Scientists say they have measured a significant slowing in the Atlantic currents that carry warm water toward Northern Europe. If the trend persists, they say, the weather there could cool considerably in coming decades.
Some climate experts have said the potential cooling of Europe was paradoxically consistent with global warming caused by the accumulation of heat-trapping "greenhouse" emissions. But several experts said it was premature to conclude that the new measurements, to be described today in the journal Nature, meant that such a change was already under way.
The currents, branching off from the Gulf Stream, are part of an oceanic system that disperses tropical heat toward the poles and makes Northern Europe far warmer than its latitude would suggest.
Warming, in theory, could stall the salty, sun-heated, north-flowing currents by causing fresh water to build up in high-latitude seas as ice melts and more precipitation falls.
The scientists, from the National Oceanography Center in Britain, measured sea temperature, currents and other conditions across the Atlantic from the Bahamas to Africa last year and found a 30 percent drop in the flow of warming waters since a similar set of measurements were taken in 1957.
The team, led by Harry L. Bryden, wrote that even though they had measurements from only 5 years out of the past 50, the pattern of change seen at various depths supported the idea that the shift was a significant trend and not random variability.
They also cited independent measurements of a long-term decline in the flow of water between some Arctic seas and the North Atlantic as evidence that a slowing of the overall Atlantic circulation was under way.
In an accompanying commentary in Nature, Detlef Quadfasel of the University of Hamburg, who was not involved with the British study, said it provided "worrying support for computer models that predict just such an effect in a world made warmer by greenhouse-gas emissions."
Other scientists were more cautious. Gavin A. Schmidt, a climate modeler at the National Aeronautics and Space Administration Goddard Institute for Space Studies, said that the estimated decline in ocean circulation should have produced a perceptible decline in surface temperatures, but that no such dip had yet been measured...
They're confusing three different events, which are theoretically linked.
1) The planetary increased retention of solar-derived heat due the greenhouse effect of a high carbon dioxide atmosphere content.
2) The slowdown and possible eventual cessation of the global conveyor current due to desalinization at the melting poles.
3) The subsequent re-cooling of the Northern hemisphere and poles due to lower conveyor current activity.
The Nature paper they're citing is pretty specific. They only measured the current flow. Which looks to be unequivocally slowing in the deeps and accelerating at the surface.
Slowing of the Atlantic meridional overturning circulation at 25° N
Harry L. Bryden, Hannah R. Longworth and Stuart A. Cunningham
The Atlantic meridional overturning circulation carries warm upper waters into far-northern latitudes and returns cold deep waters southward across the Equator1. Its heat transport makes a substantial contribution to the moderate climate of maritime and continental Europe, and any slowdown in the overturning circulation would have profound implications for climate change. A transatlantic section along latitude 25° N has been used as a baseline for estimating the overturning circulation and associated heat transport. Here we analyse a new 25° N transatlantic section and compare it with four previous sections taken over the past five decades. The comparison suggests that the Atlantic meridional overturning circulation has slowed by about 30 per cent between 1957 and 2004. 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.
The editors of Nature summarize this group's results as follows:
Most warm waters in the upper ocean circulate clockwise in a giant horizontal swirl in the subtropics, but some flow farther north and cross the Greenland–Scotland Ridge (GSR). This branch warms the northern North Atlantic and Europe, and keeps most of the Nordic Seas free of ice. Here the water sinks (indicated by the star) and flows back southwards at depth, mostly down the western edge of the Atlantic basin. The Scandinavian monitoring array tracks only the northern limb of the overturning circulation, but more deep water is added south of the GSR and in the Labrador Sea (stars). The 25° N section covers all of the overturning circulation, and also includes the horizontal recirculation in the subtropics. According to Bryden and colleagues' results, the former is weakening and the latter strengthening.
So why hasn't the reflexive cooling of the Northern hemisphere started if this has been going on since 1998? In fact, the deglaciation seems to be accelerating, if anything.
Notice, as the Editor at Nature points out, by the Bryden results, the surface recirculation from the North is increasing. In fact, this current is what sends hurricanes born off the west coast of Africa towards us. The Gulf Stream is the return current at the surface, which is what keeps Europe warm. In the past, a lot of the water has returned southward by sinking as it cooled in the North. Now, it simply seems to be recirculating at the surface.
Increasing storms, possibly.
The other parameter playing with the models is the carbon dioxide content of the atmosphere which won't let it radiate heat as fast as it usually does. The Northern hemisphere may not be cooling on average because it can't.
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