Global warming hiatus unlikely to last, says draft UN report

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Published: September 26, 2013

Warming trend to resume Report offers explanations for documented slowdown in global warming

OSLO, Norway (Reuters) — A recent hiatus in global warming is partly caused by natural variations in a chaotic climate and is unlikely to last, says a draft United Nations report by leading climate scientists.

The 127-page draft and a shorter summary for policymakers 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.

It’s vital that governments explain the reasons for the hiatus. They have promised to act on a UN deal to limit temperature rises by 2015, largely by shifting from fossil fuel.

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The fact that temperatures have risen more slowly in the past 15 years, despite rising emissions of greenhouse gases, has emboldened skeptics who challenge the evidence for man-made climate change and question the need for urgent action.

However, the draft reports from the UN’s Intergovernmental Panel on Climate Change do not project a long-term respite. Instead, they forecast a resumption in the warming trend, which is likely to cause more heat waves, 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 technical summary dated June 7.

Temperatures will likely be 0.3 to 0.7 C higher from 2016-35 than from 1986-2005, it says. The reports, 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 said.

However, 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 4.8 C this century, far above a ceiling set by governments of 2 C above pre-industrial times to avoid dangerous changes to nature and society.

Deep cuts in greenhouse gas emissions could keep the rise to just 0.3 C, the draft added.

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 buildup of carbon dioxide in the atmosphere.

“(The hiatus) is not a sign that the warming trend has gone,” said Guy Brasseur, director of the Climate Service Center in Germany.

He said the climate was comparable to Wall Street, which often has long-term trends with unpredictable daily swings.

Brasseur and other experts contacted were stating their own views rather than 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 said 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 cool the surface of the planet. The sun was in a downward cycle of output during most of the period, meaning it was emitting less energy.

The technical summary said that warming from 1998-2012 slowed to 0.05 C per decade compared to 0.12 per decade from 1951-2012. However, the decade to 2012 was the warmest since records began in the mid-19th century.

The summary said 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 from earlier this summer 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 said the planet may be less sensitive than expected to a buildup 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 1.5 to 4.5 C, down from 2 to 4.5 C estimated in 2007, it said.

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