ATLANTIC OCEAN — A new study published in the journal Science Advances suggests that runoff from the Greenland ice sheet could reduce the strength of the Atlantic Meridional Overturning Circulation (AMOC). By 2100, Greenland meltwater could cause an additional 10% to 20% reduction in AMOC strength beyond the weakening already expected by many scientists.

The AMOC moves heat from the tropics to the Northern Hemisphere and returns cold water south toward Antarctica. Previous research indicates the AMOC is at its weakest point in more than 1,000 years due to Arctic ice melt and rising ocean temperatures.

Meltwater from the Arctic and Greenland slows the AMOC by preventing the formation of deep currents in the North Atlantic. Deep currents form when cold, salty water sinks to the seabed, but fresh water inputs and rising temperatures block this process by diluting and warming surface waters.

Models indicate that continued weakening of the circulation could cause freezing weather in Northwest Europe, sea-level rise along the U.S. East Coast, and droughts around the equator. The study used the global climate model EC-Earth3, which does not include the Greenland ice sheet, and added data from the Community Ice Sheet Model version 2 (CESM2).

Researchers have been slower to include Greenland ice sheet runoff in climate models partly because it may not be as disruptive as Arctic meltwater on shorter timescales. "In order to represent correctly the Greenland melt, you need a specific, dynamic model for the Greenland ice sheet. It's nice to have, but it requires a lot of resources to develop," said Jost von Hardenberg, professor in the Department of Environment, Land and Infrastructure Engineering at the Polytechnic University of Turin.

"By the year 2100, yes, we have something, but the greatest fraction is what we add later," von Hardenberg said. "Greenland starts really melting strongly only after that date."

By 2300, the Greenland ice sheet could cause 40% of AMOC weakening on top of declines projected by some climate models if greenhouse gas emissions continue. "If you go farther in our simulation — we go up to the year 2300 — that is also when this meltwater really becomes extremely strong," he said.

In the model simulations, the AMOC recovered partially when meltwater input was turned off and fully when greenhouse gas emissions were stopped. It's consistent with a strong weakening, but not a shutdown, he said.

"It's really something which is worthwhile doing to better constrain the uncertainty of these future simulations," he added. "If we have a larger set of models which include this component, we will be in a better position to try to answer the question, 'How much do we expect AMOC to decrease in different scenarios?'"

The Atlantic Meridional Overturning Circulation is a zonally-integrated component of surface and deep currents in the Atlantic Ocean that regulates global climate patterns. The study shows the long-term impact of Greenland ice sheet runoff, which researchers have historically underrepresented in climate models due to resource constraints.

Understanding the specific contribution of Greenland meltwater helps clarify the uncertainty in future climate projections. The findings suggest that while the AMOC will weaken significantly, a complete shutdown is not projected under the simulated conditions if emissions are eventually halted.