Climate Change

The government of Tuvalu, a Pacific Island nation, made a plea last summer for countries to take in Tuvalu evacuees, fearing a rising sea level will ultimately sink the country. New Zealand is considering the request. Lowland flooding and salt-water intrusion into drinking water are already happening.

Researchers at Iowa State University have started work on corn hybrids that would thrive in significantly different growing conditions from those common today, including different temperatures, hours of daylight, and precipitation levels.

The Alaska Department of Transportation is testing ways of preserving permafrost under roads to prevent the sudden formation of sinkholes. One idea, painting highways white to reflect the sun's heat, failed because drivers had trouble with the glare.

These efforts are not unrelated, but are signs of preparations being made to deal with the increase in global mean temperatures expected by the end of the century, a change of 1.4 ÐŽ to 5.8 ÐŽC from 1990, that will have impacts in the lifetimes of current generations. (By comparison, the difference between global mean temperatures today and during the Ice Age some 20 000 years ago is roughly 4 ÐŽC.) Global mean temperature is the area average of the surface temperature over the globe.

Debate in decline

Though for decades arguments have raged over whether human activities cause changes in climate, these battles may be nearing an end. It is hard to dispute that the earth's climate is getting warmer. The apparent reason is a measurable increase in greenhouse gases, most notably carbon dioxide, but also methane, nitrous oxide, chlorofluorocarbons (CFCs), and ozone.

Some do disagree. And this group, while not large, is vocal. Some accept the evidence for a warming planet, but not that it is due to human activities. Others think a negative feedback effect will kick in or that the effects will be minor or even positive.

For example, Richard S. Lindzen, Alfred P. Sloan Professor of Meteorology of the Massachusetts Institute of Technology in Cambridge, dismisses the existence of a connection between the rises in atmospheric carbon dioxide and global mean temperatures. This is a key point, for if global temperature increases do not depend on an increase in carbon dioxide, then plans to reduce the amount of it entering the atmosphere, as proposed in the Kyoto Protocol, are pointless. Also doubtful are Sallie L. Baliunas and Willie Soon, researchers at the Harvard-Smithsonian Center for Astrophysics, Washington, D.C., who contest linking increased industrial activities to increased atmospheric carbon dioxide. Meanwhile, the National Tidal Facility in Australia has questioned whether the sea level change seen at Tuvalu represents more than anomalies caused by weather patterns.

But many scientists say global warming is real and will have serious effects. They also believe that nothing we do now can immediately stop it. Our best efforts, though important, will only slow it down. The questions of today are how well the effects can be predicted and how to cope with them.

According to the 2001 report of the International Panel on Climate Change (IPCC), a group of some 3000 scientists from around the world convened by the United Nations, "there is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

Global warming is a catch phrase for the increase in the globe's mean temperature due to a buildup of atmospheric greenhouse gases. It also refers to the negative effects caused by that temperature rise, like melting glaciers, higher oceans, or different precipitation patterns.

The evidence mounts

Holding the biggest piece of the pie in terms of greenhouse gases is carbon dioxide. It is fairly easy to measure because it mixes uniformly through the layers of the atmosphere on time scales of a year or two. So it is no surprise that researchers, looking to quantify evidence supporting global warming, checked to determine whether the amount of carbon dioxide has been increasing at any one location over time. Since 1958, these measurements have been made directly at a site in Hawaii; data for prior years is gathered by sampling bubbles of air trapped in ice cores.

The measurements indicate that while carbon dioxide levels have varied over hundreds of thousands of years, the upswing that began with the industrial age about 200 years ago shows an unprecedented rate of change.

The next bit of evidence is global mean temperature. This data is gathered from instrument records back to 1860, plus indicators that are sensitive to climate, such as tree rings, ice layers as measured in cores of ice from glaciers, ice caps, and ice sheets, and annual coral rings from cores in coral colonies. Over the last hundred years, the data shows an increase of at least 0.55 ÐŽC, a larger fluctuation than in any other past millennium.

"That increase," said Kevin Trenberth, head of the climate analysis section for the National Center for Atmospheric Research [NCAR] in Boulder, Colo., "is one of the main reasons we believe we have detected climate change. The trends in the temperature record, particularly in the last 20 years, are now outside the realm of natural variability. They're not caused by variations in solar radiation. They're not caused by pollution from volcanic eruptions. The [increase in] global mean temperature is outside the realm of anything that can be accounted for except by the increases in greenhouse gases."

Other evidence indicates the world is getting warmer. With a few exceptions, glaciers are melting. Oceans, measured consistently since the 1950s, are warming up. Sea ice in the Arctic Ocean, to cite recently declassified submarine data, has thinned by about 40 percent since the 1970s and diminished in extent. Sea level rose about 15 cm in the past 100 years, what with glaciers melting and oceans warming. The freezing season, or how long lakes and rivers around the world remain frozen in winter, has decreased by one to two weeks. Vegetation is creeping up mountains. And the list goes on.

Making the connection

The theory behind global warming starts with the Greenhouse Effect, first defined over 100 years ago. Greenhouse gases do not stop the sun's radiation from penetrating the atmosphere and reaching the earth, where it is converted into heat. But once that happens, the gases act as a blanket, reducing the amount of heat that can escape. This natural effect makes the planet habitable.

If the amount of greenhouse gases increases, according to the next step in this theory, the earth gets warmer. Therefore, since human activities--including