Which Statement Describes the Impact of Ocean Currents on Climate

Which Statement Describes the Impact of Ocean Currents on Climate

Ocean Currents and Climate Change

Note: I suggest you read this more upwardly-to-date article on the same topic instead!

Talk presented at the Symposion “Climate Impact Research: Why, How and When?”

Berlin-Brandenburg Academy of Sciences and German Academy Leopoldina, Berlin, 28 October 1997

Sea currents have a profound influence on climate

Sea currents were different in the by

The thermohaline circulation is a strongly non-linear system

The ocean apportionment may change in the future


How will man-made climate change affect the ocean circulation? Is the present organisation of ocean currents stable, and could it be disrupted if we proceed to fill the atmosphere with greenhouse gases? These are questions of great importance not only to the coastal nations of the world. While the ultimate cause of anthropogenic climatic change is in the atmosphere, the oceans are notwithstanding a vital factor. They do non respond passively to atmospheric changes simply are a very active component of the climate arrangement. There is an intense interaction between oceans, atmosphere and ice. Changes in body of water circulation appear to have strongly amplified past climatic swings during the ice ages, and internal oscillations of the ocean apportionment may be the ultimate cause of some climate variations.

Our understanding of the stability and variability of the bounding main circulation has greatly avant-garde during the by decade through progress in modelling and new data on past climatic changes. I will not attempt to give a comprehensive review of all the new findings here, but rather I will emphasise four key points.

Ocean currents take a profound influence on climate

Roofing some 71 per cent of the Globe and arresting about twice as much of the sunday’south radiation as the temper or the country surface, the oceans are a major component of the climate system. With their huge heat chapters, the oceans damp temperature fluctuations, simply they play a more active and dynamic role as well. Body of water currents move vast amounts of heat across the planet – roughly the same amount equally the atmosphere does. But in dissimilarity to the atmosphere, the oceans are confined past state masses, so that their heat transport is more localised and channelled into specific regions.

The present El Niño event in the Pacific Ocean is an impressive sit-in of how a change in regional ocean currents – in this case, the Humboldt current – can affect climatic conditions effectually the world. As I write, severe drought atmospheric condition are occurring in a number of Western Pacific countries. Catastrophic wood and bush fires have plagued several countries of Due south-Eastern asia for months, causing dangerous air pollution levels. Major floods accept devastated parts of East Africa. A like El Niño issue in 1982/83 claimed nigh 2,000 lives and global losses of an estimated US$ xiii billion.

Another region that feels the influence of body of water c?????eurrents particularly strongly is the Northward Atlantic. Information technology is at the receiving end of a circulation system linking the Antarctic with the Arctic, known as ‘thermohaline circulation’ or more than picturesquely as ‘Dandy Body of water Conveyor Belt’ (Fig. 1). The Gulf Stream and its extension towards Scotland play an important part in this organization. The term thermohaline circulation describes the driving forces: the temperature (thermo) and salinity (haline) of bounding main h2o, which determine the water density differences which ultimately bulldoze the menstruation. The term ‘conveyor belt’ describes its function quite well: an upper co-operative loaded with heat moves n, delivers the heat to the atmosphere, then returns south at almost 2-3 km below the sea surface as North Atlantic Deep Water (NADW). The rut transported to the northern North Atlantic in this fashion is enormous: it measures around 1 PW, equivalent to the output of a million power stations. If we compare places in Europe with locations at like latitudes on the Northward American continent, the issue becomes obvious. Bodö in Kingdom of norway has boilerplate temperatures of -2°C in January and 14°C in July; Nome, on the Pacific Coast of Alaska at the aforementioned latitude, has a much colder -15°C in January and only 10°C in July. And satellite images show how the warm current keeps much of the Greenland-Norwegian Body of water free of ice even in winter, despite the residue of the Arctic Bounding main, even much further south, being frozen.

Effigy ane. Europe’s heating organization. This highly simplified cartoon of Atlantic currents shows warmer surface currents (ruby-red) and cold northward Atlantic Deep Water (NADW, bluish). The thermohaline circulation heats the North Atlantic and Northern Europe. Information technology extends right up to the Greenland and Norwegian Seas, pushing ba?????eck the wintertime sea ice margin. Reproduced from Rahmstorf 1997.

We now have computer models that give fairly realistic simulations of the ocean apportionment, and these models tin be used to examine the effects of the currents on climate. For the Atlantic ‘conveyor belt’ this task is made particularly straightforward by a peculiarity of the climate system: at that place are two stable climate states, one with the Atlantic conveyor, 1 without it. Simply by using different initial conditions, all else remaining the aforementioned, the models can come up with either of these 2 different climates. This makes it like shooting fish in a barrel to compare what the world would look similar without the sea circulation that warms Europe. Manabe and Stouffer 1988 were the starting time to analyse what happens when the familiar conveyor apportionment is absent in an bounding main-atmosphere circulation model. They plant that the sea surface temperatures in the northern N Atlantic dropped up to 7°C in this case. Air temperatures dropped even more, up to x°C over the Arctic seas near Scandinavia, even though the root crusade for the atmospheric cooling was the lower bounding main surface temperatures. The reason for this amplification of the cooling was the advance of bounding main ice, which reflects sunlight dorsum into infinite and thus led to farther cooling. The air temperature changes in the model are roughly consistent with the observed difference between Bodö and Nome, confirming that this difference is indeed mainly caused by the warmth brought north by the Atlantic ocean currents in the nowadays climate.

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Sea currents were different in the past

Painstaking detective work involving sediments of the deep body of water has enabled scientists to derive a wealth of information on ocean currents of the distant ?????epast. What is just mud to a lay-person, provides a valuable archive of past climate data to the expert. Like tree rings or the annual layers of snow accumulated on glaciers, the sediments at the sea bottom preserve information on the environmental conditions from the time they were formed. Information technology is fifty-fifty possible to distinguish between conditions at the sea surface and at the bottom, as the prime source of data are the shells of tiny organisms. Under the microscope, the abundance of unlike species can be counted and identified as surface or lesser dwellers. The chemic composition of their shells has been determined by the temperature, salinity and nutrient content of the waters the organisms lived in, which in plough reveals information on the body of water currents of the fourth dimension.

Looking back over the oceanic records of the past 100,000 years or so, it is hitting how variable the currents must take been. Just the final 8,000 years, i.e. most of the Holocene, were a relatively stable menstruation. Before then, throughout the last ice age, sudden jumps and jolts occur in the record roughly every 1,000 years. These are consistent betwixt different sediment cores, and what is more, most of the spikes in the oceanic conditions represent to synchronous climate shifts on land as recorded in the Greenland water ice cap (Bond et al. 1993). Some cold climate episodes started with a temperature drop over Greenland of 5°C happening over a few decades or fifty-fifty less. The most plausible explanation for these sudden climatic changes are rapid shifts or breakdowns in the ocean currents of the N Atlantic. The exact timing and sequence of events and the ultimate causes are still under investigation, but at that place is widespread agreement that the ‘conveyor belt’ apportionment of the Atlantic played an agile and dynamic role in the climatic roller coaster of the past.

Adequately detailed reconstructions of the Atlantic ocean apportionment (Labeyrie et al. 1992; Sarnthein et al. 1994) at the height of terminal Ice Age, the Concluding Glacial Maximum (LGM) effectually 21,000 years before present, show that North Atlantic Deep Water (NADW) so formed south of Iceland (today much of it forms by convection to the north of Iceland). It sank to intermediate depths only, and Antarctic Lesser H2o (AABW), which comes in from the south below the NADW, pushed further north than it does today, filling most of the abyssal North Atlantic. Recently the first coupled ocean-temper simulation of glacial climate was performed at the Potsdam Institute (Ganopolski et al. 1998), accurately reproducing these features of the glacial body of water apportionment (Fig. 2). Through a sensitivity experiment using the nowadays-day ocean heat transports (instead of taking the glacial apportionment changes into account), the authors were able to demonstrate that the change in Atlantic sea currents played a major role in surface climate, amplifying the glacial cooling of the Northern Hemisphere by 50%. In the North Atlantic the southward shift of deep h2o formation sites and the corresponding advance of sea-water ice led to an air temperature drop of xx°C.

Figure ii. Stream functions of meridional sea transport in the Atlantic, for the present climate (left) and for the last glacial maximum (right), from the coupled ocean-atmosphere model simulation of Ganopolski et al. 1998.
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Both model experiments and paleo-data thus demonstrate that the ocean circulation has undergone important alter?????es in the by, and that these have led to major perturbations of the climate, at to the lowest degree in the Northward Atlantic region. It is possible that there are other regions of the world, such as the Southern ocean, in which the dynamic ocean caused major climate variations, but until now they accept not been studied nearly as well every bit the North Atlantic.

The thermohaline circulation is a strongly not-linear arrangement

Understanding the office of the ocean in climate change requires an understanding of the dynamics of ocean circulation changes. Systematic computer simulations have led to of import advances in our cognition of circulation dynamics in recent years. We have found that at that place are two distinct mechanisms that can cause non-linear transitions in the country of the Atlantic sea circulation, a ‘fast’ and a ‘ho-hum’ machinery. The dull mechanism is quite well understood, and tin can exist described past a simple stability diagram (discussed beneath).

The modelling studies have confirmed Stommel’s 1961 thought that at that place are two states which are stable under present climatic conditions, namely with and without deep water germination in the North Atlantic (see department 1). Stommel described the positive salt advection feedback responsible for this strange behaviour: salinity in the high latitudes needs to be high plenty for deep h2o to form, but it is only loftier enough considering the thermohaline circulation continually brings in salty water from the south. The arrangement is therefore self-maintaining.The flow depends on precariously balanced forces: cooling pulls in one direction, while the input of freshwater from pelting, snow, melting ice and rivers pulls in the other. This freshwater threatens to reduce the salinity, and therefore the density, of the surface waters; merely by a conti?????enuous flushing abroad of the freshwater and replenishing with salty water from the south does the conveyor survive. If the flow slows down as well much, there comes a point where it can no longer keep up at all and the conveyor breaks downwards. This ‘spin-down’ takes many decades or fifty-fifty centuries: this is the ‘slow’ transition machinery.

Figure three. Stability diagram showing how the meridional transport in the Atlantic (‘forcefulness of the conveyor’) depends on the amount of freshwater (atmospheric precipitation and river run-off minus evaporation) inbound the Atlantic. Notation the bifurcation bespeak S, across which no North Atlantic Deep Water formation tin can be maintained. For a detailed discussion, see Rahmstorf 1996.

A look at a simple stability diagram shows how it works (Fig. 3). The key characteristic is that there is a definite threshold for how much freshwater input the conveyor tin can cope with. Such thresholds are typical for complex, non-linear systems. The diagram is based on Stommel’due south theory, adapted for the Atlantic conveyor, merely experiments with global circulation models besides show the same behaviour (Rahmstorf 1996). Different models locate the present climate at dissimilar positions on the stability curve – for instance, models with a rather potent conveyor are located further left in the graph, and require a larger increase in precipitation to button the conveyor ‘over the edge’. The stability diagram is a unifying framework that allows u.s. to understand and compare different estimator models and experiments.

The model st?????eudies as well revealed another kind of threshold where the conveyor flow can change or pause down (Rahmstorf 1995). While the vulnerability in Stommel’south theory arises from the large-scale transport of table salt by the conveyor, this second type of threshold depends on the vertical mixing in the convection areas (east.g. the Greenland Sea and the Labrador Sea). If the mixing is interrupted, so the conveyor may break downward completely in a thing of years, or the locations of the convection sites may shift. This process is known equally ‘convective instability’, and is the ‘fast’ transition machinery. We do not withal know where the critical limits of convection are, nor what information technology would take to set off such an event. Current climate models are non powerful enough to resolve such regional processes accurately. Convective instability could be the mechanism responsible for some of the very fast climatic changes seen in the paleo-climate records. Both mechanisms are summarised in table 1.

Advective Spindown

Convective Instability

Time Scale

gradual (~100 y)

rapid (~10 y)


large-scale salt advection

local convection physics

Cause (forcing)

basin-calibration heat and freshwater budget

local forcing in convection region


conveyor winds down

shift of convection locations or complete breakdown of conveyor


conveyor ‘on’ or ‘off’

several equilibria with different convection patterns


modelled quite well by climate models

large incertitude in forcing and response

Tabular array 1: Overview over properties of the two instability mechanisms relevant to the Atlantic ocean circulation.

The ocean circulation may modify in the future

Given the past instability of ocean currents and our understanding of their non-linear behaviour, the future of the Atlantic circulation in the changing climate of the next century is a natural business organisation. Manabe and Stouffer 1993 published scenario simulations with a coupled ocean- temper model in which the carbon dioxide content of the atmosphere was gradually increased to both twice and 4 times the pre-industrial value and and so kept abiding (Fig.4). With a doubling of CO2, the Atlantic conveyor circulation declined strongly but subsequently recovered. If the CO2 content was increased fourfold, h?????eowever, the thermohaline circulation in the model was interrupted completely. Other model scenarios for a greenhouse earth generally show a reduction in thermohaline circulation between 20% and 50% for a carbon dioxide doubling in the atmosphere (Rahmstorf 1997). A systematic sensitivity study with a simpler model revealed that not only the total amount of carbon dioxide, simply too its charge per unit of increment determines the effects on the body of water (Stocker and Schmittner 1997).

Figure 4. Time series of meridional send in the Atlantic for two greenhouse scenarios of Manabe and Stouffer 1993. Superlative panel: carbon dioxide forcing of the runs. For the scenario leading to a quadrupling of carbon dioxide in the temper, the thermohaline body of water circulation winds downwardly nearly completely.

The circulation changes in all these experiments happen on the boring, advective time scale over one or two centuries; rapid changes as seen during the last glacial were not triggered in these scenarios. This is the main reason why the furnishings on regional temperatures are only moderate in these models; the reduced ocean oestrus ship then falls in a time of stiff greenhouse warming and is partly cancelled by this. The furnishings of such circulation changes on marine ecosystems are largely unexplored and will probably be serious. Furthermore, a weakened apportionment reduces the power of the ocean to absorb carbon dioxide, making the climate arrangement fifty-fifty less forgiving of human emissions (Sarmiento and Le Quéré 1996).

The lack of rapid apportionment changes ?????east in the model scenarios does not rule out that they could happen. Due to poor resolution, present climate models cannot capture the fast convective instability very well; this process depends on regional details. The latest study of the Intergovernmental Panel on Climate Change (Houghton et al. 1995) ended: “Future climate changes may too involve ‘surprises’. (…) Examples of such non- linear behaviour include rapid apportionment changes in the North Atlantic.” This is still a valid conclusion today.


Bond, G., West. Broecker, S. Johnsen, J. McManus, L. Labeyrie, J. Jouzel and G. Bonani, 1993: Correlations betwixt climate records from Northward Atlantic sediments and Greenland ice. Nature, 365, 143-147.

Ganopolski, A., S. Rahmstorf, V. Petoukhov and M. Claussen, 1998: Simulation of modernistic and glacial climates with a coupled global model of intermediate complication. Nature, 391, 350-356.

Houghton, J. T., L. Yard. Meira Filho, B. A. Callander, N. Harris, A. Kattenberg and K. Maskell, 1995: Climate Modify 1995. Cambridge University Press, Cambridge, 572 pp.

Labeyrie, 50. D., J. C. Duplessy, J. Duprat, A. Juillet-Leclerc, J. Moyes, Due east. Michel, N. Kallel and Northward. J. Shackleton, 1992: Changes in the vertical structure of the North Atlantic ocean between glacial and modernistic times. Fourth Science Review, 11, 401-413.

Manabe, Due south. and R. J. Stouffer, 1988: Two stable equilibria of a coupled ocean-atmosphere model. J. Clim., 1, 841-866.

Manabe, S. and R. J. Stouffer, 1993: Century-scale effects of increased atmospheric CO2 on the sea-atmosphere organisation. Nature, 364, 215-218.

Rahmstorf, South., 1995: Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle. Nature, 378, 145-149.

Rahmstorf, S., 1996: On the freshwater forcing and transport of the Atlantic thermohaline circulation. Clim. Dyn., 12, 799-811.

Rahmstorf, S., 1997: Run a risk of body of water-change in the Atlantic. Nature, 388, 825- 826.

Sarmiento, J. L. and C. Le Quéré, 1996: Oceanic carbon dioxide uptake in a model of century-scale global warming. Science, 274, 1346-1350.

Sarnthein, M., K. Winn, Southward. J. A. Jung, J. C. Duplessy, Fifty. Labeyrie, H. Erlenkeuser and 1000. Ganssen, 1994: Changes in east Atlantic deepwater circulation over the last 30,000 years: Eight time slice reconstructions. Paleoceanography, 9, 209-267.

Stocker, T. and A. Schmittner, 1997: Influence of CO2 emission rates on the stability of the thermohaline circulation. Nature, 388, 862-865.

Stommel, H., 1961: Thermohaline convection with two stable regimes of flow. Tellus, thirteen, 224-230.

Which Statement Describes the Impact of Ocean Currents on Climate

Source: http://www.pik-potsdam.de/~stefan/Lectures/ocean_currents.html

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