Should this prove to be a sustained decline (there is a degree of uncertainty estimated in the paper), the findings would be extremely significant, and could mark an intensification of European winters in a relatively short period of time.

The Atlantic meridional overturning circulation—the current system that includes the warm Gulf Stream current—is a major contributor to the relatively mild weather experienced by Northern and Western Europe, even at its comparatively high northerly latitude. Both salinity and water density are key to the functioning of the transport.

The weakening of the system is likely caused by the additional fresh water flowing into the northern ocean from rivers, rain and melting ice, and this is thought to be linked to global warming. Despite no indication of climate cooling—on the contrary, average temperatures in Western Europe have increased—climate modellers are worried that the resulting weakening of ocean currents could ultimately lead to substantial cooling of the North Atlantic. Cooling resulting from warming, in other words.

A direct impact of the weakening circulation on air temperatures in western Europe has so far not been observed. Average temperatures have increased by around . Whether or not the true warming is partly eclipsed by an opposite oceanic cooling trend is not clear.

—Detlef Quadfasel, University of Hamburg

During a cruise in spring 2004 from the Bahamas to the Canary Islands, on board the British research vessel RRS Discovery, the research team measured water temperature and salinity along a latitude of 25º North, taking samples roughly every 50 kilometers.

Station positions for transatlantic hydrographic sections taken in 1957, 1981, 1992, 1998 and 2004. Note Cuba and Florida to the left, Africa to the right. The 1957 and 1992 sections each went zonally along 24.5° N from the African coast to the Bahama Islands. Because of diplomatic clearance issues, the 1981, 1998 and 2004 sections angled southwestward from the African coast at about 28° N to join the 24.5° N section at about 23° W. The 1998 and 2004 sections angled northwestward at about 73° W to finish the section along 26.5° N.

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 measurements along the same latitude were previously made in 1957, 1981, 1992 and 1998. Until now, the data never showed any significant decline in circulation.

Now, however, although the near-surface, and mostly wind-driven, Gulf Stream has remained almost constant since 1957, 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.

The warmer 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.

Whereas the northward transport in the Gulf Stream across 25° N has remained nearly constant, the slowing is evident both in a 50 per cent larger southward-moving mid-ocean recirculation of thermocline waters, and also in a 50 per cent decrease in the southward transport of lower North Atlantic Deep Water between 3,000 and 5,000 m in depth. In 2004, more of the northward Gulf Stream flow was recirculating back southward in the thermocline within the subtropical gyre, and less was returning southward at depth.

—Bryden et. al.

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.

—Detlef Quadfasel

Resources:

• “Slowing of the Atlantic meridional overturning circulation at 25° N”; Harry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham; Nature 438, 655-657 (1 December 2005) | doi:10.1038/nature04385

• Ocean circulation and climate during the past 120,000 years

• Discussion of the Nature article at Real Climate

• Discussion of the Nature article at The Oil Drum