We already know that melting sea ice in the Arctic is bad news. Less ice means less habitat for animals like polar bears, and it also means there are fewer reflective surfaces in the North to bounce sunlight back into space, allowing the planet to absorb more heat. And as global warming continues to warm up the Earth, we're only going to lose more ice.
A study released this week in Nature Climate Change is drawing attention to yet another ice-related problem — one that could cause some large-scale consequences. According to the study, retreating sea ice could disrupt a major ocean circulation pattern and even affect climate patterns in Europe.
As it turns out, sea ice in the Greenland and Iceland seas is an important player in the workings of a powerful ocean current known as the Atlantic Meridional Overturning Circulation. This current acts as a kind of conveyor belt, carrying warm water from the equator to the poles, and then shuttling cold water back to the tropics where the cycle starts all over again. The Atlantic overturning circulation, in turn, is the Atlantic branch of a much larger global overturning circulation, which shuttles water all over the globe.
When warm water arrives in the North, it becomes involved in a process called ocean convection — the transfer of heat from the water to the air. As heat moves out of the seawater and into the atmosphere, the water starts to cool down. Cold water is denser than warm water, so it sinks toward the bottom of the ocean and joins the "return" leg of the Atlantic overturning circulation conveyor belt, which carries it southward again. This whole process, in which water is carried northward, cools, sinks and returns south, is known as overturning.
Overturning requires some special oceanic conditions in order to work, and it's most pronounced in parts of the ocean known as "gyres," or swirling systems of circular ocean currents. These swirling currents make heat transfer a little easier and help the cold water sink deep enough to join the deep, southward-moving current of the Atlantic overturning circulation. The researchers in this study focused their attention on two particular gyres, one in the Greenland Sea and one in the Iceland Sea. Historically, these two gyres have helped feed a major leg of the Atlantic overturning circulation by turning over large amounts of water.
Here's where the sea ice comes into play. Heat transfer is greatest when there's a large difference in temperature between the water and the surrounding air — that is, when the water is warmer and the air is colder. And unsurprisingly, air near the ice is very cold. "So the heat loss from the ocean is largest close to the ice edge," says Kent Moore, lead author of the paper and professor of physics at the University of Toronto.
Several decades ago, there was still a lot of sea ice very close to these two gyres, Moore says. This allowed for maximum heat transfer in a region where overturning was already occurring, making the process even more efficient. But during the last 30 to 40 years as the region has warmed, the sea ice has started to melt and retreat north, meaning there's a greater distance between the sea ice and gyres than there used to be. This is particularly true in the summer, but Moore points out that the range of winter sea ice is also shrinking in the region.
"What that means is that heat loss over these gyres is now reduced," Moore says. In fact, by examining data from the European Centre for Medium-Range Weather Forecasts, the researchers estimated that heat flux in these regions has been reduced by 20 percent since 1979.
Given that oceanic data is generally sparse, Stefan Rahmstorf, a professor of physics of the oceans at the University of Potsdam in Germany, says the paper puts forth solid evidence. "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," he says.
As far as the consequences outlined by Rahmstorf, the researchers believe this reduction in heat transfer could disrupt overturning in the Greenland and Iceland seas and spell trouble for the Atlantic overturning circulation, which previous studies have shown is already weakening. The researchers used models to try and figure out just what kinds of effects the current might see as a result. They found that less heat flux in the region could mean eventually only shallow waters will experience convection and overturn, and deeper waters will be left out. This could weaken the supply of dense, cold water that the region usually feeds into the Atlantic overturning circulation — and so weaken the circulation as a whole.
Scientists are already worried about the Atlantic overturning circulation, whose circulation appears to have been slowing down for several decades. "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, from the University of Potsdam. And weakening heat flux in the Arctic is probably not even the only cause.
A March 2015 study led by Rahmstorf concluded that the Atlantic overturning circulation has experienced an unprecedented weakening since the 1970s, which the authors believe has been partly caused by melting ice in Greenland pouring fresh water into the region. Fresh water is less dense than salt water, so it doesn't sink as well. This can weaken the overturning process.
According to Rahmstorf, these two factors — an influx of fresh water and a weakening of heat transfer in the region — are likely working on top of each other to slow down the Atlantic overturning circulation. "In our paper we have pointed to the meltwater from Greenland, not claiming that this is the most important mechanism, but rather saying this is an additional factor that has so far been neglected," he says.
A weakened Atlantic overturning circulation has the potential to cause some unexpected consequences. If the current slows down and less warm water gets transported north, then less heat will be transferred in regions such as Western Europe. According to Moore, Europe could actually experience a cooling effect in the future as a result of this — although how pronounced this cooling will be remains unclear. Climate change is expected to continue raising temperatures across the globe, so overturning-related cooling effects in Europe will likely be offset by global warming. It may be that Europe will continue to heat up, but at a slower pace than the rest of the world.
But there are other consequences to consider as well. Waters to the east of the northward-flowing Gulf Stream — waters on the European side, that is — tend to be warmer than waters to the west, on the North America side. If the current weakens, waters on the west side may start to warm up and become less dense, causing them to expand and take up more room. This process could lead to an increase in sea-level rise along the U.S. East Coast.
However, Moore cautions, we can only make so many inferences using historical data and models. The next step will be to start monitoring changes in the Atlantic overturning circulation (and their consequences) in real time, research that he and his colleagues hope to start conducting in the next couple of years.
And in the meantime, both Moore and Rahmstorf stress the need for continued cuts to our global carbon emissions, the dominant cause of climate change. Putting a cap on global warming will mean fewer sea ice losses down the road (and less melting of the planet's glaciers), which can help reduce disruptions to major oceanic currents.
"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."
Chelsea Harvey is a freelance journalist covering science. She specializes in environmental health and policy.