Atlantic Circulation Weakening - Is Western Europe in Trouble?

The North Atlantic's circulation, the region's natural heating system, which brings warmer weather to western Europe, is showing signs of decline, according to a recent article in Nature. Researchers report that warm Atlantic Ocean currents, which carry heat from the tropics to high latitudes, have substantially weakened over the past 50 years. According to Nature, oceanographers surveying the 'Atlantic meridional overturning circulation', the current system that includes the warm Gulf Stream current, report that it seems to be 30% weaker than half a century ago.

This is bad news for residents of western Europe who experience a climate far more temperate than others at their latitude due to the warming effects of the circulation. Failures of the Atlantic Ocean's circulation system are thought to have been responsible for abrupt and extreme climate changes in the past. [If any of this sounds familiar, a fictional shutdown of the Gulf Stream inspired the 2004 Hollywood blockbuster The Day after Tomorrow.]

According to the researchers, the likely cause of the weakening circulation is more fresh water flowing into the ocean from rivers, rain and melting ice. This is thought to be linked to global warming and this is thus an example of a potentially dire global climate change feedback scenario. Regardless of the cause, climate modellers are worried that the resulting weakening of ocean currents could ultimately lead to substantial cooling of the North Atlantic, and of course the climate of those countries that boarder it.

The researchers behind the new study are the first to spot these signs of decline in Atlantic currents. Harry Bryden of the National Oceanography Centre in Southampton, UK, and his team report their results in this week's Nature [see BrydenH., LongwortH. & CunninghamS. Nature, 438. 665 - 657 (2005)].

Nature reports that during a cruise in spring 2004 across the Atlantic from the Bahamas to the Canary Islands, on board the British research vessel RRS Discovery, Bryden's team measured water temperature and salinity along a latitude of 25º North, taking samples roughly every 50 kilometres. They then calculated from the density and pressure differences between each sample, the volume and velocity of the circulation at various depths, assuming that from coast to coast the balance of water flowing north and south must be zero.

Similar studies were conducted along the same latitude were in 1957, 1981, 1992 and 1998. Until now, the data never showed any significant decline in the strength of the circulation. "In 1998 we saw only very small changes," says Bryden. "I was about to give up on the problem."

However, the results of this study were very different. The Gulf Stream, which is near the surface and mostly wind driven, has remained almost constant since 1957. However, the deep-ocean return flow of cooler water has decreased dramatically. This cycle usually returns water to more southerly latitudes from as far north as Greenland and Scandinavia. Instead, much of this water now seems to be trapped in a loop in the subtropical Atlantic, instead of cycling all the way to the ocean's northern extremity. Bryden and his colleagues estimate that, overall, the circulation has slowed by about 30% since 1957.

As Detlef Quadfasel, an oceanographer at the University of Hamburg in Germany astutely recognizes, "This is quite sensational information in itself. But it is also an important message to politicians who negotiate the future of the Kyoto agreements: we do change our climate."

This report comes out at nearly the same time as the European Environment: State and Outlook, 2005 which indicates that the EU is not on track to meet its 2010 Kyoto committments for greenhouse gas reductions, and will hopefully serve as an impetus to EU leaders to take seriously their task of cutting back GHG emissions and combatting climate change. The Atlantic circulation is a fairly well understood phenomena and it is clear that western Europe would be impacted severely if global warming were to furthe weaken or even stop its flow.

It must be noted however that a direct impact of the weakening circulation on air temperatures in western Europe has so far not been observed. In fact, average temperatures have increased by around 0.6 ºC since 1900. Whether or not the true warming is partly eclipsed by an opposite oceanic cooling trend is not clear, says Quadfasel.

Other oceanographers also warn that this is not yet proof of a long-term trend. According to Nature, possible disturbances including ocean eddies and natural fluctuations in the strength of the circulation system must be considered.

"Something is clearly going on," says Jochem Marotzke, an oceanographer at the Max Planck Institute for Meteorology in Hamburg. "But we still have only a series of snapshots. The crux is to determine how representative they really are." He adds that the chances of imminent collapse of the circulation system is small.

Nature reports that sensor-equipped moorings installed at 25 locations across the subtropical Atlantic have now begun to monitor continuously the circulation at all depths. The next four years or so should thus provide better data and help us determine whether the Atlantic heating system is still working well, says Marotzke.

[A hat tip to Green Car Congress again]

Resources:

Nature: Atlantic currents show signs of weakening

discussion of the article at RealClimate

discussion of the article at The Oil Drum

Good luck, Europe. This seems like a pretty timely reminder to get their act together. Too bad there isnt a similarly scary reminder for the U.S. who has a long way to go to catch up with the EU in the getting their act together department...