POSTED JULY 11, 2002

2. Ultimate cooker

3. See rising sea level

4. Know for sure?

5. What to do?

Are the ominous predictions about Earth's climate from 10 to 20 years ago finally coming true? Are we entering a "greenhouse century" of rising seas, shriveled crops, animal extinctions and intense heat waves?

Or is the present warming just another blip on the murky, ever-changing pattern we call climate?

Researchers predict that the sea will rise 40 to 60 centimeters in the next century. Atoll nations, which sit only a few feet above sea level, worry about rising seas. Sea level rise may also affect fresh water, sanitation, agriculture, fishing, housing and the rest of the economy. National Institutes of Health.

Don't know enough?
A decade ago, if you asked the average climate scientist if global warming was a fact or a prediction, you got this kind of mealy-mouthery: "Our computer models predict global warming, but we can't say for sure whether it has started. Best to be cautious and reduce greenhouse gas emissions anyway."

Today, the imagined response differs only in intensity. "The past decade has matched our predictions for global warming, but we can never know for sure. Climate is a terribly complex thing, and it would be smartest to assume that warming is happening. We need to reduce greenhouse gas emissions."

So here's the critical question: If climatologists can't give a definitive answer, is it wisest to watch and wait, or to act on what we do know?

The simple science
As the name implies, greenhouse gases cause the greenhouse effect by trapping heat in the atmosphere, much as glass traps heat in a greenhouse. Nobody disputes that the natural greenhouse effect warms surface air temperatures by a hefty 33 degrees C, allowing life as we know it to survive.

The debate concerns the extra warming caused by human actions (including the release of greenhouse gases, primarily carbon dioxide, and big changes on Earth's surface).

The Bush Administration says the United States can't afford to limit greenhouse gases. Perhaps they haven't noticed how rich the country is... Data: World Bank, April 2002

Because most economic activities cause the release of greenhouse gases, reducing emissions would be expensive. Some evoke this argument in the country's rejection of the Kyoto Protocol, a treaty designed to limit emissions of greenhouse gases. The United States, they say, simply cannot afford to limit its economy in this fashion.

Status quo sauté
Doing nothing about global warming could be expensive, however. The Intergovernmental Panel on Climate Change (IPCC), the United Nations group set up to study the issue, predicts that by 2100, temps will rise by 1.4 C to a staggering 5.8 degrees C. Although that would almost represent a return to dinosaur temperatures, carbon dioxide levels and global warming might well continue rising past that point.

A reconstruction of Northern Hemisphere temperature. Blue: inferred temps from tree rings, corals, ice cores, and historical records; red: temps from instruments; black, a smoothed version of the data. IPCC: Technical Summary, Working Group I, 2001, p. 29 29_temp_reconstruct.

Since the first IPCC report in 1991, planetary conditions have given little support to those who doubt global warming. According to Kevin Trenberth, head of the Climate Data Analysis Section at the National Center for Atmospheric Research, the 1990s were the hottest decade in at least 1,000 years.

This spring, amid rising concern from citizens around the world, the Bush Administration renounced Kyoto -- even as the Environmental Protection Agency admitted that much of the ongoing global toast-up can indeed be blamed on human activities, especially burning fossil fuels.

President Bush blamed "the bureaucracy" for his Administration's report.

But consider the following:

Average air temperatures have increased 1.2 degrees F over the past century. The IPCC says about 0.8 degrees of that is likely due to human activities.

The hottest 10 years in the last century have all occurred since 1980.

1998 was the hottest year on record, and 2001 was the second-hottest.

January-March, 2002, was the hottest such period on record.

Glaciers are retreating in many locations around the world.

Ditto for permafrost in Alaska (severe warming at high latitudes was one of the first regional forecast of climate models).

Sea level continues to rise due to the expansion of warming ocean water and the melting of glaciers.

Being simple-minded creatures, we Why Filers figured that since the global warming debate heated up more than a dozen years ago, enough time would have passed to check the accuracy of predictions about temperature and sea level.

Warming to the task, we did the obvious thing
We read the latest IPCC report, which stated that the human hand was increasingly obvious in climate. But when we talked to people who spend their lives studying warming, something odd happened.

Certainly, we found no support for Patrick Michaels, a climatologist and prominent skeptic of global warming. He and others have highlighted supposed errors and inconsistencies in climate data. How, for example, can you be certain about the historic temperatures that were often recorded on urban thermometers? Cities, after all, are warmed by energy released from vehicles, industry and buildings, and this "urban heat island" effect exaggerates temperatures.

Thus any warming recorded in cities says nothing about the globe as a whole. Not so, says Stephen Schneider, a professor of geosciences at Stanford University and an early voice in the warming debate. "If you eliminate large cities or correct for them, it does not change the answer more than 10 percent. The urban heat island has been completely and thoroughly put to bed, it's been analyzed and reanalyzed."

In 1992, Michaels wrote that the gloomy predictions of global havoc are "wrong. The most internally consistent case that can be generated from billions and billions of bits of climatic data simply do not support that vision... The likelihood that we are creating a better world far outweighs the probability of climate apocalypse" (see "Sound and Fury" in the bibliography).

That does not reflect the consensus of climatologists, as reflected in the IPCC.

Red: increases; blue: decreases; green: little or no change. Trends were calculated only for areas where average temp changed significantly during 66 of the 100 years. IPCC: Technical Summary , Working Group I, 2001, p. 27 27_annual temperature.

However, we did find support for a frustrating truth that's plagued the warming debate since the start: We may not have a definitive, scientific answer about the human impact on climate until far too late.

First things first. What have thermometers told us about climate?

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Terry Devitt, editor; Pamela Jackson, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2002, University of Wisconsin, Board of Regents.