Shifting Continents and Climates |
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By Mike Carlowicz, Science Writer
Communications Department
Woods Hole Oceanographic Institution Sixty-five millions years ago, dinosaurs had just become extinct,
and mammals were starting to dominate the planet. Tropical conditions
extended to northern Spain and the heartland of North America. Large
trees grew in Greenland and Antarctica, and alligators and primates
could be found on Ellesmere Island in Arctic Canada. Global
temperatures were 6° to 10°C (11° to 18°F) warmer than today, and the
polar regions were free of ice.
Since then, Earth’s history
has been marked by a sustained and nearly continuous cooling trend,
punctuated by abrupt shifts and transitions. Today, Homo sapiens now
dominate the landscape, the poles are blanketed in ice, and over the
past 3 million years, massive continental glaciers have waxed and waned
in an ongoing era of ice ages. Our modern climate is a brief, temperate
respite from an otherwise cold cycle in Earth’s geologic life.
So how did our hothouse planet turn into an icehouse planet?
Tectonic causes, climatic effects
One
explanation for the change is the steadily and substantially decreasing
levels of carbon dioxide and other greenhouse gases in the atmosphere
(at least until the anomalous and very recent post-Industrial
Revolution era). Less greenhouse gas means that less heat is trapped in
Earth’s atmosphere.
But changes in Earth’s atmosphere cannot
explain the full extent of global cooling or periods of acute change.
Nor can scientists fully explain the causes of the atmospheric changes
themselves.
So what other forces or processes might have rearranged Earth’s climate so dramatically?
In
recent years, scientists have been building a persuasive, but still
controversial case that changes in the solid earth (the crust and
mantle) spurred changes in the liquid earth (the oceans and
atmosphere). In other words, so-called tectonic forces—the drifting and
collisions of Earth’s tectonic plates—may lead to climate changes.
Rising mountains, closing gateways
The
following articles outline two theories that link tectonic and climatic
changes. One theory, outlined by Gerald Haug of the Eidgenossiche
Technische Hochschule (ETH) in Zürich, Switzerland, and colleagues,
proposes that the opening and closing of oceanic gateways between land
masses—a result of continental drift—may have altered global ocean
circulation patterns, which, in turn, led to climate changes. According
to another theory, outlined by Peter Clift of Woods Hole Oceanographic
Institution, the uplift of great mountain belts—caused by continental
collisions—may have disrupted atmospheric circulation and triggered a
cascade of other climate changes.
“Understanding the links
between solid and liquid Earth systems is a first-order scientific
problem for the 21st century,” says Clift, a marine geologist.
The best evidence to reveal those links, he notes, is buried under the seafloor.
Posted: February 23, 2004 [top] |
