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Shutdown of thermohaline circulation

 

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Shutdown of thermohaline circulation


 
 

Shutdown or slowdown of the thermohaline circulationThermohaline circulation

The thermohaline circulation is a term for the global density-driven circulation of the oceans....
is a postulated effect of global warmingEffects of global warming

The predicted effects of global warming for the environment and for human life are numerous and varied....
.

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 IcelandIceland

Iceland, officially the Republic of Iceland is a volcanic island nation in the northern Atlantic Ocean between Greenl...
, IrelandIreland

Ireland is the third largest island in Europe....
, the Nordic countriesNordic countries

The Nordic countries, sometimes also the Nordic region or in English usage Scandinavia, comprise a region in Northern...
, and BritainUnited Kingdom Summary

The United Kingdom of Great Britain and Northern Ireland is a country and sovereign state that lies off the northwest coast...
 that are warmed by the North Atlantic driftNorth Atlantic Drift Overview

North Atlantic Drift is:* A synonym for a component of the ocean circulation: North Atlantic Current...
. The chances of this occurring are unclear; there is some evidence for the stability of the Gulf StreamGulf Stream

The Gulf Stream, together with its northern extension, the North Atlantic Drift, is a powerful, warm, and swift Atlant...
 but a possible weakening of the North Atlantic drift; and there is evidence of warming in northern EuropeEurope

Europe is one of the seven traditional continents of the Earth....
 and nearby seas, rather than the reverse.

Measurements in 2004 and 2005

In April 2004, the hypothesisHypothesis

A hypothesi is a suggested explanation of a phenomenon or reasoned proposal suggesting a possible correlation between multi...
 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 WadhamsPeter Wadhams Summary

Peter Wadhams is professor of Ocean Physics, and Head of the Polar Ocean Physics Group...
 reported to The TimesThe Times

The Times is a national newspaper published daily in the United Kingdom since 1785, and under its current name since 178...
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 2005

The NewScientist.com news service reported on 30 November 2005 that the National Oceanography CentreNational Oceanography Centre, Southampton

The National Oceanography Centre, Southampton is a purpose-built, joint venture between the University of Southampton and th...
 in the UKUnited Kingdom

The United Kingdom of Great Britain and Northern Ireland is a country and sovereign state that lies off the northwest coast...
 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 palaeoclimatePaleoclimatology

Paleoclimatology is the study of climate change taken on the scale of the entire history of the Earth....
 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 sciencesAtmospheric sciences

Atmospheric sciences is an umbrella term for the study of the atmosphere, its processes, the effects other systems have ...
 at the University of Illinois at Urbana-ChampaignFacts About University of Illinois at Urbana-Champaign

The University of Illinois at Urbana-Champaign, also known as UIUC and the U of I, is the flagship campus in the...
, 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 magnitudeOrder of magnitude

    An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to...
     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 DryasYounger Dryas

    The Younger Dryas stadial, named after the alpine / tundra wildflower Dryas octopetala, and also referred to as the Bi...
    ) 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 planktonPlankton

    Plankton are drifting organisms that inhabit the water column of oceans, seas, and bodies of fresh water. ...
     stocks, warming or rainfall changes in the tropics or AlaskaAlaska Overview

    Alaska is a U.S. state, located on the northwest tier of North America....
     and AntarcticaAntarctica Overview

    Antarctica is the southernmost continent and encompasses the South Pole....
     (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 eventsExtinction event

    An extinction event occurs when a large number of species die out in a relatively short period of time....
    ).


Further measurements support the interpretation of natural variation.

See also

  • Global warmingGlobal warming

    Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades....
  • Effects of global warmingEffects of global warming

    The predicted effects of global warming for the environment and for human life are numerous and varied....
  • Loop CurrentLoop Current

    Part of the Gulf Stream, the Loop Current is a warm ocean current in the Gulf of Mexico that flows northward between Cuba an...
     of the Gulf of Mexico
  • Oceanic anoxic event
  • El Niño-Southern OscillationEl Niño-Southern Oscillation Summary

    El Nio-Southern Oscillation is a global coupled ocean-atmosphere phenomenon....
  • 8.2 kiloyear event8.2 kiloyear event

    The 8.2 kiloyear event is the term that climatologists have adopted for a sudden decrease in global temperatures that occurr...
  • The Day After TomorrowThe Day After Tomorrow Summary

    The Day After Tomorrow is a 2004 apocalyptic science-fiction film that depicts catastrophic effects of global cooling an...


External links

  • Peter Schwartz and Doug Randall, Global Business Network -
  • W. S. Broecker from Lamont-Doherty Earth ObservatoryLamont-Doherty Earth Observatory

    Lamont-Doherty Earth Observatory is a world-class research institution specializing in earth sciences and is part of Columbi...
     - [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.]