The term "thermohaline" is used to describe worldwide ocean circulation because water density (which causes it to rise or fall in the ocean) is determined by both temperature and salinity (saltiness). This flow is part of what marine scientists call the Global Conveyor, a vast submarine flow of water around the world. As part of this conveyor, water flows north from the tropics, in the Gulf Stream, which helps to keep Britain and much of Western Europe warmer than might be expected for such a northerly latitude. The Gulf Stream delivers 27,000 times more heat to British shores than all of the nation's power stations supply (Radford, 2001, 3).
The salinity of the North Atlantic is closely related to world ocean circulation. If the North Atlantic becomes too fresh (due, most likely, to melting Arctic ice), its waters could stop sinking, and the Conveyor
Worldwide ocean circulation ( Jeff Dixon)
could slow, even perhaps stop. Spencer R. Weart, in The Discovery of Global Warming, provided a capsule description of possible changes in the ocean's thermohaline circulation under the influence of sustained, substantial global warming: "If the North Atlantic around Iceland should become less salty—for example, if melting ice sheets diluted the upper ocean layer with fresh water—the surface layer would no longer be dense enough to sink. The entire circulation that drove cold water south along the bottom could lurch to a halt. Without the vast compensating drift of tropical waters northward, a new glacial period could begin" (Weart, 2003, 64).
A report presented at the annual meeting of the American Association for the Advancement of Science during February 2005 by Ruth Curry, a scientist at the Woods Hole Oceanographic Institute, indicated that massive amounts of freshwater from melting Arctic ice were flowing into the Atlantic Ocean. According to Curry's research, between 1965 and 1995, about 4,800 cubic miles of freshwater (more water than Lake Superior, Lake Erie, Lake Ontario, and Lake Huron combined) melted from the Arctic region and poured into the northern Atlantic.
Curry projected that if this pattern continued at current rates, the thermohaline circulation might begin to shut down in about two decades. Furthermore, said Curry, Greenland's ice, which had not been melting quickly, is now thawing at more rapid rates. "We are taking the first steps," Curry said in a news conference. "The system is moving in that direction" (Borenstein, 2005, A-6). In the longer run, according to calculations by Curry and colleagues, "at the observed rate, it would take about a century to accumulate enough freshwater (e.g., 9,000 cubic thousand cubic meters) to substantially affect the ocean exchanges across the Greenland-Scotland Ridge, and nearly two centuries of continuous dilution to stop them (Curry et al., 2005, 1774).
By 2007, however, other studies were discounting this possibility, as changes earlier believed to be long-term erosion were being regarded, instead, as part of natural variation. Stuart Cunningham of the National Oceanography Centre at Southampton, U.K., and colleagues found that the thermohaline circulation had varied by as much as 25 percent in a year.
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