Just another sci-fi flick?
2. Climatic roller-coaster
3. Furious feedback
4. Warming Southern
Icebergs in Greenland. If the ice cap melts,
sea level rises. Photo: NOAA
and Julian Dowdeswell, Centre for Glaciology, Institute of Earth
Studies, University of Wales
A Chinese scientist removes a core of
ice from thick Arctic ice pack. Photo:
and Emory Kristof, National Geographic Society
The abrupt warming documented in the Greenland
ice cores is generally viewed as an indication of regional warming,
not necessarily global warming. But what could have caused such
a rapid shift in the North Atlantic climate? Scientists -- in both
The Day After Tomorrow and real life -- have speculated about a shutdown
in the ocean conveyor that moves heat from the equator toward the
poles. This conveyor, which includes the Gulf Stream, gives Northwest
Europe a tolerable climate, considering that the United Kingdom
is at the latitude of Labrador.
About 10 years ago, as the Greenland data were emerging, Wallace Broecker and others began warning that the rapid warming seen in the ice could have caused massive melting of glaciers, causing a deluge of freshwater into the Labrador Sea.
Part of the driving force for the ocean conveyor
is the sinking of dense, cold and salty water in the Labrador Sea.
Broecker warned that freshwater, being less dense, might not sink,
slowing or stopping the conveyor, and freezing Europe. A shutdown
in the ocean conveyor, he suggested, could have started cold snaps
(like the Younger Dryas) that punctuated the warming trend after
the last ice age.
In that sense, The Day After Tomorrow has a credible premise -- warming causes cooling. But, says climate modeler Stephen Vavrus, the flick immediately goes Hollywood. "It's plausible that a large increase in meltwater could slow or shut down the conveyor circulation," he says, "but most climate models... are predicting only a slowdown, not a shutdown."
In any case, the slowdown would not have the global effects seen in the movie. Against the backdrop of rapid warming caused by rising levels of greenhouse gases in the atmosphere, Vavrus adds, climate models are generally predicting "either a reduction in warming, or maybe no change in temperature. Very few climate models predict an outright cooling from this effect."
Richard Alley doesn't expect another ice age to emerge from the growing levels of greenhouse gases in the atmosphere. "Global warming will warm Earth on average," he says. "Warm makes warm, not cold." Alley was a principal investigator with GISP 2, a 3,053 meter drill hole that recorded 110,000 years of Greenland's climate.
Still, a shutdown of the Atlantic conveyor
was threatening enough to interest the Pentagon, which explored
the national-security implications -- famines, refugee flows and
economic chaos -- of climate change (see "The Pentagon's ..." in
The Atlantic conveyor is just one mechanism of abrupt climate change. Taking a broader view, Reid Bryson, a climatologist who has a skeptical view of global warming, sees rapid change as unsurprising in view of the way that weather forms.
In the Midwestern United States, he points out, the onset of spring is related to the position of pressure waves surrounding the north pole. During winter, a big low-pressure zone around the Aleutian Islands directs cold air south into the center of the continent. With spring, a second low appears over eastern Canada. Location matters, Bryson says. On the east side of a low, prevailing winds come from the North, on the west, from the south. So when the second low pressure appears over Hudson's Bay, Bryson says, "it brings up this soft spring air."
The sudden break is just physics at work, he points out: Pressure waves must occur in whole numbers. "It happens rapidly," says Bryson. "You can't change the waves fractionally -- the number has to be an integer, so you get a rapid change from winter to summer."
Similar mechanisms may also mark the longer-term systems that determine climate, Bryson says. The many factors that contribute to climate are poorly understood, but it's becoming clear that climate may have some "regimes" where things work together; after a seemingly slight change, the climate may "jump" to a new stable regime. That, after all, is what happened before and after the quick warming that ended the Younger Dryas.
In considering how a warming could get more warming, let's visit feedback mechanisms, starting with a simple example: If continued warming melts ice near the poles, Earth will absorb more solar heat. Warming will cause more warming.
That's positive feedback.
Feedback can also affect carbon dioxide, the primary greenhouse gas. Currently, about a third of carbon dioxide released by burning gets trapped in soil, wood or other organic matter, greatly slowing global warming. Eventually this natural carbon storage could fill, or even start to empty. For example, continued warming could dry the Amazon Basin, allowing fires to eradicate forests. Suddenly, instead of being a source of oxygen and a sink for carbon dioxide, the Amazon would reverse its role, causing accelerated warming that could hasten the disappearance of Amazonian forests.
Methane, which is a stronger greenhouse gas than carbon dioxide, is another possible source of feedback. Vast amounts of methane are trapped under the seabed as methane hydrates. A recent study suggested that big releases of methane may have triggered past temperature spikes.
Severinghaus says atmospheric methane rose 50
percent immediately after the Younger-Dryas warming; the timing
indicates that methane did not cause the warming. Severinghaus is
measuring the radioactive isotope carbon-14 in Greenland ice to
see if that methane came from swamps or methane hydrates. Carbon-14,
made by cosmic rays, has a 5,000-year half-life, so it appears in
swamp methane but not in the much older methane hydrates.
A large release of methane from a warming Earth could be part of a positive feedback mechanism. "In the next 50 years," Severinghaus says, "there will be a lot of global warming. The oceans will warm; they are already warming. If the ocean bottom water warms a little bit, methane hydrates will turn to a gas, and come leaking out into the ocean. It might exacerbate global warming."
Methane hydrates contain the planet's largest store of organic carbon. Over the next few centuries, they could eventually double the pace of warming.
Be nice to return to ice . What's its role in warming?