The apparent halt to global warming so far this century is unlikely to last much longer according to draft technical report by UN’s Intergovernmental Panel on Climate Change (IPCC). The report, by 259 authors in 39 countries, is the first of four due in the next year about climate change by the IPCC.

The 127-page draft, which was obtained by Reuters, and a shorter summary for policymakers that are being finalized for release in Stockholm this Friday, 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.

The IPCC drafts also say 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 1.5º and 4.5º C, down from 2.0º to 4.5 º C estimated in 2007.

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 IPCC draft, dated 7 June, forecasts 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," said the draft report. Temperatures are likely be 0.3º to 0.7º C higher from 2016-35 than from 1986-2005, it says.

Increased certainty
Based on advances in the study of climate change, the IPCC is preparing the strongest warning yet that climate change is man-made. The drafts seen by Reuters say that human activities, primarily the burning of fossil fuels, are "extremely likely" - at least a 95 percent probability - to be the main cause of warming since the 1950s. The likelihood is up from 90 percent in the last report in 2007 and 66 percent in 2001.

"There is high confidence that this has warmed the ocean, melted snow and ice, raised global mean sea level, and changed some climate extremes," the draft says of man-made warming, projecting that most impacts will get worse unless sharp cuts are made to greenhouse gas emissions.

The draft says temperatures could rise by up to 4.8º C this century, but could be held to a rise of 0.3 º C with deep cuts in greenhouse gas emissions. Governments have promised to limit a rise in temperatures to 2º C above pre-industrial times. The range differs from the IPCC’s 2007 scenarios of global temperatures increasing by between 1.1º and 6.4º C by 2100, largely because of new computer models.

The draft also says that sea levels, which rose 19 cm in the 20th Century, could rise by an extra 26 to 81 cm towards the end of this century, in a threat to coasts. That rise is more than was projected in 2007 as that report did not take full account of the risks of melting ice sheets in Greenland and Antarctica.

Constraining factors

The technical summary says that warming from 1998-2012 slowed to 0.05º C per decade, against 0.12º per decade from 1951-2012. It says this 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.

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.