A. Sport utility vehicles; B. Rice fields; C. Increased solar output.
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.
That's when the roulette wheel started to look rigged. The temperature increase since the 1950s "is not like anything seen in the paleoclimate data," says atmospheric scientist Joyce Penner of the University of Michigan. "It's very clear that the last 50 years are very unusual." Temperatures in the Northern Hemisphere during the second half of the 20th century were even farther out of line with natural variability. They are warmer than during "any other 50-year period in the last 500 years," found the IPCC report, "and it is likely that this was the warmest period in the past [1,300 years]."
The case for natural variability founders on another shoal. When natural cycles such as El Niño cause unusual warming, they also cause unusual cooling. One place heats up and another gets a chill, as if Peter were robbed of heat to warm Paul. The result is no net global change. To warm both Peter and Paul in a closed system violates the laws of thermodynamics. But according to the latest IPCC report, which assesses hundreds of climate studies, temperatures have risen on every continent except one (there are not enough data from Antarctica to draw a conclusion about its climate history). "You can detect an anthropogenic imprint on all continents where we have adequate observations," says Francis Zwiers, director of the Climate Research Division of Environment Canada, a government agency, who is also an author of the IPCC report. Peter and Paul both got warmer. Or, as the IPCC put it, "No known mode of internal variability leads to such widespread, near universal warming as has been observed in the past few decades. Although modes of internal variability such as El Niño can lead to global average warming for limited periods of time, such warming is regionally variable, with some areas of cooling."
The conclusion that observed climate change is our fault rests on the pattern of warming, too. As it happens, "human and natural factors that affect climate have unique signatures," says climatologist Ben Santer of Lawrence Livermore National Laboratory, part of the U.S. Department of Energy. The clearest signature is differences in the warming of different layers of the atmosphere. According to satellites and weather balloons, the lower atmosphere, or troposphere, has warmed; the upper atmosphere, or stratosphere, has cooled. That's not what a hotter sun would do. "If you increase output from the sun, you increase the amount of energy that arrives at the top of Earth's atmosphere," says Santer. "And you get heating throughout the whole column. Have we observed anything like that? The answer is a very clear no." Greenhouse gases such as carbon dioxide from the burning of coal, oil and natural gas and methane from, among other surprising sources, rice fields (where bacteria thriving in the submerged paddies release this and other gases) act from the bottom up. That is, they warm the troposphere and cool the stratosphere by trapping heat waves wafting off the planet's surface. The warm troposphere and cool stratosphere "is entirely consistent with our best understanding of how temperatures would change with an increase in greenhouse gases," says Santer.
Another problem for the blame-the-sun idea is that the climate balance sheet doesn't, well, balance. Solar output rises and falls over an 11-year cycle. The high point in the cycle raises surface temperatures 0.2 degree F, at most—much less than the increase that has been measured between the late 1800s and now.
More recent changes are even tougher to blame on a hotter sun. From 1955 to 2000, the world's oceans warmed .7 degree F, Tim Barnett of Scripps Institution of Oceanography and colleagues reported in 2005. That may seem small, but the immensity of the oceans means the amount of heat required to warm them even a little is enormous. In the same period, the sun has increased its energy output less than 0.1 percent, according to satellite measurements. That's not nothing, but it's not enough to explain the warmer seas. The extra solar output can no more account for that than holding a candle under a pot can account for boiling a gallon of water. Extra heat pouring out of the planet's core could warm the oceans, except for one problem: it would heat the oceans from the bottom up. In fact, the greatest warming is at the waters' surface. "And if it were natural variability, then a couple of oceans might warm but others would cool, and the net would be zero," says Barnett. "All the oceans are warming, and for that you need a net heat source. We've ruled out everything but greenhouse gases."
One by one, climatologists have gone through the signs of climate change and exonerated both natural variability and natural outside sources as the main culprits. Extremely warm summers, such as the 2003 European heat wave that killed thousands of people? A human fingerprint. Glacial retreat? Ditto, though it is partly natural. Stronger tropical storms, such as Katrina? Possibly our fault, though on this one the evidence is murkier. Heavy precipitation that alternates with dry spells, so that when it rains it pours? That also conforms to models of man-made climate change. And neither natural variability nor more solar heat can explain the warmer surface waters in the hurricane breeding grounds of the Atlantic and Pacific, which have heated up .5 to 1 degree since 1906. Natural oscillations have never been that great, says Santer. And extra solar heat "is way, way too small, an order of magnitude too small."
This is not to say that every sign of climate change reflects the heat-trapping effects of the gases we spew out of our utility plants, cars and planes. Increases in the sun's output were probably responsible for the warming that occurred in the early 20th century. And natural variability explains some sea-level rise and loss of arctic sea ice. Undoubtedly, other natural explanations for climate change will pop up; one currently getting some buzz (as well as lots of criticism) is that changes in the sun's output alter the barrage of cosmic rays that strike Earth, producing fewer clouds and therefore a warmer world.
But that raises a question for those who emphasize nature's contributions to global warming and other aspects of climate change. Let's suppose that those are nudging the climate toward worse storms and more droughts and more heat waves, just as greenhouse gases are. In that case, you'd think the world would want to control the causes of global warming that it can. At last check, no one had figured out how to turn down the sun.