When studies are published with new findings related to global warming, they usually convey that the changes are coming faster or that the effects could be more devastating than previously thought. But now there is a cautiously optimistic American study by two scientists suggesting that freshwater input from Greenland’s melting ice cap may not play as large a role in Atlantic Ocean circulation as previously thought.
According to their latest calculations, Feng He, of the Center for Climatic Research at the University of Wisconsin-Madison and the lead author of the study, and Peter Clark, a paleoclimatologist with Oregon State University, have concluded that most simulations of our future climate may be too sensitive to Arctic ice melt as a cause of abrupt changes in ocean circulation.
Currently, climate scientists generally assume that the Atlantic Meridional Overturning Circulation (AMOC) is one of the major tipping points in climate change. This current in the Atlantic works like a conveyor belt, carrying warm surface water from the tropics — known as the Gulf Stream — northward and cooler, heavier deep water southward.
“We’ve been taught to picture it like a conveyor belt — even in middle school and high school now, it’s taught this way — that shuts down when freshwater comes in from ice melt,” Dr He said.
According to the current understanding of this process, the circulation could weaken if large amounts of freshwater are introduced into deep water formation zones, such as south of Greenland. There, the surface water is cooled down so that its density increases and it thus becomes heavier and sinks into the deep. However, more freshwater reduces the density of the surface water, which would reduce the sinking and thus affect the whole circulation.
In their study, published in the journal Nature Climate Change, Drs He and Clark revise their understanding of the relationship between AMOC and freshwater from melting Arctic ice based on previous work.
The Atlantic circulation has already stalled in the past about 14,700 years ago at the end of the last ice age, when a period of abrupt and strong global temperature rise known as the Bølling-Allerød warming began. Dr He was able to successfully reproduce this event in 2009 during his studies. “That was a success, reproducing the abrupt warming about 14,700 years ago that is seen in the paleoclimate record,” he says now. “But our accuracy didn’t continue past that abrupt change period.”
Global temperatures initially dropped after this abrupt warming before rising again and reaching new maxima in the last 10,000 years. According to Dr He, his 2009 climate model couldn’t keep up. The simulated warming in the northern regions of the Earth did not match the temperature rise detected in geologic climate archives, such as ice cores.
In their new study, Drs He and Clark describe a new model simulation that corresponds to the warmth of the last 10,000 years. In doing so, they omitted the supposed trigger that slows or halts AMOC —freshwater input.
“The problem is with the geological climate data,” Dr He says. They show large amounts of freshwater after the final melting of the ice sheets in North America and Europe, but the AMOC hardly changed, so Dr He deleted the assumption of a freshwater flood 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.”
This is critical for climate models that assess how the AMOC will respond to future increases in freshwater due to ice melt. “It’s built into many models,” Dr He explains. “Future global warming from increasing carbon dioxide in the atmosphere melts sea ice, and the freshwater from the melting ice is believed to cause the AMOC to weaken.”
The far-reaching consequences of a drastic weakening of the AMOC include a rapid rise in sea level along the eastern coast of North America, cooling over Europe that could affect agriculture, a parched Amazon rainforest and disruption of the Asian monsoon. The new modeling study assumes a much smaller decrease in AMOC strength, but does not rule out an abrupt change.
“We suggest until this challenge is solved, any simulated AMOC changes from freshwater forcing should be viewed with caution,” Dr He said. “We can’t be certain why the AMOC shut down in the past. but we are certain it did change. And it can change again.”
Julia Hager, PolarJournal
Link to the study: Feng He, Peter U. Clark. Freshwater forcing of the Atlantic Meridional Overturning Circulation revisited. Nature Climate Change, 2022; DOI: 10.1038/s41558-022-01328-2.
