Shutdown of thermohaline circulation

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.

Thermohaline circulation and fresh water

Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream, a wind-driven gyre, which transports warm water from the Caribbean northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe. Other factors are also important, such as atmospheric waves that bring subtropical air further north, which have been suggested to influence the Climate of the British Isles more than the Gulf Stream. [Seager, R., [http://www.americanscientist.org/template/AssetDetail/assetid/51963/page/1 The Source of Europe's Mild Climate.] American Scientist, July/August 2006] [Rhines, P.B. and Häkkinen, S. [http://www.realclimate.org/Rhines_hakkinen_2003.pdf Is the Oceanic Heat Transport in the North Atlantic Irrelevant to the Climate in Europe?] OSAF Newsletter, September 2003] The evaporation of ocean water in the North Atlantic increases the salinity of the water as well as cooling it, both actions increasing the density of water at the surface. The formation of sea ice further increases the salinity. This dense water then sinks and the circulation stream continues in a southerly direction. Global warming could lead to an increase in freshwater in the northern oceans, by melting glaciers in Greenland and by increasing precipitation, especially through Siberian rivers. [Turrell, B. [http://www.bbc.co.uk/science/horizon/2003/bigchilltrans.shtml The Big Chill] Transcript of discussion on BBC 2, 13 November 2003] . It is by no means clear that sufficient freshwater could be provided to interrupt thermohaline circulation — climate models indicate not, but research continues.

Some even fear that global warming may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events — rapid climate fluctuations — may be attributed to freshwater forcing at high latitude interrupting the THC. The Younger Dryas event may have been of this sort, too. (See the discussion of chaos theory for related ideas.) However, these events are believed to have been triggered by massive freshwater discharges from the Laurentide ice sheet, rather than from the melting of polar sea-ice and precipitation changes associated with the increased open water in global warming. Also, in coupled Atmosphere-Ocean General Circulation Models the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even locally: the IPCC Third Annual Report notes that "even in models where the THC weakens, there is still a warming over Europe". [ [http://www.grida.no/climate/ipcc_tar/wg1/357.htm Thermohaline circulation changes] . From IPCC Working Group 1, chapter 9.] Model runs in which the THC is forced to shut down do show cooling — locally up to 8 degrees Celsius [Vellinga, M. and Wood, R.A. [http://www.ocean.washington.edu/people/faculty/luanne/classes/pcc586/papers/vellingawood_thc2002.pdf Global climatic impacts of a collapse of the Atlantic thermohaline circulation] . Climatic Change 54: 251 267, 2002.] — although the largest anomalies occur over the North Atlantic, not over land.

Measurements in 2004 and 2005

In 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. [ [http://www.nasa.gov/centers/goddard/news/topstory/2004/0415gyre.html Satellites record weakening North Atlantic Current] . NASA, 15 April 2004.]

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. [Jonathan Leake, [http://www.timesonline.co.uk/article/0,,2087-1602579,00.html Britain faces big chill as ocean current slows] , The Sunday Times, 8 May 2005.] [ [http://www.realclimate.org/index.php?p=159 Gulf Stream slowdown?] RealClimate.org, 26 May 2005.]

Bryden measurements reported late 2005

The NewScientist.com news service [F. Pearce. [http://www.newscientist.com/article.ns?id=dn8398 Failing ocean current raises fears of mini ice age.] NewScientist, 30 November 2005] 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 [ [http://www.bjerknes.uib.no/pages.asp?id=169&kat=2&lang=2 Unravel extensive changes in the North Atlantic Ocean] . Bjerkness Centre for Climate Research press release.] . 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. [Gavin Schmidt and Michael Mann. [http://www.realclimate.org/index.php?p=225 Decrease in Atlantic circulation?] RealClimate.org, 30 Nov 2005.]

The New Scientist article was based on an article in Nature [Harry L. Bryden "et al.". [http://www.nature.com/nature/journal/v438/n7068/abs/nature04385.html Slowing of the Atlantic meridional overturning circulation at 25° N] . Nature 438, 655-657, 1 December 2005.] 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. [Detlef Quadfasel. [http://www.nature.com/nature/journal/v438/n7068/abs/438565a.html Oceanography:The Atlantic heat conveyor slows] . "Nature" 438, 565-566, 1 December 2005]

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. [James E. Kloeppel. [http://www.news.uiuc.edu/news/05/1206climate.html Global warming could halt ocean circulation, with harmful results] . Press release, University of Illinois at Urbana-Champaign, 6 December 2005.] 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, [Quirin Schiermeier. [http://www.nature.com/nature/journal/v439/n7074/full/439256a.html Nature 439, 256-260.] (subscription required); see also [http://www.realclimate.org/index.php?p=187 Atlantic circulation change summary] . RealClimate.org, 19 Jan 2006.] 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 (oxygen (O₂) 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. [ cite journal | last = Schiermeier | first = Quirin | authorlink = | coauthors = | year = 2007 | month = | title = Ocean circulation noisy, not stalling | journal = Nature | volume = 448 | issue = 7156 | pages = 844–845 | doi = 10.1038/448844b | url = | accessdate = | quote = ]

ee also

* Global warming
* Effects of global warming
* Loop Current of the Gulf of Mexico
* Oceanic anoxic event
* El Niño-Southern Oscillation
* 8.2 kiloyear event
* "The Day After Tomorrow"

References

External links

* Peter Schwartz and Doug Randall, Global Business Network - [http://www.environmentaldefense.org/documents/3566_AbruptClimateChange.pdf "An Abrupt Climate Change Scenario and Its Implications for United States National Security"] (Environmental Defense Fund)
* 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.]
* [http://www.bjerknes.uib.no/pages.asp?id=169&kat=2&lang=2 Increased temperature and salinity in the Nordic Seas]
* [http://www.americanscientist.org/template/AssetDetail/assetid/51963?fulltext=true The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth]
* [http://www.stabilisation2005.com/Schlesingerm_Thermohaline.pdf Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation, Michael E. Schlesinger et al]