Climate scientists have noticed a disturbing pattern in the North Atlantic. This is the relative cooling of surface waters in the area fed by the Gulf Stream. This pattern has emerged over recent decades, and may portend very rapid and potentially disruptive climate change in the upcoming decades. The cooling is not subtle at all, and looks like this:

Map based on NASA GISS data of warming 1901-2013

So what does this mean? A paper out just today describes, explains, and discusses this odd anomaly and its potential consequences. First, a bit of context.

The Earth’s climate follows certain patterns. Most obviously it is warmer at the equator, colder at the poles. Less obvious if you’ve not looked into this is the presence of a very wet band around the middle of the earth, flanked to the north and south by irregular dry bands (that’s where most of the deserts are), with these flanked by the temperate zone, where you have more moisture and highly seasonal temperatures, and so on.

This pattern emerges as a complex response to two major inputs. First the Earth is spinning, and second, the Earth is heated more at the equator than the poles, so heat must move through air and water currents towards the north and south.

One of the major systems that moves heat away from the equator is known sometimes as the Atlantic Conveyor, which is really part of a lager system of sea currents that includes the Gulf Stream. Notice that the Indian Ocean is sequestered mostly in the Southern Hemisphere, bordered along the west by Africa and the north by Asia. Extra warm water in the Indian ocean tends to make its way around the southern tip of Africa, and up the Atlantic, which is a round about route. This water eventually makes its way to the North Atlantic, where it cools, and owing to evaporation, becomes extra salty. This drives the formerly warm surface water into the depth of the ocean, where it flows along the bottom of the Atlantic south, eventually returning (I oversimplify a bit) to the Indian Ocean and elsewhere.

This system is also known as the AMOC (Atlantic Meridional Overturning Circulation) and is part of the global “Thermohaline Circulation” system.

Meanwhile, a smaller but similar aspect of this system starts with the Gulf of Mexico. This water becomes quite warm from the Sun, but is blocked from moving directly north by the presence of North America, with Florida adding to the captive nature of those waters. But the water does make its way around Florida and flows north along the East coast of the US, and eventually also reaches the North Atlantic, and similarly, contributes to the saline deep currents.

Because salinity partly, even largely, drives this system, adding fresh water to the North Atlantic may interfere with this system of currents. How do you get enough fresh water to do this? In the past, huge volumes of fresh water probably entered the North Atlantic every now and then as large outflows of giant inland lakes, formed by melting glaciers, broke through barriers of ice or sediment. There is some evidence that in the past this sort of thing may have partly, or even completely, shut down the Atlantic Conveyor system, which would have had huge impacts on climate.

Today there seems to be two main sources of extra fresh water in the area. One is during years (or decades) when there is a larger than usual number of ice bergs floating into the North Atlantic from the Arctic. The other, potentially, is from melting of Greenland’s fast glaciers, a process that has recently speeded up because of human caused greenhouse gas pollution warming the Earth.

By now you may recognize this scenario as the basis for the Hollywood disaster movie “The Day After Tomorrow.” In that movie the thermohaline circulation system shut down and an ice age instantly gripped the planet. Giant frozen tornadoes came plummeting down from the Stratosphere. One of them hit the helicopter the British Royal Family was escaping in. Everybody in the US ended up in Mexico.

Every one who survived, that is.

The thing is, now, this can’t happen. Well, that particular scenario can’t ever really happen. But yes, the shutting down of this system can theoretically cause the onset of an ice age, or at least a mini-ice age, and has done so in the past. But no, it can’t now because our planet has warmed too much from human greenhouse gas pollution to allow that to happen. That may be the one good thing about global warming.

The new research does suggest, though, that this major pattern of circulation appears to be slowing down. This will have a number of effects. It will likely change the weather in Europe a bit. It will likely cause an increase in sea level along the US East Coast, because the current (and former) system piles up water towards the east and lowers it in the west, within the North Atlantic. That could be worth a few inches.

According to lead author Stefan Rahmstorf, “It is conspicuous that one specific area in the North Atlantic has been cooling in the past hundred years while the rest of the world heats up. Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,” says Rahmstorf. If the slowdown of the Atlantic overturning continues, the impacts might be substantial. Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe.”

The researchers used a combination of sea surface, atmospheric, and proxy (mainly coral) indicators of temperature to indirectly measure changes in ocean currents over time.

According to climate scientist Jason Box, “Now freshwater coming off the melting Greenland ice sheet is likely disturbing the circulation. So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning – and this effect might increase if temperatures are allowed to rise further.” Michael Mann, another author of the paper, adds, “Common climate models are underestimating the change we’re facing, either because the Atlantic overturning is too stable in the models or because they don’t properly account for Greenland ice sheet melt, or both. That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding.”

What happens if the system actually turns off completely? It was formerly thought that the chances of this happening were small, but this research, conforming to a growing body of expert opinion, suggest that the chances of that may be higher than previously thought. Were this to happen the main characteristic of any effects would be rapidity. Whatever happens would happen fast, and rapidly changing climate is generally regarded as bad no matter what the change itself really is.

UPDATE ADDED:

A criticism of this work has emerged, suggesting that another study indicates that there is no a long-term slowdown of the Atlantic Meridional Overturning Circulation (as suggested by the research covered here). That criticism is incorrect. Michael Mann, one of the AMOC study's author has written a clarification on his facebook page. He begins:

Some critics have tried to make hay over a previous article from last year by URI Graduate School of Oceanography scientist Tom Rossby (see: http://www.gso.uri.edu/b…/rossby-gulf-stream-is-not-slowing/) they claim contradicts our recent Nature Climate Change study finding evidence for a long-term slowdown of the Atlantic Meridional Overturning Circulation ("AMOC"). ... Rossby employs direct measurement of Gulf Stream transport using a ship-board acoustic Doppler current profiler (ADCP) over the interval 1993-2012. I have no reason at all to doubt Rossby's findings. And they do *not* conflict with our own findings (though some have misleadingly sought to assert they do) for two fundamental reasons:

Mann's entire post is HERE and you should go read it.

Additional Resources:

The article:

Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015): Evidence for an exceptional 20th-Century slowdown in Atlantic Ocean overturning. Nature Climate Change (online) [DOI: 10.1038/nclimate2554 ]

Stefan Rahmstorf, lead author, has this blog post at RealClimate: What’s going on in the North Atlantic?

Figure caption from the original article, goes with the graphic at the top of the post:

Figure 3. Surface temperature time series for different regions. Data from the proxy reconstructions of Mann et al.12,13, including estimated 2-σ uncertainty bands, and from the HadCRUT4 instrumental data49. The latter are shown in darker colours and from 1922 onwards, as from this time on data from more than half of all subpolar-gyre grid cells exist in every month (except for a few months during World War II). The orange/red curves are averaged over the subpolar gyre, as indicated on Fig. 1. The grey/black curves are averaged over the Northern Hemisphere, offset by 3 K to avoid overlap. The blue curves in the bottom panel show our AMOC index, namely the difference between subpolar gyre and Northern Hemisphere temperature anomalies (that is, orange/red curves minus grey/black curves). Proxy and instrumental data are decadally smoothed.

A neat video of the thermohaline circulation system.

A movie produced by Peter Sinclair, that goes along with THIS blog post.

Coverage by Chris Mooney at the Washington Post: Global warming is now slowing down the circulation of the oceans — with potentially dire consequences

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Other posts of interest:

- [Important new meta-study of sea level rise in the US.](http://scienceblogs.com/gregladen/2014/09/05/important-new-meta-study-o…)

- [Whatever you thought about sea level rise, it’s worse than you were thinking.](http://scienceblogs.com/gregladen/2013/12/05/whatever-you-thought-about…)

- [How high can the sea level rise if all the glacial ice melted?](http://scienceblogs.com/gregladen/2013/06/18/how-high-can-the-sea-level…)

- [Bangladesh and Sea Level Rise](http://scienceblogs.com/gregladen/2013/04/29/bangladesh-and-sea-level-r…)