If ocean circulation has an Achilles’ heel, it’s the North Atlantic. Here, salty surface water from the south cools, becoming dense enough to mix with the deep water below. Once in the deep, it turns to make a return journey southward. This mixing is a delicate balance—dump enough lower-salinity water at the surface, and it can become too buoyant to sink. That slows, and can even halt, the great conveyor belt of circulation that carries warm surface water around.

This conveyor belt, called the Atlantic Meridional Overturning Circulation (AMOC for those who don’t want to have to say that all the time), is unlikely to shut down any time soon. (It’s certainly not going to happen The Day After Tomorrow…) But while the last IPCC report judged a shutdown to be “very unlikely” this century, climate models consistently predict that the AMOC would slow down in the neighborhood of 12 to 52 percent by 2100, provided “business as usual” greenhouse gas emissions.

So what has the AMOC done in recent decades, given the warming we’ve already experienced? It’s hard to say. We don’t have a continuous record of this circulation, in part because it isn’t the easiest thing to measure. Studies that have tried to assess the recent past using one indicator or another have produced conflicting results, with some suggesting a slowdown and others seeing no signs of change. Real monitoring has only been in place since 2004. It shows a small decrease, but the time period is too short to know if that’s just natural variability.

Rather than waiting for future measurements, a new study led by Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research tries to create a robust reconstruction of the past behavior of the AMOC. (Rahmstorf also wrote about the study for the RealClimate blog.) The work relies on the AMOC’s surface temperature footprint. If you look at warming trends around the globe for the last century, there’s a conspicuous hole in the North Atlantic that hasn’t warmed at all. It’s in just the place where climate models simulate cooling when the AMOC, with its northward transport of warmer water, slows down.

The researchers calculated an “AMOC index” from the difference between the temperature in that region and the Northern Hemisphere average. Applying their index to a climate model simulation of a warming planet, they find that it correlates excellently with the strength of the circulation.

In addition to applying this index over the last hundred years of the instrumental temperature record, they also attempt to go back a thousand years using temperature reconstructions based on physical climate records like tree rings and ice cores. The results indicate that the AMOC was pretty stable for most of the millennium, but show a marked slowdown of roughly 14 percent over the 20th Century, culminating in a low point between the 1970s and mid-1990s. After that, it strengthened a bit, but is still short of its strength in the early 20th Century.

The researchers suggest that melting Greenland ice over the 20th Century could be the cause of this slowdown in circulation, as this adds less-dense freshwater to the North Atlantic.

While climate models show AMOC slowdown in future warming scenarios, they don’t show the 20th Century weakening seen in this study. If this index is a reliable indicator of AMOC strength in both the recent and more distant past, and if that slowdown really was due to something like Greenland meltwater rather than natural variability, this could suggest that climate models are underestimating future changes.

That could mean larger than expected changes in coming decades. A slowing AMOC means faster sea level rise on the eastern coast of the US, due to the change in surface water movement. It would also reduce phytoplankton growth in the North Atlantic and alter the paths of storms in Europe.

Researchers will no doubt look for ways to confirm the findings of this study. There’s no shortage of interest in the special role the AMOC has in Earth’s climate system—past, present, and future.

Nature Climate Change, 2014. DOI: 10.1038/NCLIMATE2554 (About DOIs).