A slowdown in warming that has provided fuel for climate sceptics is one of the thorniest issues in a report to be issued by UN experts on Friday.

Over the past 15 years, the world's average surface temperature rose far slower than many climate models have predicted.

According to projections, global warming should go in lockstep with the ever-rising curve of heat-trapping carbon emissions.

But in recent years, warming has lagged. So, where has the missing heat gone?

For climate sceptics, the answer is clear. Either the computer models used to project temperature rise are flawed, or man-made global warming is just a green scam, they say.

The report of the Intergovernmental Panel on Climate Change (IPCC) will confirm warming has recently slowed.

The document, being debated line-by-line in Stockholm, is the first volume of a vast trilogy that will be released by the Nobel-winning group over the coming months, and only its fifth overview in a quarter of a century.

Over the past 50 years, the mean global temperature rise was 0.12 degrees Celsius (0.2 degrees Fahrenheit) per decade, slowing to an average 0.05 C (0.09 F) per decade over the past 15 years.
Half of the slowdown could be attributed to volcanic eruptions, whose particles reflect sunlight, and a bigger-than expected drop in heat from the Sun's changing activity cycle, said a summary of the report.
The other half is attributed to a "cooling contribution from internal variability".

Laurent Terray with the French computer modelling agency Cerfacs said the term is used to explain a shift in the way heat is distributed between land, sea and air.
Still unclear is what causes the variation or determines its duration.

New research by Britain's Met Office suggests the "missing" heat, or some of it, is being transferred from the ocean surface to the deeps.
Temperatures at depths below 3,000 metres (10,000 feet) have been rising since the 1990s, implying a source of heat-trapping today will contribute to warming tomorrow.

Governments, which have the right to vet and amend the summary but not the main text on which it is based, are looking with concern at the brief section on the warming pause.

This reflects jitters after the IPCC's last big report in 2007 was shown to contain several background errors, denting the agency's credibility. The panel's main conclusions were not affected but the mistakes were a windfall for sceptics.

Together with the 2008 financial crisis and the disastrous 2009 Copenhagen UN climate summit, this almost sent global warming into political limbo.

In comments of the IPCC summary draft, China, India and Norway want to know why the section dealing with the warming pause fails to refer to the role of the deep ocean. Others complain the text is dangerous gobbledegook.

(Source: AFP)