Restricted circulation

Dec 1st 2005
From The Economist print edition

An ocean current in the North Atlantic is getting weaker. That may be bad news for north-west Europe

THOSE who worry about climate change worry about many things: rising temperatures, rising sea levels, changes in rainfall and stronger storms, for example. One of the things they worry about most, though, is changes in the circulation of the ocean's currents. That is because these currents are the main way that heat is redistributed from the tropics, where there is a lot of it, to the polar regions, where there is not. If the currents shifted, it would mean that temperatures in some parts of the world changed much more than they would merely as a result of the local atmosphere warming up as heat-trapping greenhouse gases accumulate. Indeed, it could mean that in some places temperatures fell, rather than rising.

One of the places that both history, in the form of sediment records and ice cores, and computer models suggest is vulnerable to such a fall is north-west Europe. And a paper in this week's Nature, by Harry Bryden and his colleagues at the National Oceanography Centre in Southampton, Britain, suggests that history may be about to repeat itself. That is because Dr Bryden and his team have found evidence that the North Atlantic currents which keep the region warmer than its latitude suggests it deserves have weakened significantly over the past decade.

 

Most readers of this newspaper will be familiar with the Gulf Stream, which carries warm water from the Gulf of Mexico to the edge of the Arctic Ocean. They may be less familiar with the Deep Southerly Return Flow (DSRF), an ocean-bottom current that returns part of this water back to warmer climes. As the map shows, the Gulf Stream splits in two. Part of its water returns past the coast of Africa in a loop known as the subtropical recirculation. The rest takes the DSRF route. The northern arm of the Gulf Stream and the DSRF together constitute what is known as the Atlantic Conveyor Belt.

Since 1957, the International Geophysical Year, the south part of the North Atlantic has been studied from time to time by taking measurements along latitude 25°N from the Canary Islands to Florida. This line marks the point where the maximum amount of heat is being shifted around, and is also a place where the separation of the individual currents is easy to follow (seabed features further north tend to break the currents into separate streams).

Five such transects have been taken, the three most recent being in 1992, 1998 and the one Dr Bryden is reporting, which was done in 2004. Within the usual margins of error, the results in 1992 were the same as those of the earlier two. In 1998, though, the DSRF showed an appreciable drop in flow. And in 2004 it had dropped further. The surface flow of the Gulf Stream past Florida, though, was unchanged. And, to complete the equation, the volume carried by the subtropical recirculation had increased to compensate for the drop in the DSRF. The result, when the numbers were crunched, suggests that the volume of water being carried by the Atlantic Conveyor Belt has dropped by 30%.

If that is correct, and more importantly, if it were sustained, the result for places such as Britain would be a 1°C drop in average temperature—enough to be noticeable. If it were not merely sustained, but got bigger (and the 2004 figure was larger than that for 1998), the temperature drop would be greater. And if the conveyor belt stops altogether, as it has in the past on more than one occasion, Britain's climate would come to resemble that of Newfoundland. The questions, of course, are why is this happening, and can anything be done?

The big chill?

The reason to believe global warming is the cause is that the computer models suggest it. The conveyor belt is powered by the pull of water sinking from the surface to the seabed in three places. It sinks because it is salty and, by the time it has reached northern latitudes, cold. It is therefore dense. But a warming climate will, the models agree, put more fresh water into the sea around these so-called sinking regions. This water will come from increased rainfall feeding rivers that flow into the areas, and also from the melting of the Greenland ice cap. Fresh water is less dense than brine, and the result is a mixture that will remain on the surface and therefore slow the conveyor down.

Dr Bryden's observations do not extend that far north, so whether this is actually the cause he cannot say. But measurements at one of the sinking regions, which were published four years ago, suggest the turnover rate has dropped significantly since 1957, and a three-year-old paper suggests that the surface waters of the North Atlantic are, indeed, getting fresher.

Everything, therefore, points to global warming being the culprit. It is probably no coincidence that the editors of Nature, who are no slouches when it comes to publicity, have published Dr Bryden's paper in the week that a meeting in Montreal is reviewing the implementation of the Kyoto protocol on climate change. But even if Kyoto were implemented in full, it would not save the conveyor belt if it really is slowing down.

The one positive thing about Dr Bryden's study is that it is the first of what should become a series of annual reports on the state of the North Atlantic. It should thus become clear soon whether this result is just a blip, and reasonably soon whether it is a shift that has run its course, or is part of a downward trend. If it is part of a trend, and you live in north-west Europe, it may soon be time to wrap up warm.