STOCKHOLM—Scientists working on a landmark United Nations 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 forests that absorb carbon dioxide.

The IPCC report is expected to affirm the human link with greater certainty than ever, but the panel is under pressure to 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 then have been cooler. Using 1999 or 2000 as a starting year would yield a more upward-pointing curve.

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 19 centimetres since 1901.

Many researchers say the slowdown in warming is related to the natural ocean cycles of El Niño and La Nina. 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.