Rajendra Pachauri also said the panel did not plan to issue more frequent reports as suggested by some governments, reckoning that several years were needed to come up with robust findings. The last series of reports was in 2007.

"We would certainly have much more greater detail," in the next reports, Pachauri told in a telephone interview from Venice, where leading scientists have been meeting from July 13-17 to work on an outline to be approved later this year.

"In the case of clouds we will certainly provide much greater emphasis in this report -- clouds, aerosols, black carbon. These are issues that we will certainly cover in much greater detail," he said.

The 2007 report pointed to cloud formation as a big uncertainty in climate change. Warmer air can absorb more moisture and so lead to more clouds in some regions -- the white tops can reflect heat back into space and offset any warming.

In an opposite effect, black carbon -- or soot from sources such as factories or forest fires -- can blanket ice and snow with a heat-absorbent dark layer and so accelerate a thaw.

"Sea level rise is another issue that...will get much greater in-depth attention," he said.

Scenarios for sea level rise this century in the 2007 report ranged from 18 to 59 cm (7-24 inches). But it said that 59 cm should not be considered an upper limit because of uncertainties about a possible melt of Greenland and Antarctica.

And the panel, the Intergovernmental Panel on Climate Change (IPCC), is also planning an extra report on extreme events such as droughts, floods, heatwaves or mudslides projected because of global warming.

Pachauri said the next report by the IPCC, which shared the 2007 Nobel Prize with former US Vice President Al Gore, was intended to guide nations after the planned agreement of a new UN climate treaty in Copenhagen in December.

He welcomed an agreement by major economies at a Group of Eight summit in Italy last week to recognise a broad scientific view that world temperature rises should not exceed 2 Celsius (3.6 Fahrenheit) above pre-industrial times.

But he said too little was being done to achieve the limit. "It's a step forwards. I wish they would have made some commitments on what would ensure limiting the temperature increase to 2 degrees," he said.

"In the (2007 report) we said if you want to limit temperatures to that range all we have is up to 2015 as the year when global emissions must peak and they must decline thereafter," he said.

Greenhouse gas emissions, mainly from burning fossil fuels, have risen fast in recent years although recession is now curbing industrial activity in many nations. China has overtaken the United States as top emitter.

Of 177 scientific scenarios in the 2007 report, only 6 looked at tough emissions curbs needed to keep temperature rises below 2 Celsius.

Governments have put more funding into scientific research into higher emissions limits that they judge to be more likely.

"We're certainly going to look at much more stringent mitigation," Pachauri said, when asked if governments were still reluctant to put money into looking at curbs needed to achieve the 2 Celsius limit.