One of the most important currents is the North Atlantic Current, which moves warm surface waters north toward the Arctic. When the current reaches northern latitudes, it gets cooler and denser, and sinks. Then it turns south, pulling deeper cold waters toward the equator. Eventually, in the south, the water warms, rises, and turns back north. The system as a whole essentially functions like a giant, ever-turning conveyor belt.
The North Atlantic Current and the warm water it moves northward are often credited with keeping the weather mild in the north, especially in western Europe. Ruth Curry of the
Woods Hole Oceanographic Institution, for example, noted, "the Atlantic circulation moves heat toward the Arctic, and this helps moderate wintertime temperatures in the high-latitude Northern Hemisphere."18
Some scientists have argued that global warming might have an effect on the North Atlantic Current. As Arctic glaciers melt, freshwater may be dumped into the current. As discussed earlier, both temperature and salt-water content have an impact on currents; a giant dump of cold, freshwater could therefore have an effect on how a current flows. The worry is that the current could "rapidly collapse, turning off the huge heat pump and altering the climate over much of the Northern Hemisphere."19 Some scientists have suggested that the result could be a new ice age.
There does seem to be some possibility that ice-water melt might affect the North Atlantic Current. Science writer John Roach reported that a survey of scientists found that some believed there to be a more than 50 percent chance that the North Atlantic Current might shut down, while others said that there was "no chance" of such a failure.20
Even if the North Atlantic Current is affected, however, it is far from clear that the result would be a new ice age. In fact, according to Richard Seager, a climate researcher at Columbia University, Europe's mild climate is not primarily a result of the North Atlantic Current. Instead, in an article in American Scientist in August 2006, Seager explained that Europe has mild weather mostly because it has a maritime climate caused by winds blowing off the ocean (maritime climates are discussed more fully in Chapter 1). Seager noted that the climate in Europe, in fact, is similar to the climate in Seattle, a city that is also northerly and mild, but which has no warm currents nearby.
Seager points out that the North Atlantic Current does have some moderating effect on climate. If it were slowed or stopped, he says, it "would have a noticeable but not catastrophic effect on climate. . . . Temperatures will not drop to ice-age levels. . . . The
Apocalyptic scenarios based upon global warming have been prominent in popular culture. For example, the idea of an ice age caused by global warming served as the basis for the 2004 film The Day After Tomorrow. In that film, ice dumped into the North Atlantic Current triggered catastrophically low temperatures, tornadoes, hurricanes, and a giant hailstorm that leveled Tokyo.
An even more prevalent end-of-the-world scenario involves sea-level rise. Thus, as just one example, in the graphic novel JLA: American Dreams by writer Grant Morrison and illustrators John Dell and Howard Porter, published in 1997, there is a scene set in the near future that shows the superhero Aquaman swimming past an almost entirely submerged Statue of Liberty. Only the statue's upraised arm breaks the surface; the head is completely underwater. The scene is meant to suggest that global warming has caused the seas to rise and cover the earth.
A submerged Statue of Liberty is a memorable symbol of the dangers of rising sea levels, and it has appeared in both fictional stories, such as comic-book tales, and in actual news stories. For instance, a Boca Raton paper called the Sun ran on the cover of its July 25, 1995, issue a picture of water risen to the chest of the Statue of Liberty. Despite their popularity, however, these images have no basis in fact. To immerse the Statue of Liberty to its chest, sea levels would have to rise by about 76m (250 feet), according to University of Victoria climate scientist Andrew Weaver. This is a greater sea level rise "than would occur if all the ice on our planet were to melt," Weaver notes in his book Keeping Our Cool.
North Atlantic will not freeze over, and English Channel ferries will not have to plow their way through sea ice." Instead, Seager argues, a North Atlantic slowdown would probably serve merely to provide slight mitigation for human-caused global warming in Europe.21 In Keeping Our Cool, Andrew Weaver observes that "While much has been made in the media of global warming potentially leading to the next ice age, it's simply not possible." Weaver argues in addition that, although a slowdown of the North Atlantic Current might reduce sea-surface temperatures, it probably will not have a significant mitigating effect on global warming in Europe itself.22
Changes in currents might not cause a new ice age, but they may nonetheless have other important effects. For example, one research study led by University of Colorado scientists Scott Lehman and Thomas Marchitto suggested that the melting of ice in the Atlantic and resulting changes in currents could result in the ocean releasing stored carbon in a giant burp. Such burps occurred in the past when currents were altered; and future burps might dangerously intensify global warming. "This study provides strong indicators of just how intimately coupled the connection between the ocean and atmosphere can be," said Kent State University professor Joseph Ortiz, a coauthor of the study.23
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