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Global warming slowing: What does that mean?

Global warming is slowing but greenhouse gases keep rising. What's happening? Scientists say the global warming lull is likely due to heat going deep into the ocean and natural climate fluctuations.

By Karl RitterAssociated Press / September 19, 2013

Halldor Thorgeirsson, right, a senior director with the United Nations Framework Convention on Climate Change, and researcher Brian Hoskins take questions at London's Imperial College on Tuesday, Sept. 17, 2013. Thorgeirsson said international leaders are failing to fight global warming, appealing directly to the world's voters to pressure their politicians into taking tougher action against the buildup of greenhouse gases.

(AP Photo/Raphael Satter)

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Stockholm

Scientists working on a landmark U.N. report on climate change are struggling to explain why global warming appears to have slowed down in the past 15 years even though greenhouse gas emissions keep rising.

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Leaked documents obtained by The Associated Press show there are deep concerns among governments over how to address the issue ahead of next week's meeting of the Intergovernmental Panel on Climate Change.

Climate skeptics have used the lull in surface warming since 1998 to cast doubt on the scientific consensus that humans are cooking the planet by burning fossil fuels and cutting down CO2-absorbing forests.

The IPCC report is expected to affirm the human link with greater certainty than ever, but the panel is under pressure to also address the recent lower rate of warming, which scientists say is likely due to heat going deep into the ocean and natural climate fluctuations.

"I think to not address it would be a problem because then you basically have the denialists saying, 'Look the IPCC is silent on this issue,'" said Alden Meyer, of the Washington-based Union of Concerned Scientists.

In a leaked June draft of the report's summary from policy-makers, the IPCC said the rate of warming in 1998-2012 was about half the average rate since 1951. It cited natural variability in the climate system, as well as cooling effects from volcanic eruptions and a downward phase in solar activity.

But several governments that reviewed the draft objected to how the issue was tackled, in comments to the IPCC obtained by the AP.

Germany called for the reference to the slowdown to be deleted, saying a time span of 10-15 years was misleading in the context of climate change, which is measured over decades and centuries.

The U.S. also urged the authors to include the "leading hypothesis" that the reduction in warming is linked to more heat being transferred to the deep ocean.

Belgium objected to using 1998 as a starting year for any statistics. That year was exceptionally warm, so any graph showing global temperatures starting with 1998 looks flat, because most years since have been cooler. Using 1999 or 2000 as a starting year would yield a more upward-pointing curve.

Hungary worried the report would provide ammunition for skeptics.

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