STOCKHOLM (AP) -- 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.
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.
Many skeptics claim that the rise in global average temperatures stopped in the late 1990s and their argument has gained momentum among some media and politicians, even though the scientific evidence of climate change is piling up: the previous decade was the warmest on record and, so far, this decade is even warmer. Meanwhile, Arctic sea ice melted to a record low last year and the IPCC draft said sea levels have risen by 7.5 inches (19 centimeters) since 1901.
Many researchers say the slowdown in warming is related to the natural ocean cycles of El Nino and La Nina. Also, a 2013 study by Kevin Trenberth at the National Center for Atmospheric Research found dramatic recent warming in the deeper oceans.
Stefan Rahmstorf, a German climate scientist, said it was possible that the report's authors were feeling pressured to address the warming slowdown because it's received so much attention recently.
"I think a lot of the interest in this topic in the science community has been triggered by the public debate about it," said Rahmstorf, who was a reviewer for the report's chapter on sea levels.
Jonathan Lynn, a spokesman for the IPCC, declined to comment on the content of the report because it hasn't been finalized, but said it would provide "a comprehensive picture of all the science relevant to climate change, including the thousands of pieces of scientific research published since the last report in 2007 up to earlier this year."
The IPCC draft report says it's "extremely likely" that human influence caused more than half of the warming observed since the 1950s, an upgrade from "very likely" in the last IPCC report in 2007.
The panel also raised its projections for sea level rise to 10-32 inches (26-81 centimeters) by the end of the century. The 2007 report predicted a rise of 7-23 inches (18-59 centimeters).
Continued carbon emissions at or above current rates "would induce changes in all components in the climate system, some of which would very likely be unprecedented in hundreds to thousands of years," the IPCC said in the draft. A final version will be presented at the end of the panel's meeting in Stockholm next week.
The IPCC's conclusions are important because they serve as the scientific underpinnings of U.N. negotiations to rein in emissions of CO2 and other greenhouse gases. A global climate treaty is supposed to be adopted in 2015.
AP Science Writer Seth Borenstein in Washington contributed to this report.