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Shutdown of thermohaline circulation
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Shutdown or slowdown of the thermohaline circulation is a postulated effect of global warming.
There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localized cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would affect in particular areas like Iceland, Ireland, the Nordic countries, and Britain that are warmed by the North Atlantic drift. The chances of this occurring are unclear; there is some evidence for the stability of the Gulf Stream but a possible weakening of the North Atlantic drift; and there is evidence of warming in northern Europe and nearby seas, rather than the reverse.
Measurements in 2004 and 2005In April 2004, the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.
In May 2005, Peter Wadhams reported to The Times about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.
Bryden measurements reported late 2005The NewScientist.com news service reported on 30 November 2005 that the National Oceanography Centre in the UK found a 30% reduction in the warm currents that carry water north from the Gulf Stream from the last such measurement in 1992. The authors note that currently the observed changes are "uncomfortably close" to the uncertainties in the measurements. However, the North Atlantic is currently warmer than in the earlier measurements . This suggests that either the circulation is not weakening, or that, even if it is weakening, the weakening is not having the hypothesised cooling effect, or that other factors are able to overwhelm any cooling.
The New Scientist article was based on an article in Nature In News and Views in the same issue, Detlef Quadfasel reinforces the point that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results. Quadfasel continues by pointing out the significance of the possible implications, with palaeoclimate records showing drops of air temperature up to 10°C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concludes that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation.
On 6 December 2005 Michael Schlesinger, a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign, leading a research team, said “The shutdown of the thermohaline circulation has been characterized as a high-consequence, low-probability event. Our analysis, including the uncertainties in the problem, indicates it is a high-consequence, high-probability event. This remains a minority opinion based on unpublished research.
On 19 January 2006, a News Feature Climate change: A sea change by Quirin Schiermeier appeared in Nature, detailing reactions to the Bryden results (439, 256-260, Points made by Schiermeier include the following:
- The results are a surprise to scientists in the field.
- Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring, and such increases are unlikely to become critical within the next hundred years; this is hard to reconcile with the Bryden measurements.
- The Bryden results could be caused by natural variation, or "noise", that is, coincidence.
- If the results are correct, perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling.
- While previous shutdowns (e.g. the Younger Dryas) have caused cooling, the current overall climate is different; in particular sea-ice formation is less because of overall global warming.
- However, a thermohaline circulation shutdown could have other major consequences apart from cooling of Europe, such as an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica (including those from intensified El Niño effect), more frequent and intense El Niño events, or an oceanic anoxic event]] below surface levels of the stagnant oceans becomes completely depleted - a probable cause of past mass extinction events).
Further measurements support the interpretation of natural variation.
See also
External links- Peter Schwartz and Doug Randall, Global Business Network -
- W. S. Broecker from Lamont-Doherty Earth Observatory - [ftp://rock.geosociety.org/pub/GSAToday/gt9901.pdf What If the Conveyor Were to Shut Down? Reflections on a Possible Outcome of the Great Global Experiment.]
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