Every five years, the Intergovernmental Panel on Climate Change—the IPCC—issues a thick report summarizing recent scientific progress in the study of climate change—more widely known as global warming. The latest IPCC Report came out in 2007.

Very few people read the actual IPCC Report, which is written by and for specialists in climatology and the fields related to it, such as geochemistry, atmospheric physics, remote sensing science, meteorology, and so on. More people read its "Summary for Policymakers," a separate, less detailed document that is hammered out word by word among hundreds of authors and dozens of editors.

Perhaps you remember the 1995 IPCC Report, which gave birth to the following widespread belief among the media and the public: Scientists of the world agree that human greenhouse gases are heating up the Earth's surface, and they predict the climate will heat up by 1 to 3.5 degrees Celsius by the year 2100. That's a simplification of the Summary for Policymakers' simplified conclusions. Here's the actual soundbite from the 1995 Summary, on the IPCC Web site:

The balance of evidence, from changes in global mean surface air temperature and from changes in geographical, seasonal and vertical patterns of atmospheric temperature, suggests a discernible human influence on global climate.

That's not a very definitive statement—the balance (not the majority or preponderance) of evidence suggests (not indicates or documents) a discernible (not substantial or even measurable) influence. But it accurately reflects the state of science at that date. The temperature projections came from selected computer models, but these models range widely in their methodologies, and the numbers are clouded by a thick haze of uncertainties.

The 21 words of the October 2000 media storm were supposedly in the Summary for Policymakers of the upcoming 2001 IPCC Report. A draft of the Summary was being circulated among high-ranking scientists and nonscientist officials, part of a long and rigorous review process before finalizing the statement. Someone leaked the draft to an Associated Press writer, Joseph Hebert, and on October 26 he quoted it very selectively in this single sentence:

"While there remain uncertainties, studies of the last five years and more sophisticated computer modeling shows 'there is now stronger evidence for a human influence' on the climate and more certainty that man-made greenhouse gases 'have contributed substantially to the observed warming over the last 50 years.' "

Interpreting this handful of words, taken out of their context, is like the wild scientific guessing that followed the first pictures from the surface of Mars. Hebert interviewed a few atmospheric scientists who would only comment on the fact that the global temperature has been rising recently, which is not disputed. A reporter for Reuters could not get one scientist to comment on the record. CNN gave even less substance and included a quote that doesn't match what Hebert reported.

But the hint of smoke has prompted some over-the-top "flames" from people who aren't experts, like politicians. If you pay attention to the exact words that people are using, as in this summary of recent political jousting about environmental issues, you can watch the soundbite grow and change as people repeat it and stretch it.

Without being able to consult the IPCC report, we're left with gaping questions about those 21 words:

• What kind of evidence is "stronger" now, and how much stronger is it?
• What specific "human influence" is meant—land use, greenhouse gases, sulfates, aerosols?
• What does "substantially" mean in detail?
• What about the temperature record of the previous 50 years, which included a cooling period, not just the most recent 50 years of uninterrupted warming?

As it happened, the Summary for Policymakers came out on schedule and those 21 words were nowhere to be found. All the heat and furor was baseless; all the political points scored were arms waving in the air.

Better that we should read a balanced, authoritative article by a genuine atmospheric scientist. It's up to us to listen for scientists' voices, because most reporters aren't up to speed on science and won't give us what we need to know.

For more exploration of the science behind climate change, see my list of links on the subject. And as for that election, you can read Al Gore and George Bush's statements on global warming and other science issues on the American Institute of Physics site.