Which of These Is Not Causing Global Warming Today?

When 600 climate scientists from 40 countries reported in February that there was, for the first time, "unequivocal" evidence that the world is warming and greater than 90 percent certainty that man-made greenhouse gases have caused most of the warming since 1950, at least one expert demurred. "We're going to see a big debate on it going forward," said Vice President Dick Cheney. By "it," he meant "the extent to which [the warming] is part of a normal cycle versus the extent to which it's caused by man."

There is no denying the intuitive appeal of the idea that climate change is natural. After all, local temperatures can rise or fall by 40 degrees from one day to the next; violent storms can barrel in over the course of only minutes. It's little surprise, then, that many laypeople look at much tinier and subtler changes—the 1 degree Fahrenheit increase in global mean temperature since the 1970s, say—and figure those, too, could well be natural. As for 11 of the 12 hottest years on record occurring in the last 12? Well, everyone has experienced a run of statistics-defying weather. Besides, some signs of climate change are undeniably the work of Mother Nature's whims and not man's "addiction" (as President Bush called it) to fossil fuels, at least in part. For instance, glaciers in East Africa, including Mount Kilimanjaro, began shrinking around 1880—well before the greenhouse effect ramped up. And the 1936 heat wave is still the worst many American cities ever experienced, never mind the (globally) record-setting 1990s. No wonder that, in the NEWSWEEK Poll, only 17 percent of those surveyed correctly absolved a hotter sun of responsibility for recent global warming.

That impression is at odds with the science, however. As the February report from the Intergovernmental Panel on Climate Change concluded, greenhouse gases have caused most of the recent warming. "Without greenhouse gases and other [man-made] forcings," says climatologist Gabriele Hegerl of Duke University, an author of the report, "we cannot really explain the observed climate changes."

Climatologists did not reach that conclusion lightly. They know full well that climate change can arise from any of three basic causes. One is what they call "internal, natural variability" (a fancy name for "s—t happens," climate-wise). Because there is as much randomness in climate as there is in a roulette wheel, droughts and heat waves and killer storms are to be expected, just like a run of evens or reds in Las Vegas. Around 1880, for instance, atmospheric circulation over the Indian Ocean strengthened in such a way that less rain and snow fell on East Africa, including Kilimanjaro, finds glaciologist Stefan Hastenrath of the University of Wisconsin. No one knows why the circulation changed. But the result of this natural hiccup was glacial retreat that has gotten worse through the decades. Changes in the Niño cycle can also reflect internal, natural variability. A second cause of climate change, "natural external forcings," refers to random, or at least hard-to-predict, shifts in outside influences, such as more heat coming from the sun or from Earth's core.

And then there is the hand of man.

The first hint that natural variation fails to explain recent climate change comes from the climate version of noticing that the roulette ball has clattered into an even number three times in a row. That is, you compare seemingly weird weather to what has come before to see if it might not be as strange as it seems. (The chance of three evens in a row in roulette is about 1 in 8, so when it happens you don't automatically conclude the wheel is rigged.) When scientists measured a rise in Earth's average temperature of 1 degree F over the past 50 years, they therefore scurried to the record books, both man's and nature's—that is, to historical weather archives as well as tree rings and ice cores that preserve records of ancient temperatures—to search for precedents.