As climatologists gather here this week to discuss new research on global warming, a disquieting idea has been gaining currency--the possibility that small shifts in global temperature could lead to sudden and abrupt climate changes.
What makes such projections important is not their likelihood, which is uncertain, although a growing number of scientists believe that sudden changes in climate are a possibility. Instead, the chief significance for policymakers and the public lies in what the new research suggests about scientific uncertainty and risk.
Until recently, much of the climate debate has centered on whether global warming is occurring at all. Most climate models had assumed a slow, steady increase in temperature and forecast gradual changes with gradual effects.
But newer, more sophisticated models suggest that the Earth's climate system is "nonlinear"--in other words, small changes can have large effects on everything from ocean and land temperatures to drought and monsoon patterns, icecaps and tropical rain forests.
Though loath to cry wolf, more and more experts are beginning to publicly discuss--and personally fear--changes that are far more dramatic, and potentially faster, than those at the center of discussion so far. Some events could permanently alter life on Earth.
For example, one projection is that melting Arctic ice could cause a flow of fresh water into the North Atlantic that would shut down the Gulf Stream this century. That warm current moderates the European climate, and turning it off would make a swath of land from London to Stockholm miserable.
"Sometimes very small, innocent changes can trigger huge changes," said Will Steffen, executive director of the Sweden-based International Geosphere-Biosphere Program, or IGBP, which is coordinating the Amsterdam conference. "Sometimes you hit it with a hammer and nothing happens. We simply do not know. We are heading into uncharted waters."
In the global warming debate, a chief argument of industry, joined by Bush administration officials and some scientists, is that the U.S. and its allies should not rush into potentially costly measures to head off possible climate change because our knowledge of the subject is limited.
Many scientists, however, say that argument is precisely backward. The possibility of sudden, dramatic climate shifts means that, although there is a risk that current models are too pessimistic, there is also a substantial risk that they are too optimistic.
A prominent advocate of the go-slow school of thought is Sallie Baliunas of the Harvard-Smithsonian Center for Astrophysics, an expert on how the sun and its heat output have varied through time.
Her research is funded by federal agencies but she accepts money--to "travel around and speak"--from firms that have advocated a go-slow approach on global warming. She argues that computer models are unreliable, exaggerate warming trends, fail to adequately take into account natural fluctuations in temperature and do not explain why no warming has been seen in the upper atmosphere.
"The best evidence says [climate change] is slow to work, so we have a window of opportunity," she said.
As advocates of that school of thought note, many climate scientists a decade ago feared that global warming could cause a catastrophic melting of the massive West Antarctic ice sheet. Such an event would release huge amounts of water into the seas, devastating many of the world's highly populated, low-lying coastal areas.
Recent studies, though, suggest that the Antarctic icecap is stable--and actually growing as more precipitation falls there.
Other scientists argue that because knowledge is uncertain, it is crucial to begin cutting emissions of carbon dioxide and other so-called greenhouse gases to slow the rate of climate change.
"We could be either under- or overestimating the effect of human activities on climate," said Robert Watson, chief scientist at the World Bank and head of the U.N. Intergovernmental Panel on Climate Change. "So why should we be complacent?"
Paul Crutzen, an atmospheric chemist working in Germany who won the Nobel Prize for explaining the hole in the atmosphere's ozone layer, makes a similar point. There is not enough room to take chances with the climate, he argues.
The chief cause of the hole, which appeared over Antarctica in the final decades of the 20th century, was chlorofluorocarbons--chemicals used as refrigerants and as propellants in spray cans. Had chemists earlier in the century decided to use bromine instead of chlorine to produce coolants--a mere quirk of chemistry--the ozone hole would have been far larger, occurred all year and severely affected life, he said.
"Avoiding that was just luck," he said, noting that no scientist had predicted the hole or its impact. "We missed something very important. There may be more of these things around the corner."
What climate watchers fear most are shifts that could "kick the climate system" into an entirely new state, said Berrien Moore III, chairman of the IGBP. That could cause "unpredictable consequences with cascading effects."
Such shifts have occurred before. A tiny change in the Earth's orbit, for example, altered precipitation and temperature patterns enough to convert what was once fertile African savanna into today's dry Sahara. "There are caves in today's desert that show giraffes and all kinds of other animals," said Robert J. Scholes, a South African climatologist.
"Abrupt changes in the Earth's systems can occur when thresholds are crossed," said Moore, a climate researcher at the University of New Hampshire. "Those changes may involve rather distant, telegraphed connections."
One current possibility is the melting of the Arctic sea ice.
Arctic snow and sea ice moderate the climate by covering a massive portion of the Earth's surface. This white, frozen blanket reflects sunlight and heat back into space, cooling the planet. If much of the ice melted and the Arctic Ocean became an open sea, the resulting big, dark patch would absorb heat and lead to even more warming.
Oleg Anisimov, an expert on the planet's icy "cryosphere" at the State Hydrological Institute in St. Petersburg, Russia, said Thursday that such a shift is already occurring. The snow and sea-ice cover in the Arctic has decreased 10% since the 1970s, and the ice has thinned markedly in that time, he said. "Such changes are already enhancing the greenhouse effect," he said.
Research published June 21 in the journal Nature suggests that freshwater flows in the Nordic seas are increasing and may be slowing the crucial circulation of warm water, said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany.
Anisimov said the increased flow of Siberian rivers also provides evidence that Arctic waters are freshening. Thawing permafrost in the region, he said, could also fuel warming by allowing decomposing material to emit greenhouse gases now trapped in frozen soil.