Shutdown of thermohaline circulation

Shutdown or slowdown of the thermohaline circulation
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

is a postulated effect of global warming

Effects of global warming

This article is about the effects of global warming and climate change. The effects, or impacts, of climate change may be physical, ecological, social or economic. Evidence of observed climate change includes the instrumental temperature record, rising sea levels, and decreased snow cover in the...

.

There is some speculation that global warming could, via a shutdown or slowdown of the thermohaline circulation, trigger localised cooling in the North Atlantic and lead to cooling, or lesser warming, in that region. This would particularly affect areas such as Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, Britain

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

and Nordic countries

Nordic countries

The Nordic countries make up a region in Northern Europe and the North Atlantic which consists of Denmark, Finland, Iceland, Norway and Sweden and their associated territories, the Faroe Islands, Greenland and Åland...

that are warmed by the North Atlantic drift

North Atlantic Drift

North Atlantic Drift is:* An ocean current that continues from the North Atlantic Current* An album by Ocean Colour Scene: North Atlantic Drift this doesn't make any goddamn sense....

. The chances of this occurring are unclear; there is some evidence for the stability of the Gulf Stream

Gulf Stream

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic ocean current that originates at the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean...

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. The future is undecided, as studies of the Florida Current

Florida Current

The Florida Current is a thermal ocean current that flows generally from the Gulf of Mexico to the Atlantic Ocean. The current was discovered by Spanish explorer Juan Ponce de León in 1513....

suggest that the Gulf Stream weakens with cooling and strengthens with warming, being weakest (by ~10%) during the Little Ice Age

Little Ice Age

The Little Ice Age was a period of cooling that occurred after the Medieval Warm Period . While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939...

and strongest during 1,000–1,100 years BP

Before Present

Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...

(Before Present), the Medieval Warm Period

Medieval Warm Period

The Medieval Warm Period , Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region, that may also have been related to other climate events around the world during that time, including in China, New Zealand, and other countries lasting from...

Thermohaline circulation and fresh water

Heat is transported from the equator polewards

Geographical pole

A geographical pole is either of the two points—the north pole and the south pole—on the surface of a rotating planet where the axis of rotation meets the surface of the body...

mostly by the atmosphere

Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

but also by ocean current

Ocean current

An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...

s, 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

Gyre

A gyre in oceanography is any large system of rotating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis Effect; planetary vorticity along with horizontal and vertical friction, which determine the circulation patterns from the wind curl...

, which transports warm water from the Caribbean

Caribbean

The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...

northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation

Thermohaline circulation

The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

(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 wave

Atmospheric wave

An atmospheric wave is a periodic disturbance in the fields of atmospheric variables which may either propagate or not . Atmospheric waves range in spatial and temporal scale from large-scale planetary waves to minute sound waves...

s that bring subtropical air further north, which have been suggested to influence the Climate of the British Isles

Climate of the British Isles

The British Isles are an archipelago off the north-west of Europe, consisting of the islands of Great Britain and Ireland along with smaller surrounding ones. Its position allows dry continental air from Eurasia to meet wetter air from the Atlantic Ocean, which causes the weather to be highly...

more than the Gulf Stream. The evaporation

Evaporation

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

of ocean water in the North Atlantic increases the salinity

Salinity

Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

of the water as well as cooling it, both actions increasing the density of water at the surface. The formation of sea ice

Sea ice

Sea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....

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

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

in the northern oceans, by melting glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

s in Greenland

Greenland

Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...

and by increasing precipitation

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

, especially through Siberian rivers. It is by no means clear that sufficient freshwater could be provided to interrupt thermohaline circulation – yet the Younger Dryas

Younger Dryas

The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

are a case where this might have been the cause, however 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 event

Dansgaard-Oeschger event

Dansgaard–Oeschger events are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists claim that the events occur quasi-periodically with a recurrence time being a multiple of 1,470 years, but this is debated...

s – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. The Younger Dryas

Younger Dryas

event may have been of this sort, too. (See the discussion of chaos theory

Chaos theory

Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...

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. Meltwater events aside, the climate deterioration into the last ice age appears to have taken about 5,000 years. Also, in coupled Atmosphere-Ocean General Circulation Models

Global climate model

A General Circulation Model is a mathematical model of the general circulation of a planetary atmosphere or ocean and based on the Navier–Stokes equations on a rotating sphere with thermodynamic terms for various energy sources . These equations are the basis for complex computer programs commonly...

the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even locally: the IPCC

Intergovernmental Panel on Climate Change

The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

Third Assessment Report notes that "even in models where the THC weakens, there is still a warming over Europe". Model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F)— although the largest anomalies occur over the North Atlantic, not over land. However, climate models are not sufficiently sophisticated at present to include climatic factors which give these predictions veracity; e.g., the recent return of deep convection to the subpolar gyre in both the Labrador and Irminger seas and the growing ice mass of Greenland.

Measurements in 2004, 2005, 2008 and 2010

In April 2004, the hypothesis

Hypothesis

A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

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

North Atlantic Gyre

The North Atlantic Gyre, located in the Atlantic Ocean, is one of the five major oceanic gyres. It includes the Gulf Stream and contains the Sargasso Sea. This gyre is similar to the North Pacific Gyre in the way it traps man-made ocean debris in the North Atlantic Garbage Patch, similar to the...

, the northern swirl of the Gulf Stream.

In May 2005, Peter Wadhams

Peter Wadhams

Peter Wadhams is professor of Ocean Physics, and Head of the Polar Ocean Physics Groupin the Department of Applied Mathematics and Theoretical Physics, University of Cambridge...

reported to The Times of London
The Times
The Times is a British daily national newspaper, first published in London in 1785 under the title The Daily Universal Register . The Times and its sister paper The Sunday Times are published by Times Newspapers Limited, a subsidiary since 1981 of News International...

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.

In 2008, Vage et al. reported "the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008," employing "profiling float data from the Argo program to document deep mixing," and "a variety of in situ, satellite and reanalysis data" to set the context for the phenomenon. This might have a lot to do with the observations of variations in cold water chimney behaviour.

In January 2010, the Gulf Stream briefly connected with the West Greenland Current

West Greenland Current

The West Greenland Current is a weak cold water current that flows to the north along the west coast of Greenland. The current results from the movement of water flowing around the southernmost point of Greenland caused by the East Greenland Current....

after fluctuating for a few weeks due to an extreme negative phase of the Arctic oscillation

Arctic oscillation

The Arctic oscillation or Northern Annular Mode/Northern Hemisphere Annular Mode is an index of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered...

, temporarily diverting it west of Greenland.

Bryden measurements reported late 2005

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

National Oceanography Centre, Southampton

The National Oceanography Centre, Southampton describes the integrated collaboration between the Southampton-based part of the Natural Environment Research Council’s National Oceanography Centre, and University of Southampton Ocean and Earth Science...

in the UK

United Kingdom

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

Nature (journal)

Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

. 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

Paleoclimatology

Paleoclimatology is the study of changes in climate taken on the scale of the entire history of Earth. It uses a variety of proxy methods from the Earth and life sciences to obtain data previously preserved within rocks, sediments, ice sheets, tree rings, corals, shells and microfossils; it then...

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 19 January 2006, a News Feature Climate change: A sea change by Quirin Schiermeier appeared in Nature, detailing reactions to the Bryden results. 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
Order 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 the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the exponent being applied to this amount...

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
Younger Dryas
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief period of cold climatic conditions and drought between approximately 12.8 and 11.5 ka BP, or 12,800 and 11,500 years before present...

) are thought by some scientists to have caused cooling, the current overall climate is believed to be different; in particular, sea-ice formation is hypothesized to be less because of overall so-called 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
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

stocks, warming or rainfall changes in the tropics or Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

and Antarctica (including those from intensified El Niño effect), more frequent and intense El Niño events, or an oceanic anoxic event (oxygen (O₂)
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

below surface levels of the stagnant oceans becomes completely depleted - a probable cause of past mass extinction events
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

).

Further measurements support the interpretation of natural variation.

External links

Peter Schwartz and Doug Randall, Global Business Network - "An Abrupt Climate Change Scenario and Its Implications for United States National Security" (Environmental Defense Fund)
W. S. Broecker from Lamont-Doherty Earth Observatory
Lamont-Doherty Earth Observatory
The Lamont–Doherty Earth Observatory is a research unit of Columbia University located on a campus in Palisades, N.Y., north of Manhattan on the Hudson River.- History :...

- [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.]
The notion that the Gulf Stream is responsible for keeping Europe anomalously warm turns out to be a myth
Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation, Michael E. Schlesinger et al.

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Thermohaline circulation and fresh water

Measurements in 2004, 2005, 2008 and 2010

Bryden measurements reported late 2005

See also

External links