducing the abrupt warming about 14,700 years ago that is seen in the paleoclimate record,\u201d says He, now. \u201cBut our accuracy didn\u2019t continue past that abrupt change period.\u201d<\/p>\n\n\n\n
Instead, while Earth\u2019s temperatures cooled after this abrupt warming before rising again to plateau at new highs for the last 10,000 years, the 2009 model couldn\u2019t keep pace. The simulated warming over the northern regions of the planet didn\u2019t match the increase in temperatures seen in geological archives of climate, like ice cores.<\/p>\n\n\n\n
In a study published this week 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.<\/p>\n\n\n\n
Warming temperatures on Earth\u2019s surface cause sea ice in the Arctic Ocean and the Greenland Ice Sheet to melt, releasing freshwater into the ocean. Scientists widely believed that the freshwater influx disrupts the density differences in the North Atlantic that make the AMOC\u2019s north-bound water sink and turn back south.<\/p>\n\n\n\n
\u201cThe problem,\u201d says He, \u201cis with the geological climate data.\u201d<\/p>\n\n\n\n
Though the climate record shows an abundance of fresh water 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.<\/p>\n\n\n\n
\u201cWithout 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,\u201d He says. \u201cThe 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.\u201d<\/p>\n\n\n\n
This is particularly important to climate models that evaluate how the AMOC will respond to future increases of freshwater from ice melt.<\/p>\n\n\n\n
\u201cIt\u2019s built into many models,\u201d He says. \u201cFuture global warming from increasing carbon dioxide in the atmosphere melts sea ice, and the fresh water from the melting ice is believed to cause the AMOC to weaken.\u201d<\/p>\n\n\n\n
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\u2019t rule out abrupt change.<\/p>\n\n\n\n
\u201cWe suggest until this challenge is solved, any simulated AMOC changes from freshwater forcing should be viewed with caution,\u201d He says. \u201cWe can\u2019t be certain why the AMOC shut down in the past. but we are certain it did change. And it can change again.\u201d<\/p>\n