Update: 18 DEC 2001

Warming: Here at last?
If all the news about soaring temperatures is starting to make a global warming believer out of you, join the club. Those ominous temperature charts. The fact that, according to James Hansen of the Goddard Institute for Space Studies, virtually the entire globe was warmer than average last year.

It's enough to make you want to buy air-conditioner stock, or invent a new solar cell or some device to make energy without burning stuff.

Yet despite the warming trend, a surprising mini-dispute remains about whether the atmosphere is really warming at all. John Christy, of the University of Alabama at Huntsville, says trends in the lower atmosphere, when measured from satellites, show no warming over the past 20 years, exactly when global warming has seemed most acute. In the lower five miles of the atmosphere, the temperature trend was "zero" from 1979 to 1997, Christy and fellow NASA researchers concluded. And for the stratosphere (9 to 12 miles up), the cooling was 0.6 degrees per decade. We e-mailed Christy to ask how the climate could be warming if the atmosphere isn't? "Good question, no one really knows," he responded.


In the troposphere (the lower atmosphere) there's no clear signal of global warming-- in fact, no signal at all -- in these satellite measurements. Courtesy NASA.

Sherwood Rowland, an atmospheric chemist at the University of California at Irvine, questions the satellite data. "If there's a debate about whether to trust satellite data or the old-fashioned data," he said, "remember the satellite was 800 kilometers away from where you were trying to measure temperature, and problems could arise in between."

Whose fault
Most scientists accept that the atmosphere is warming. But is the warming due to the carbon dioxide and other greenhouse gases we're releasing into the atmosphere? Hansen, who made headlines in 1988 by telling the U.S. Senate that warming had indeed begun, is convinced that the higher setting on the thermostat reflects human activity. "The rapid warming of the past 25 years undercuts the argument of 'greenhouse skeptics' who have maintained that most of the global warming occurred early this century while greenhouse gases were increasing more slowly-- in fact, the fastest warming is occurring just when it is expected," he wrote on the web in a link that is now dead.

Other climatologists are a bit more circumspect, echoing the consensus of the Intergovernmental Panel on Climate Change that "the balance of the evidence" points to a human role in the ongoing warming. Francis Bretherton, director of the Space Science and Engineering Center at University of Wisconsin-Madison, says, "I have no unequivocal interpretation, but the most probable explanation is that it's a combination of a long-term anthropogenic [human-caused] trend toward global warming with a shorter-term natural fluctuation." Having said that, he then nods toward the surging global thermometer: "The longer [the current heat wave] goes on, the less likely it is that the latter part is playing a role."

A fickle friend
Climate is also a complicated creature, reflecting interactions between air chemistry, physics, water in clouds, rain and the ocean, the sun, the Earth's reflected heat, plants and human activity.

Bretherton stresses that climate is a variable beast. "A feature of our natural climate is the year to year changes," he points out. "There are cold winters and warm winters, and have always been." Regional climate -- the stuff we notice in our daily lives -- changes more rapidly than the global temperature, which typically changes from year to year by fractions of a degree -- at least over the 150 years of modern climate record.

While that record is a mere eyeblink in history, it's the best we have. And it contains no heat wave comparable to the past 30 years, Bretherton says. "What is new, is when you average temperatures around the world, the systematic change from year to year seems to be inexorably upward."

Anybody know how the greenhouse effect works?


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