The ink had barely dried on the Clinton administration's signature on the Kyoto Protocol on greenhouse-gas emissions this month when Vice President Al Gore declared, "In the 11 months since Kyoto (was completed), the evidence of global warming has grown only stronger."
Some scientists who study the climate aren't so sure. Over the past decade, as they have learned more about how the climate works, they've scaled back their predictions of how much the Earth will warm over the next century.
Even James Hansen - director of the Goddard Institute for Space Studies of the National Aeronautics and Space Administration who put the phrase "global warming" on the map - has backed off.
He has said that scientists don't fully understand all the factors that influence the climate. Until they do, they won't be able to predict the future clearly.
But few backers of the theory that human actions cause global warming have changed their minds. Right now, they say, the warming is being masked by temporary factors.
Until it's clear that man-made emissions increase temperatures, though, it's hard to make a case that governments should quickly move to restrict greenhouse gases.
The theory that man-made global warming is a threat emerged in '88, when then-Sen. Al Gore, D-Tenn., convened a Senate committee hearing on the issue.
Several witnesses, including Hansen, testified that greenhouse gases were already warming the planet. "The greenhouse effect has been detected, and it is changing our climate now," Hansen said then.
But in the Aug. 18 issue of Proceedings of the National Academy of Sciences this year, Hansen wrote, "The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change."
In other words, scientists aren't sure how the components that make up the atmosphere interact. And they need to understand those connections before they know anything about climate change.
These factors are a "major area of uncertainty," Hansen said.
Such uncertainty is no knock against science.
The climate is affected - literally - by everything under the sun. It is an incredibly complex system that depends on millions of variables.
As climate models have been refined, they've tended to predict less global warming.
Take the forecasts produced by the U.N. Intergovernmental Panel on Climate Change.
In '90, the first IPCC report predicted that the average global temperature would rise 3.3 degrees Celsius over the next century. In '92, the IPCC projected a smaller increase of 2.8 degrees.
The group's '95 report cut the forecast to 2.0 degrees. When the natural causes that affect the climate were accounted for, the group said warming related to human activity would be only 1.0 degree over the next century.
That's about how much temperatures have gone up since the 1880s.
Gas Buildup Slower
Why have the forecasts of global warming been trimmed?
For one, greenhouse gases - notably carbon dioxide and methane - are not building up as quickly as expected. If that pattern continues, the Earth will take longer to warm than first thought.
The concentration of carbon dioxide in the atmosphere is rising by about 0.4% a year, rather than the 1.0% annual pace first projected. The concentration of methane has actually decreased.
"We don't know nearly as well as we'd like to why this is happening," Hansen said.
One possibility: More land is being covered with the plants and trees that soak up carbon dioxide as they grow. Oceans may also absorb carbon dioxide.
Role Of Factories, Volcanoes
There are several possible reasons the planet hasn't warmed as fast as predicted. The climate may just not be very sensitive to a greenhouse-gas buildup. Or other factors like sulfate aerosol emissions from factories and volcanoes may mask global warming.
Hansen believes it's the latter. Sulfur emissions, both natural and man-made, may shade the planet and cool it -offsetting greenhouse warming.
Hansen says volcanoes have been unusually active over the past two decades.
But there haven't been any big eruptions over the past couple of years, he says. As sulfate emissions from earlier eruptions fall to Earth, they won't shade the planet any more.
Sunspots A Factor?
Changes in the sun's intensity could also reduce warming. Sunspots can make the sunlight that reaches the Earth more or less intense.
And sunspots appear to rise and fall on a cycle that averages 11 years, notes Sallie Baliunas, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
In an interview in the October '98 Reason magazine, Baliunas said the cycle can vary - from as few as eight years to as many as 15.
When the cycle is shorter, she says, sunlight gets more intense. That may drive global temperatures up.
Hansen argued the effect of sunspots "is going to go away," in which case global warming would be obvious to "the man on the street" within a decade.
Baliunas disagrees.
Why Not More Warming?
Over the past two decades, the Earth's surface temperature has risen at a pace of 0.15 degree C per decade. But climate models from 10 years ago predicted that temperatures would rise three times as fast.
"We are left in a scramble trying to explain why this large warming didn't occur," said Patrick Michaels, a climatologist at the University of Virginia. Michaels thinks a combination of factors is at work, one of which is the slower-than-expected buildup of greenhouse gases in the atmosphere.
And early computer climate models weren't very accurate, he says. Faulty predictions resulted.
Satellite Readings Debated
Satellite measurements of temperature in the troposphere - the layer of atmosphere from the Earth's surface to about 20,000 feet up - have risen at only a 0.06 degree-per-decade pace over that same time.
Without the warming that resulted from El Nino in '98, the troposphere would have shown no temperature change, says John Christy, professor of atmospheric sciences at the University of Alabama, Huntsville.
"All global-warming models show that this atmospheric layer will warm as fast or even faster than the surface of the Earth," Christy said. "The fact that it has not suggests that the typical climate model is not accounting for what happens in the real world."
Christy believes the temperature readings are correct. But the planet may have a way of releasing heat from the Earth's surface to space, he says.
Other scientists dismiss that possibility. Rather, a recent article in Nature magazine argued that satellite measurements of temperature are not accurate.
Christy points out that he, along with Roy Spencer of NASA's Marshall Center, have corrected their satellite data to reflect some inaccuracies.
But they haven't yet published their revised data. (Their corrected data are used above).
Also, the trend shown by the satellite data matches a different set of temperature readings that were taken on weather balloons.
Uncertainties Remain
Plenty of questions surround climate science. Another is how clouds and the oceans affect the Earth's temperature.
For example, clouds reflect sunlight away from the Earth, helping to cool it. They also trap some heat in the atmosphere, helping to warm it.
The net effect? That depends on the types of clouds they are, as well as on their altitude.
Scientists may not understand the net effects of all of these factors.
But they have learned much more over the past few decades about each piece of the climate puzzle. And it seems that the more they learn, the less they know for sure.
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