Scientists' use of computer models to predict climate change is under attack

Then, the computer imagines effects in one box spilling into the next, and so on.

As the model runs, imaginary cold fronts sweep over virtual oceans, simulating weather at rates such as five years per day. In some cases, the models are re-run with different weather conditions, until a pattern emerges in global temperatures.

The pattern is the point. It is man's signature, a guide to what could happen in the real world. All the major climate models seem to show that greenhouse gases are causing warming, climate scientists say, although they don't agree about how much. A 2007 United Nations report cited a range of estimates from 2 to 11.5 degrees over the next century.

"It's an educated, scientifically based guess," said Michael Winton, an oceanographer at the National Oceanic and Atmospheric Administration. "But it's a guess nonetheless."

Raining on their parade

But Warren Meyer, a mechanical and aerospace engineer by training who blogs at www.climate-skeptic.com, said that climate models are highly flawed. He said the scientists who build them don't know enough about solar cycles, ocean temperatures and other things that can nudge the earth's temperature up or down. He said that because models produce results that sound impressively exact, they can give off an air of infallibility.

But, Meyer said -- if the model isn't built correctly -- its results can be both precise-sounding and wrong.

"The hubris that can be associated with a model is amazing, because suddenly you take this sketchy understanding of a process, and you embody it in a model," and it appears more trustworthy, Meyer said. "It's almost like money laundering."

Last month, a Gallup poll provided the latest evidence of a public U-turn on climate change. Asked if the threat of global warming was "generally exaggerated," 48 percent said yes. That was up 13 points from 2008, the highest level of skepticism since Gallup started asking the question in 1997.

But scientists say that, during this time, they have only become more certain that their models work.

Put in the conditions on Earth more than 20,000 years ago: they produce an Ice Age, NASA's Schmidt said. Put in the conditions from 1991, when a volcanic eruption filled the earth's atmosphere with a sun-shade of dust. The models produce cooling temperatures and shifts in wind patterns, Schmidt said, just like the real world did.

If the models are as flawed as critics say, Schmidt said, "You have to ask yourself, 'How come they work?' "