A study released on Monday draws attention to another ice-related problem that could be particularly devastating to Western Europe, by disrupting a major ocean circulation pattern and possibly affecting climate patterns.
Climate change has already affected large portions of the Earth, and as a warming climate continues to melt the Arctic sea ice, this means there isn’t as much cold, dense water, generated through a process known as oceanic convection, created to flow south and feed the Gulf Stream.
“A warm western Europe requires a cold North Atlantic Ocean, and the warming that the North Atlantic is now experiencing has the potential to result in a cooling over western Europe,” says professor G.W.K. Moore of UTM’s Department of Chemical & Physical Sciences.
Sea ice in the Greenland and Iceland Seas is a major player in the workings and function of a very powerful ocean current called the Atlantic Meridional Overturning Circulation (AMOC). This current is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic Ocean.
The AMOC current basically brings warmer water from the equator toward the poles, in turn delivering colder water back to the equator, and the cycle starts all over again. The AMOC is actually the Atlantic branch of only one part of a much larger, global overturning circulation, moving water all over the world.
First study to document sea-air heat exchange in Arctic region
While previous studies have looked at changing salinity in the northern seas and its effect on ocean circulation, the study conducted by the University of Toronto was the first attempt to examine and document the changes brought on by global warming in sea-air heat exchange in the Arctic region.
Using winter-based data from 1958 to 2014 provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) and model simulations, the research team can now estimate that heat loss has been reduced by 20 percent since 1970. Professor G.W.K. Moore of UTM’s Department of Chemical & Physical Sciences, and lead author of the study explained what this means.
Heat transfer is greatest when there is a large difference in temperature between water and the surrounding air, such as when the water is warmer and the air is colder. Moore says surprisingly, the air nearest the ice is very cold. “So the heat loss from the ocean is largest close to the ice edge,” he says.
Moore points out that several decades ago, there was a lot of sea ice very close to the gyres in the Greenland Sea and the Iceland Sea. Maximum heat transfer was excellent then, and very efficient. But over the past 30 to 40 years, as the region has gotten warmer, the sea ice has melted and retreated north.
This means there is a greater distance between the sea ice and the two gyres. “What that means is that heat loss over these gyres is now reduced,” Moore says. And based on the data from ECMWF, this is why the researchers estimated the 20 percent loss of heat flux since 1979.
Stefan Rahmstorf, a professor of physics of the oceans at the University of Potsdam in Germany, says the study gives us solid evidence. He adds, “I think [the authors] have done a good job based on data about the heat flux to first of all show there is a trend in this heat flux and secondly, look at the oceanic consequences, especially for convection, an integral part of deep water renewal and large-scale overturning.”
A weakening circulation and the consequences
The prospect of reductions in heat transfer disrupting overturning in the Greenland and Iceland seas leads to the worry about trouble with the Atlantic overturning circulation, which previous studies have shown is already weakening.
What the researchers figured out was that the weakening circulation caused by the reduction in Arctic sea ice would mean that only shallower waters would be warmed and experience convection and overturn, leaving deeper, denser waters left out. This would weaken the supply of colder, dense water that is usually fed into the AMOC, weakening the circulation pattern as a whole.
“There has been this concern since the 1980s that the ocean overturning circulation would respond to global warming by a weakening, and I think now evidence is accumulating that this process has indeed started,” says Rahmstorf.
As for consequences, Moore says If the current slows down and less warm water gets transported north, then less heat will be transferred in regions such as Western Europe. It is not known how much colder it would get, and many scientists think that as the Earth continues to warm, any effects from the weakening Atlantic circulation would be offset by the rising temperatures.
“There’s no fix — you can’t go out and spray ice or something in the North Atlantic,” Moore says. “The only long-term solution is to essentially restrict our future emissions of carbon so that we mitigate the additional warming that’s going to happen in the system.”
The study was published on June 29, 2015 in the journal Nature Climate Change under the title, Decreasing intensity of open-ocean convection in the Greenland and Iceland seas
