FEARS that a shutdown of ocean currents is about to plunge Europe into a mini ice age receded last week. New measurements have failed to show clear evidence that the current is weakening, and models of the North Atlantic show that a shutdown would not occur in the way oceanographers had expected.
Currents in the North Atlantic, dominated by the Gulf Stream, carry warm water north from the tropics towards Europe. During the winter, this water warms the westerly winds travelling from America, keeping the climate in western Europe milder than it would otherwise be.
The circulation is driven by density differences in the water arising from variations in temperature and salinity. Global warming reduces temperature differences, because higher latitudes warm more than the tropics, and salinity differences could be affected by increased meltwater from the Greenland ice sheet flowing into the sea. Climatologists had feared that together these factors could shut down the North Atlantic circulation altogether.
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Last year, Harry Bryden at the National Oceanography Centre (NOC), in Southampton, UK, heightened fears that the worst was about to happen when he reported what seemed to be a 30 per cent reduction in these currents (New Scientist, 3 December 2005, p 6). His controversial conclusion drew on very few measurements. 鈥淭he results were based on just five snapshots of the circulation patterns over the past 50 years,鈥� says Meric Srokosz, also at the NOC.
Now an array of ocean sensors set up in the Atlantic in 2004 is allowing more thorough monitoring to take place, and last week at the Rapid Climate Change Conference in Birmingham, UK, Bryden announced the results from the first year鈥檚 measurements. 鈥淲e鈥檝e got a variable signal, but it鈥檚 too early to detect any trends,鈥� he says.
鈥淎brupt climate change initiated by ice sheet melting is not a realistic scenario for the 21st century鈥�
While the measurements are inconclusive on whether ocean circulation is shutting down, climate modellers are emphatic. 鈥淭he answer is simply no,鈥� says Johann Jungclaus at the Max Planck Institute for Meteorology in Hamburg, Germany, who modelled ice sheet effects. 鈥淎brupt climate change initiated by the ice sheet melting is not a realistic scenario for the 21st century.鈥�
Though global warming will slow ocean circulation, beyond a certain point it does not get worse, Jungclaus says. His conclusion is echoed by a team led by Thierry Fichefet at the Catholic University of Louvain (UCL) in Belgium, who found that additional freshwater would have very little extra effect on the circulation.
鈥淚t鈥檚 a surprising result,鈥� says Richard Woods, chief oceanographer at the Hadley Centre for Climate Change in Exeter, UK. 鈥淭he findings depend strongly on the particular models being used.鈥�
Not everyone is convinced. Michael Schlesinger at the University of Illinois at Urbana-Champaign says the models haven鈥檛 allowed for the recent discovery that crevasses in the ice sheet can allow water to flow down to the bedrock, providing extra lubrication. It is believed that this could greatly accelerate the melt-rate.
