Whether or not he has seen "The Day After Tomorrow," any self-respecting environmentalist is familiar with the doomsday scenario posited in Roland Emmerich's 2004 Hollywood blockbuster. But, in case you aren't a card-carrying member of club green or a fan of big-budget summer fare, I'll provide a brief summary. Global warming leads to a stoppage of the thermohaline circulation, also known as the ocean conveyor belt. The halted transfer of warm waters from equatorial regions toward the poles results in dramatic climate change, plunging the northern latitudes into another ice age. In the film, this transition happens unbelievably rapidly; just days after the current stops, the Statue of Liberty is buried in snow and many millions of displaced United States citizens flee south to Mexico in search of a hospitable climate. (1)
Most viewers realize that "The Day After Tomorrow" is over-the-top, but this didn't stem the tide of green endorsements surrounding the film's theatrical release. Al Gore, along with other prominent political and popular figures, trumpeted the movie as an important way to heighten people's awareness of the serious threat posed by global warming. Not long after seeing it, I wrote, "It’s not a triumph of cinema, but it does make the viewer ask some tough questions about how we spend our days here." And so it does, but what I didn't know then was how completely off-base the science behind the scenario was. After all, the thermohaline circulation has been big news in the international media for at least five years, and most articles connect the currents with our relatively temperate global climate. It didn't strike me as alarmist, then, when environmentalists voiced concern about the dangers of a slowed or dead circulation. In fact, this threat seemed to be one of the principal worries of global warming.
Not so, according to Richard Seager's fascinating essay in the July/August issue of American Scientist. Apparently the greens - myself included - have been crying wolf...yet again. This time, though, it's not entirely our fault. The hypothesis was based on accepted fact, after all, but Seager exposes this fact as myth. "The Day After Tomorrow" won't happen because it is not the Gulf Stream that keeps Europe 's climate temperate, as the screenplay suggests and so many of us have long believed. Instead, the trade winds are responsible. That is to say, winds moving from west to east blow in air from the Atlantic, warming Europe in the winter and cooling the pseudo-continent in the summer. The Pacific equivalent is observed on the west coast of the United States. In the case of the southeastern U.S., warm air from the Gulf of Mexico and the warm southwest blows over Florida, Georgia and the Carolinas, heating up the summers but keeping winters temperate. As you move northward along the east coast of the U.S., the much cooler, "continental" winds affect climate, keeping summer temperatures relatively cool, but resulting in cold winters.
As Seager explains it:
"The effect of differing heat capacities is augmented by the fact that the Sun's heat is stored within a larger mass in the ocean than on land. The heat reservoir is bigger because, as the Sun's rays are absorbed in the upper several meters of the ocean, the wind mixes that water downward so that, in the end, solar energy heats several tens of meters of water. On land, the absorbed heat of the Sun can only diffuse downward and does not reach deeper than a meter or two during a season. The greater density of soil and rock (which ranges up to three times that of water) cannot make up for this difference in volume of material that the Sun heats and for the difference in heat capacity of water compared with soil or rock.
Because sea-surface temperatures vary less through the seasonal cycle than do land-surface temperatures, any place where the wind blows from off the ocean will have relatively mild winters and cool summers. Both the British Isles and the Pacific Northwest enjoy such "maritime" climates. Central Asia, the northern Great Plains and Canadian Prairies are classic examples of "continental" climates, which do not benefit from this moderating effect and thus experience bitterly cold winters and blazingly hot summers. The northeastern United States and eastern Canada fall somewhere in between. But because they are under the influence of prevailing winds that blow from west to east, their climate is considerably more continental than maritime."
Seager does not, however, dismiss the very real possibility that a stoppage of the thermohaline circulation will result in marked global cooling. He believes this will, in fact, occur, but he argues that global warming itself would serve to mitigate the cooling.
"The germ of truth on which such hype is based is that most atmosphere-ocean models show a slowdown of thermohaline circulation in simulations of the 21st century with the expected rise in greenhouse gases. The conveyor slows because the surface waters of the subpolar North Atlantic warm and because the increased transport of water vapor from the subtropics to the subpolar regions (where it falls as rain and snow) freshens the subpolar North Atlantic and reduces the density of surface waters, which makes it harder for them to sink. These processes could be augmented by the melting of freshwater reserves (glaciers, permafrost and sea ice) around the North Atlantic and Arctic.
But from what specialists have long known, I would expect that any slowdown in thermohaline circulation would have a noticeable but not catastrophic effect on climate. The temperature difference between Europe and Labrador should remain. Temperatures will not drop to ice-age levels, not even to the levels of the Little Ice Age, the relatively cold period that Europe suffered a few centuries ago. The North Atlantic will not freeze over, and English Channel ferries will not have to plow their way through sea ice. A slowdown in thermohaline circulation should bring on a cooling tendency of at most a few degrees across the North Atlantic—one that would most likely be overwhelmed by the warming caused by rising concentrations of greenhouse gases. This moderating influence is indeed what the climate models show for the 21st century and what has been stated in reports of the Intergovernmental Panel on Climate Change. Instead of creating catastrophe in the North Atlantic region, a slowdown in thermohaline circulation would serve to mitigate the expected anthropogenic warming!"
Some die-hard greens may be reluctant to consider Seager's conclusions, but they should keep in mind that he is neither a denialist nor a recipient of industry monies. He is a senior research scientist at Columbia University's Lamont-Doherty Earth Observatory and even if his ideas are too nuanced for environmental fund raising efforts, it's always best to acknowledge complexity over simple slogans. As I wrote in "Make Me A Mutt," my response to Nicholas Kristof’s March 12, 2005 op-ed piece in the New York Times,
"The Pew Research Center statistic Kristof cites is encouraging; 3/4 of Americans polled agree that environmental protection is vital. With that sizable majority in mind, it may not be necessary to throw tantrums. Reasonable, determined methods will serve the movement well. In environmentalism, like anything else, contradictions abound. Rather than dogmatically championing every device or idea that comes out of the green camp, we should fully consider each and push for thoughtful action as a diplomat would.
As Garret Keizer writes in his recent essay, "Life Everlasting," 'We can dare to walk on this ground of dubious footing, because we are holding one another up as best we can, and because it is we ourselves and not some deterministic logic that writes our civil laws…We can sniff out our options and pick and choose among them, a birthright generally less appreciated by a dogmatist than by a dog.'"
Photo credit: ripped from "The Day After Tomorrow" promo website; 20th Century Fox
(1) The movie should be spared a critique of it's Americentrism, if only because the international coverage of climate change is similarly biased. Most every report, whether published in a newspaper or broadcast on television (not to slight magazines and radio, my favorite media), focuses on the Gulf Stream, the oceanic current that draws water from the Gulf of Mexico to Europe, passing along the eastern coast of the United States as it does so. If this current should cease to be, countless white Europeans on both sides of the Atlantic would be adversely affected. By contrast, the Kuroshio Current, the Pacific equivalent of the Gulf Stream, receives only passing mention, and even then only in regards to California's fair shores. Siberia, Japan, Africa or any other place is of much less concern to the media. Frankly, Roland Emmerich at least included some shots of Japan being pelted by enormous hail chunks.