Which of These Is Not Causing Global Warming Today?

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."