OSLO A "hiatus" in global warming so far this century is partly caused by natural variations in a chaotic climate and is unlikely to last, a draft United Nations report by leading climate scientists says.
The 127-page draft, and a shorter summary for policymakers that is due for release in Stockholm on September 27 after editing, say factors including a haze of volcanic ash and a cyclical dip in energy emitted from the sun may also have contributed to a slower warming trend.
Explaining what the Intergovernmental Panel on Climate Change (IPCC) draft calls a "hiatus" in warming is vital for governments, which have promised to agree a U.N. deal by 2015 to limit temperature rises, largely by shifting from fossil fuels.
France called on Friday for bolder EU cuts in greenhouse gases and said it would halve its own energy consumption by 2050.
The fact that temperatures have risen more slowly in the past 15 years despite rising emissions of greenhouse gases has emboldened sceptics who challenge the evidence for man-made climate change and question the need for urgent action.
But the IPCC draft reports do not project any long-term respite. Instead, they forecast a resumption in the warming trend that is likely to cause ever more heatwaves, droughts, floods and rising sea levels.
"Barring a major volcanic eruption, most 15-year global mean surface temperature trends in the near-term future will be larger than during 1998-2012," according to the 127-page Technical Summary dated June 7 and obtained by Reuters.
Temperatures are likely be 0.3 to 0.7 degree Celsius (0.5-1.3 Fahrenheit) higher from 2016-35 than from 1986-2005, it says. The reports by the IPCC, updating an overview of climate change from 2001, are the main guide for government action.
"Fifteen-year-long hiatus periods are common" in both historical records and in computer models, the technical summary says. But scientists were caught out - in one computer model, 111 of 114 estimates over-stated recent temperature rises.
The drafts predict that temperatures could rise by up to 4.8C (8.5F) this century - far above a ceiling set by governments of 2C (3.6F) above pre-industrial times to avoid dangerous changes to nature and society.
With deep cuts in greenhouse gas emissions, the rise could be kept to just 0.3C (0.5F), the draft says.
Many experts agree that natural variations in the weather, caused by factors such as shifts in ocean currents or winds, can mask a warming trend even with a continued build-up of carbon dioxide in the atmosphere.
The hiatus "is not a sign that the warming trend has gone," Guy Brasseur, director of the Climate Service Center in Germany. He said the climate was comparable to Wall Street - there were often long-term trends with unpredictable daily swings.
Brasseur and other experts contacted were stating their own views, not referring to details of the coming report.
"There are a number of explanations (for the hiatus), any one of which might be correct," said professor Myles Allen of Oxford University, who contributed to the IPCC draft. "That is very different from saying:'We have no idea what's going on'."
The drafts say that a reduction in warming for 1998-2012 compared to 1951-2012 is "due in roughly equal measure" to natural variations in the climate and factors such as "volcanic eruptions and the downward phase of the current solar cycle."
Volcanoes spew ash into the air that can dim sunlight and so cool the surface of the planet. The sun was in a downward cycle of output - meaning that it was emitting less energy - during most of the period.
The technical summary says that warming from 1998-2012 slowed to 0.05 degree C (0.09F) per decade, against 0.12 (0.2F) per decade from 1951-2012. But the decade to 2012 was the warmest since records began in the mid-19th century.
It says another factor could be that computer models consistently over-estimate warming. Some experts argued that near-term projections of temperature rises should be cut by 10 percent, it said.
Other theories include that more heat is going into the oceans or that air pollution is dimming sunlight.
An academic report last month said a cooling of the Pacific Ocean, linked to natural La Nina events that bring cooler waters to the surface, was the main explanation.
The IPCC draft also says the planet may be somewhat less sensitive than expected to a build-up of carbon dioxide in the air. A doubling of atmospheric carbon dioxide levels from pre-industrial times is likely to mean an eventual temperature rise of between 1.5 and 4.5 C (2.7-8.1F), down from 2.0 to 4.5 (3.6-8.1F) estimated in 2007, it says.
(Editing by Mark Trevelyan)