Diagram illustrating the major surface and deep water circulation components of the ocean that combine to form the Global Conveyor Belt
Credit: The M Factory © Smithsonian Institution
As we have seen, surface ocean currents are the dominant sources of deep water masses. In fact, it is a little more complicated than this, as other deep water masses also feed one another. However, in a generalized sense, the surface and deep ocean currents can be viewed as an integrated system known as the Global Conveyor Belt, a concept conceived by the brilliant Geoscientist Wally Broecker of Columbia University. Diagrams of the Global Conveyor Belt (GCB) are two-dimensional and therefore simplified and do not, for example, include all of the intermediate water masses or surface water currents. However, the key of the Global Conveyor Belt concept is that it explains the general systems of heat transport as well as bottom water aging and nutrient supply in the oceans.
The following animation traces the path of water through the surface and deep ocean, showing the dominant features of the GCB, including formation of NADW in the North Atlantic.
The Thermohaline Circulation
Adrian LeeThe GCB shows the dominant source of deep water in the oceans as North Atlantic Deep Water, and how this splits in two to flow into the Indian and Pacific Oceans. In these locations, upwelling of the deep water mass produces surface water currents that generally flow back towards the original source of deep water in the North Atlantic. For heat supply, the conveyor belt involves the transport of heat and moisture to northwest Europe by the Gulf Stream; this accounts for about 30% of the heat budget for the Arctic region, making the GCB extremely important for climate in the Arctic.
Because deep-water masses circulate very slowly, the GCB takes about years to complete, meaning that the oldest water in the oceans is about this age. In addition, because oxygen is gradually depleted in deep waters as they age, and because CO2 contents and nutrients conversely increase, the oldest water masses of the ocean in the North Pacific are among the most nutrient-rich, CO2 rich, and oxygen-depleted waters in the ocean. Conversely, the newly produced NADW waters are among the most nutrient-depleted, CO2 depleted, and well-oxygenated waters in the world.
As it turns out, recent research on the detailed configuration of surface and deep currents shows that circulation is much more complex than the GCB. Floats deployed in the ocean don&#;t always follow expected pathways in the GCB model. Wind actually plays a more significant role in causing downwelling than previously thought. Moreover, mixing by small systems or eddies plays a large role in driving surface currents.
The great conveyor belt is the circulation of waters throughout the world's oceans. It is also called the ocean conveyor belt, the great ocean conveyor, and similar phrases, but it is known technically as the thermohaline circulation. This circulation is driven by differences of temperature (which affect water density) and halinity (saltiness), hence the term thermohaline. Warm surface currents, which are less dense, move along the ocean's surface, conveying heat from the tropics to the poles, where it is radiated away to space. Near the poles, cooler and therefore denser water sinks and moves back toward the tropics in currents flowing at the bottom of the sea.
The great conveyor belt is not a single loop, but a complex network of loops whose courses are determined by the positions of the continents, the temperature differences between the poles and tropics, and by underwater land features such as the Greenland-Scotland ridge. Among its many effects on world climate, the conveyor moderates Europe's climate, which would otherwise be colder. Increased melting of Greenland and Antarctic ice, as well as atmospheric warming of the polar regions, may slow the conveyor down. Scientific understanding of the conveyor is still not good enough to say whether such slowing is already happening or whether more severe changes may be expected in the next century.
The existence of surface ocean currents, such as the Gulf Stream, has been observed and exploited by sailors for centuries. In the nineteenth century, scientists debated what elements might cause these currents, such as the wind or differences in temperature. In , Swedish oceanographer Johan Sandström performed a series of classic experiments on ocean-water samples, heating and cooling them and blowing air over them to disentangle the effects of what were then called wind-driven and thermal circulation. By the s, the term &#;thermal&#; had been changed to the word &#;thermohaline,&#; since it was learned that the currents were driven by differences not only in temperature but also in halinity (that is, saltiness or salinity when speaking of ocean water). Cooler water is denser than warmer water of the same halinity; saltier water is denser than less-salty water at the same temperature.
The thermohaline circulation is powered by a complex global machinery of polar cooling, freshwater input from land sources, tidal mixing, and other factors. Cooling near the poles, for example, tends to cause water to become denser and to sink; this deep water then flows along the ocean floor to various upwelling points. In general, water is warmed in the tropics and moved toward the poles by the conveyor belt, where it gives up its heat, becomes denser, and sinks again. Some water can take many centuries to complete its journey through the belt, while other water moves more quickly.
The Atlantic part of the conveyor belt has received much scientific and popular-media attention because its warm surface component, the Gulf Stream, makes the climate in northeastern North America and in Europe warmer than it would be otherwise. Computer studies show that if the Atlantic conveyor were shut down, average temperatures in Europe might decrease by several degrees or more, with possibly extreme cooling over the Scandinavian seas.
In , researchers believed that they had detected a 30% slowdown in the North Atlantic conveyor since the s. However, since , a line of buoys containing scientific instruments has been moored across the Atlantic, roughly in a line from Florida to Africa. The buoys float at different depths and measure temperature, salinity, and pressure, allowing water flow to be calculated. The measurements showed that the conveyor circulation is more variable over short time-scales than was thought, so what appeared to have been long-term changes may have only been short-term changes.
Scientific debate continues over how vulnerable the conveyor is to climate change and whether the conveyor might suddenly stop working, with drastic consequences for Earth's climate. Computer models of climate change predict that quadrupling of the carbon dioxide in Earth's atmosphere over the next 140 years, such as might occur if no effective measures are taken to reduce greenhouse-gas emissions, could slow the conveyor by 10-50%.
Other researchers, such as Michael Schlesinger at the University of Illinois at Urbana-Champaign, assert that these estimates are too conservative, and that if greenhouse-gas emissions continue unabated, accelerated melting of Greenland's ice cap and other factors will create a 70% chance of shutting down the North Atlantic conveyor during the next 200 years and a 45% chance of shutting it down during the twenty-first century. Even with strict control of greenhouse-gas emissions, Schlesinger and colleagues state a 25% chance of conveyor shutdown. As of , however, most climate scientists did not set the risks so high.
Scientists agree that the thermohaline circulation is nonlinear&#;that is, can turn on and off suddenly, rather than ramping smoothly up and down. However, they disagree about whether it is vulnerable to actual shutdown by climate change or merely to being slowed. There is also agreement that climate change has been adding more freshwater to Arctic seas, which may have the effect of slowing the thermohaline circulation. In , it was announced by other scientists that the rate of melting of Greenland's ice had more than doubled from to to to .
The conveyor belt is being more closely observed than ever before, and computer models of climate are improving. Uncertainties about the present and future behavior of the great conveyor belt will likely decrease over the next few years, as this is a rapidly changing area of climate knowledge.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
GREENLAND-SCOTLAND RIDGE: Underwater ridge connecting Greenland to Scotland in the North Atlantic that separates the Nordic seas, where North Atlantic deep water formation occurs. Below a depth of ft (840 m), the ridge forms a continuous barrier between the two basins; in some areas it rises to shallower depths or islands. Formation of the North Atlantic deep water is an essential part of the great conveyor belt or thermohaline circulation of the oceans, a key component of the global climate mechanism.
HALINITY: Salt content of seawater: synonym for salinity or saltiness. Not all seawater is equally salty (haline). Halinity can be increased by evaporation and decreased by the addition of freshwater from rivers or melting glaciers. More haline water is denser and tends to sink.
NONLINEAR: Something that cannot be represented by a straight line: jagged, erratic.
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THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.
TIDAL MIXING: Mixing of different layers of ocean water due to the rising and falling of the tides. Tidal mixing occurs along shorelines but also in some parts of the deep ocean, where it is crucial to the mixing of deep waters with warmer surface waters in the tropics; this allows water to flow to the surface as part of the global thermohaline circulation of the ocean, one of the defining systems of Earth's climate mechanism.
UPWELLING: The vertical motion of water in the ocean by which subsurface water of lower temperature and greater density moves toward the surface of the ocean. Upwelling occurs most commonly among the western coastlines of continents, but may occur anywhere in the ocean. Upwelling results when winds blowing nearly parallel to a continental coastline transport the light surface water away from the coast. Sub-surface water of greater density and lower temperature replaces the surface water and exerts a considerable influence on the weather of coastal regions. Carbon dioxide is transferred to the atmosphere in regions of upwelling.
This article gives a glimpse of the scientific debate about whether global warming might have drastic effects on the conveyor belt circulation of the Atlantic Ocean. Although some scientists assert that there is scant chance of a shutdown of the circulation by global warming, the scientist interviewed in this online NASA journal thinks that global climate change could disrupt the ocean conveyer belts and drastically alter some climates.
Absent any climate policy, scientists have found a 70 percent chance of shutting down the thermohaline circulation in the North Atlantic Ocean over the next 200 years, with a 45 percent probability of this occurring in this century. The likelihood decreases with mitigation, but even the most rigorous immediate climate policy would still leave a 25 percent chance of a thermohaline collapse.
&#;This is a dangerous, human-induced climate change,&#; said Michael Schlesinger, a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign. &#;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.&#;
Schlesinger will present a talk &#;Assessing the Risk of a Collapse of the Atlantic Thermohaline Circulation&#; on Dec. 8 at the United Nations Climate Control Conference in Montreal. He will discuss recent work he and his colleagues performed on simulating and understanding the thermohaline circulation in the North Atlantic Ocean.
The thermohaline circulation is driven by differences in seawater density, caused by temperature and salinity. Like a great conveyor belt, the circulation pattern moves warm surface water from the southern hemisphere toward the North Pole. The water cools between Greenland and Norway, sinks into the deep ocean, and begins flowing back to the south.
&#;This movement carries a tremendous amount of heat northward, and plays a vital role in maintaining the current climate,&#; Schlesinger said. &#;If the thermohaline circulation shuts down, the southern hemisphere would become warmer and the northern hemisphere would become colder. The heavily populated regions of eastern North America and western Europe would experience a significant shift in climate.&#;
Higher temperatures caused by global warming could add fresh water to the northern North Atlantic by increasing the precipitation and by melting nearby sea ice, mountain glaciers and the Greenland ice sheet. This influx of fresh water could reduce the surface salinity and density, leading to a shutdown of the thermohaline circulation.
&#;We already have evidence dating back to that shows a drop in salinity around the North Atlantic,&#; Schlesinger said. &#;The change is small, compared to what our model needs to shut down the thermohaline, but we could be standing at the brink of an abrupt and irreversible climate change.&#;
To analyze the problem, Schlesinger and his colleagues first used an uncoupled ocean general circulation model and a coupled atmosphere-ocean general circulation model to simulate the present-day thermohaline circulation and explore how it would behave in response to the addition of fresh water.
They then used an extended, but simplified, model to represent the wide range of behavior of the thermohaline circulation. By combining the simple model with an economic model, they could estimate the likelihood of a shutdown between now and , both with and without the policy intervention of a carbon tax on fossil fuels. The carbon tax started out at $10 per ton of carbon (about five cents per gallon of gasoline) and gradually increased.
&#;We found that there is a 70 percent likelihood of a thermohaline collapse, absent any climate policy,&#; Schlesinger said. &#;Although this likelihood can be reduced by the policy intervention, it still exceeds 25 percent even with maximal policy intervention.&#;
Because the risk of a thermohaline collapse is unacceptably large, Schlesinger said, &#;measures over and above the policy intervention of a carbon tax&#;such as carbon capture and sequestration&#;should be given serious consideration.&#;
James E. Kloeppel
kloeppel, james e. &#;global warming could halt ocean circulation, with harmful results.&#; nasa earth observatory. december7, . < http://earthobservatory.nasa.gov/newsroom/mediaalerts//.html> (accessed december3, ).
See Also Antarctica: Melting; Arctic Melting: Greenland Ice Cap; Arctic Melting: Polar Ice Cap; Greenland: Global Implications of Accelerated Melting.
Solomon, S., et al, eds. Climate Change : The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, .
Church, John A. &#;Oceans: A Change in Circulation?&#; Science 317 (): 908-909.
Clark, Peter U. &#;The Role of the Thermohaline Circulation in Abrupt Climate Change.&#; Nature 425 (): 863-869.
Quadfasel, Detlief. &#;The Atlantic Heat Conveyor Slows.&#; Nature 438 (): 555-556.
Dickson, Bob, and Steven Dye. &#;Interrogating the &#;Great Ocean Conveyor.&#;&#; Oceanus (Woods Hole Oceanographic Institute), September 6, . < http://www.whoi.edu/oceanus/viewArticle.do?id=> (accessed October 8, ).
Kloeppel, James E. &#;Global Warming Could Halt Ocean Circulation, With Harmful Results.&#; NASA Earth Observatory (U.S. National Aeronautics and Space Administration), December 7, . < http://earthobservatory.nasa.gov/Newsroom/MediaAlerts//.html> (accessed October 8, ).
Larry Gilman
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