Two years ago, a United Nations scientific panel won the Nobel Peace Prize after concluding that global warming is "unequivocal" and is "very likely" caused by man.

Then came a development unforeseen by the U.N.'s Intergovernmental Panel on Climate Change, or IPCC: Data suggested that Earth's temperature was beginning to drop.

That has reignited debate over what has become scientific consensus: that climate change is due not to nature, but to humans burning fossil fuels. Scientists who don't believe in man-made global warming cite the cooling as evidence for their case. Those who do believe in man-made warming dismiss the cooling as a blip triggered by fleeting changes in ocean currents; they predict greenhouse gases will produce rising temperatures again soon.

The reality is more complex. A few years of cooling doesn't mean that people aren't heating up the planet over the long term. But the cooling wasn't predicted by all the computer models that underlie climate science. That has led to one point of agreement: The models are imperfect.

"There is a lot of room for improvement" in the models, says Mojib Latif, a climate scientist in Germany and co-author of a paper predicting the planet will cool for perhaps a decade before starting to warm again — a long-term trend he attributes to greenhouse-gas emissions. "You need to know what you can believe and can't believe from the models."

The renewed discussion of inherent shortcomings in climate models comes on the cusp of potentially big financial commitments. In five weeks, diplomats from around the world will meet in Copenhagen to try to hash out a new agreement to curb global greenhouse-gas emissions. The science continues to evolve.

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