What is the ‘cold blob’ in the North Atlantic?

You’ve seen those maps of the global impact of climate change — a sea of red signifying that it’s getting hotter almost everywhere. But maps of ocean surface temperatures show a strange anomaly: a big blue blob floating conspicuously south of Greenland.

This area is not a fluke, a temporary cold spell nor just a patch of glacier meltwater. It is a region in the subpolar North Atlantic that has not only been resisting global warming but has actually cooled down by up to one degree Fahrenheit (a little over half a degree Celsius) over the last 120 years.

Driven by a complex series of oceanic and atmospheric interactions, this “global warming hole” or “cold blob” suggests water circulation patterns in the Atlantic Ocean may be changing in ways that could have big impacts on weather and sea levels.

“We should care about it. Not only the cold blob, but also what’s going on in the [entire] North Atlantic Ocean. That’s a region that is very, very sensitive to climate change,” says Yifei Fan, a doctoral student at Pennsylvania State University who has been studying this cooling phenomenon since writing her undergraduate thesis.

The Atlantic is a uniquely important indicator of climate change, thanks in large part to the presence of the Atlantic Meridional Overturning Circulation. AMOC is a winding system of ocean currents which carries warm water from the surface to the north and saltier, colder water from the depths to the south. In particular, the northern part of this system, the Gulf Stream, is a big part of what keeps temperatures mild in parts of the northern hemisphere.

“If you look at a map of the globe, you’ll see that Great Britain is about the same latitude as northern Canada, but it’s a much more mild place to live because of that heat that gets transported by the ocean,” explains Stephen Yeager, a project scientist at the University Corporation for Atmospheric Research.

In recent years, though, there has been concern that AMOC could be starting to run amok.

“What drives AMOC is strong cooling in the high latitude North Atlantic,” Yeager says. “And if you have a warmer atmosphere, there’s less of that cooling, and so there’s less kick to keep AMOC going.” That’s why the consensus among scientists is that the ocean currents making up AMOC will slow as the climate keeps warming, Yeager says.

Whether the slowdown has already started is a matter of long-standing debate, but climate models suggest that a clear indicator would be unusual changes in temperature over both land and sea in areas affected by AMOC.

That’s where the cold blob comes in. Stefan Rahmstorf, an oceanographer at the Potsdam Institute for Climate Impact Research, has long promoted the idea that the blob in the North Atlantic is one of these temperature changes — and a clear sign AMOC is already slowing.

“This is exactly the region where AMOC delivers much of its heat, and exactly the region where climate models have long predicted cooling as a result of the AMOC slowing down,” Rahmstorf writes in a recent article.

But not everyone is convinced. Chengfei He, a postdoctoral research fellow at Woods Hole Oceanographic Institute, notes that sea surface temperature is complex and “could be affected by a lot of other factors, like the atmosphere.”

As greenhouse gasses warm the world, wind strength over the Atlantic can increase, He says. Increases in cold westerly winds blowing over the North Atlantic, then, could be the driving force behind the warming hole. Still, He acknowledges that AMOC could certainly play a role in the North Atlantic warming hole’s cool temperatures — perhaps just deeper in the ocean.

“Maybe the subsurface temperature in the subpolar North Atlantic could be a more robust fingerprint of the AMOC,” He suggests. “Subsurface temperature is more affected by the [ocean] circulation.”

Changes in sea level along the east coast of North America are another possible indicator of an AMOC slowdown, since there is a “known connection” between the two phenomena, according to Yeager. In the Mid-Atlantic and Gulf Coast regions of the United States, for example, sea level rise is already outpacing the global average — due in part to ocean currents changes, according to U.S. Sea Level Change, an inter-agency government website launched last year.

A key reason why there’s still debate over a possible slowing of AMOC is that the changes in the complex swirl of ocean currents are notoriously difficult to measure. Yeager says the first direct measurements are only about 20 years old.

Another complication is that if AMOC is already slowing down, the deceleration would be gradual and hard to detect, explains Simon Josey, a professor of oceanography at the U.K.’s National Oceanography Centre.

To get a better sense of the extent to which changes to ocean surface temperature impact climate, we can look at much quicker changes — for example, the cold anomaly that appeared in the subpolar North Atlantic in 2015.

The 2015 anomaly — which, like the warming hole, is often referred to as a cold blob — was tied to a severe heatwave over Europe later that year, as several scientists argued in a research paper published in 2016.

It is possible that as AMOC slows, ocean currents would meander a bit more than usual and even get stuck in these new paths — with potentially chaotic consequences for weather, Josey explained. “If it gets stuck, it could lead to very cold winds from the north or warm winds from the south,” Josey says. “So, you’d get more of these warm blobs or cold blobs occurring. And if you get more of them, they’re likely to impact land temperatures, heat waves, et cetera.”

Although a recent study suggests AMOC may be more stable than previously thought, its potential slowdown remains — for many experts — a scary prospect in a world already increasingly plagued by heatwaves and strengthening storms driven by climate change.

Fan, Yeager and Josey each emphasized that more research is needed to fully understand both types of cold blobs — short-term cold anomalies like the one that appeared in 2015, as well as the long-term warming hole that is at least a century in the making.

“If the AMOC slowed down or shut down, the whole climate would be changed significantly,” Fan says. “This is why people are so interested in the cold blob. This could be a sign of a weakening AMOC, but still … there’s an open question.”