I mentioned in an entry a couple of days ago about global warming and changes in ocean currents (in particular the gulf stream) leading to cooling rather than warming in Europe, so while I'm taking a break from study I thought I'd post more about it (the article I read this in came from the BBC news website a few days back).
A British research project called Rapid, which aims to gather evidence relating to potentially fast climatic change in Europe, found that changes to ocean currents derived from the Gulf Stream in the Atlantic are weakening and bringing less heat north, and as a result may cool European weather within a few decades. Their conclusions, reported in the scientific journal Nature, are based on 50 years of Atlantic observations.
The key is the Gulf Stream. After it emerges from the Caribbean, it splits in two, with one part heading north-east to Europe and the other circulating back through the tropical Atlantic. As the north-eastern branch flows, it gives off heat to the atmosphere, which in turn warms the European landmass.
By the time it reaches the northern latitudes around Greenland and Iceland, the water has cooled so much that it sinks towards the ocean floor, a process known as overturning. This cooler water heads south, forming the return stream of a conveyor belt. The complete cycle sees warm water coming northwards on the ocean's surface, and the cold water returning hundreds or thousands of metres underwater, as pictured.
Florida-based scientists monitoring the northwards-flowing Gulf Stream have found it has remained roughly constant over the last 50 years, however NOC researchers (National Oceanography Centre at Britain's Southampton University) concentrated on the colder water flowing south, and found that over the last half century these currents have changed markedly.
"We saw a 30% decline in the southwards flow of deep cold water," said Harry Bryden (NOC), "And so the summary is that in 2004, we have a larger circulating current [in the tropical Atlantic] and less overturning." [and less heat then delivered to European shores]
The concept that the North Atlantic conveyor may well reduce in intensity or even turn off altogether was the basis behind the movie 'The Day After Tomorrow'. What happens is that as Arctic ice melts and Arctic rivers flow faster - trends which have both been documented - the northern oceans become less saline. Less salinity means a lower density - the waters then cannot sink, so the conveyor weakens. Computer models have predicted that if it turned off completely, Europe would cool by perhaps four to six degrees celsius. The NOC experiments provide the first observational evidence that such a decrease of the oceanic overturning circulation is well underway already. There is also the additional issue that the extra heat left circulating around the tropical Atlantic could have major impacts on weather systems in Africa, the Caribbean and Central America.
The NOC researchers admit that the case is not yet proven. The analysis involves only five sets of measurements, made in 1957, 1981, 1992 and 1998 from ships, and in 2004 from a line of research buoys tethered to the ocean floor. Even if the trend is confirmed by further data, it could be down to natural variability rather than human-induced global temperature change. A decade ago Professor Schlesinger from the University of Illinois at Urbana-Champaign, a leading expert in models of climate and ocean circulation, showed that the North Atlantic conveyor undergoes a natural 70-year cycle of strengthening and weakening, however the measurements from this latest research are out of phase with this cycle, indicating that the NOC team has probably come up with a link to human-induced climate change - the slowing down of the southward return more or less constitutes a 'smoking gun'.
