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UN panel: 'Extremely likely' that human activity behind most global warming

The UN Intergovernmental Panel on Climate Change concludes that the past decade has been the warmest on record and, with medium confidence, that the last three decades are the warmest in 1,400 years.

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"We got quite a few review comments on various chapters saying: What's going on here? We need to assess what we know" about the hiatus, he said during a briefing Friday morning.

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He attributed the oversight to a tendency of each group working on each of the 14 chapters to rely on some other chapter to deal with the issue. And anyone who was thinking about it at all thought some other chapter should handle the issue.

The result is a statement that the slowdown in the rate of warming over the past 15 years is – with medium confidence – due equally to natural variability in the climate system and to a combination of changes in what researchers dub climate "forcings": in this case accumulated aerosols from a spate of midsize volcanic eruptions in the late 1990s, which have a cooling effect, and a decade that spent most of its time on the downside of the sunspot cycle. As the number of sunspots fall, solar radiation reaching Earth is reduced. While those reductions are tiny in absolute numbers, researchers have uncovered mechanisms by which the climate system can amplify the effect of those small changes.

"This does not mean that global warming has stopped, because the ocean is still taking up heat, sea level is still rising, ice is still melting everywhere we look," Dr. Marotzke said. Instead, he suggested, this likely is a confluence of conditions where, in Yahtzee terms, the system rolled three dice and all came up sixes.

Still, the group, which relies on studies published in peer-reviewed journals for its overviews, didn't have much to go on, acknowledges Working Group 1's co-chairman, Dr. Stocker. "I'm afraid to say there is not a lot of published literature that allows us to delve deeper into the required depth of this emerging scientific question," he says, citing a lack of adequate measurements of ocean heating, especially in the deep ocean, as one hindrance. This is one mechanism scientists have proposed for moderating the rise in surface temperatures.

One explanation that scientists skeptical of this explanation have offered for the failure of climate models to foresee this hiatus holds that the models reconstruct a climate system that is too sensitive to rising CO2 concentrations.

Indeed, the new report modifies slightly the IPCC's estimate of how touchy the climate is to changes in greenhouse gas levels compared with the reports the IPCC issued in 2007.

Then, researchers estimated that if CO2 concentrations doubled over pre-industrial levels, one could expect global average temperatures to rise by 2 degrees to 4.5 degrees Celsius (3.6 to 8 degrees F.), with a 3-degree increase as the most likely. Friday's summary widens that to 1.5 to 4.5 degrees, with no figure given in the summary as a most-likely number.

The authors attribute this change to improved estimates of past temperatures, the factors "forcing" climate, and a better understanding of how the climate system works.

Still, under each of four emissions scenarios the working group worked with, temperatures and sea levels continue to rise through the end of the century – with the end-of-century levels determined by the magnitude of additional increases in atmospheric concentrations of greenhouse gases.

Using updated observations of land-ice loss and considering new insights into the contributions of calving icebergs compared with gradual melting, researchers now estimate the global average sea levels could rise from a quarter to half a meter by 2100, under the most rosy emissions scenario, versus half a meter to a meter under the highest emissions scenario, compared with levels between 1986 and 2005. That represents a significant increase over estimates made in the 2007 reports.

Global average temperatures are expected to increase by 0.3 and 0.7 degrees C by 2035. By the last two decades of this century, temperatures are expected to rise between 0.3 and 1.7 degrees C under tight emissions controls to between 2.6 to 4.8 degrees C with no controls, compared with global average temperatures between 1986 and 2005.

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