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“It’s a very, very broad consensus. There are a few individuals who don’t believe it, but we are talking about science and not beliefs,” Van Ypersele told AP.

Climate change skeptics say IPCC scientists have in the past overestimated the effect of the accumulation of CO2 in the atmosphere and underplayed natural cycles of warming and cooling. Others have claimed the authors, who aren’t paid for their work, exaggerated the effects that climate change will have on the environment and on human life.

Negotiators in Doha are supposed to start talks on an elusive global treaty to rein in emissions. They have set a deadline of 2015 to adopt that pact, which would take effect in 2020.

Among other topics, they are discussing how to help poor countries convert to cleaner energy sources and adapt to a shifting climate, as well as extending the expiring Kyoto Protocol, an agreement that limits the greenhouse emissions of industrialized countries.

The U.S. rejected the Kyoto deal because it didn’t cover world-leading carbon polluter China and other fast-growing developing countries. Other rich countries including Canada and Japan don’t want to be part of the extension, which means it will cover less than 15 percent of global emissions.

“Japan will not be participating in a second commitment period, because what is important is for the world is to formulate a new framework which is fair and effective and which all parties will join,” Japanese delegate Masahiko Horie said.

Meanwhile, a series of recent climate reports have underscored the depth of the challenge before the U.N. climate negotiators. A report released Tuesday by the U.N. Environment Program warned current climate projections are likely too conservative because they don’t factor in the thawing of permafrost _ a layer of soil that stays frozen year-round in cold climates.

Lead author Kevin Schaefer, of the National Snow and Ice Data Center in Colorado, said 1,700 gigatons of carbon are locked up in permafrost primarily in the U.S., China, Russia and Canada. He called for further studies on the potential climate impact if it’s released, saying up to 39 percent of total emissions could come from permafrost by 2100.

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AP Environment Writer Michael Casey contributed to this report.

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Karl Ritter can be reached at http://www.twitter.com/karl_ritter