He welcomed ratification of the Kyoto pact on Friday by Russia's lower house of parliament, paving the way for the long-delayed 1997 accord to enter into force in the 126 nations that approved it even though the world's greatest polluter, the United States, pulled out in 2001.

"This mustn't lull us into thinking that the problem is solved," Pachauri said. "Kyoto is not enough. We now have to look at the problem afresh."

Kyoto is a first step toward curbing emissions of gases like carbon dioxide, mainly from burning fossil fuels, that scientists blame for trapping heat in the atmosphere like the panes of glass in a greenhouse.

Rising concentrations could melt icecaps, swamp low-lying coastal regions and trigger catastrophic changes to the planet's climate with more volatile weather from typhoons to droughts.

Pachauri urged the world to shift strategy from Kyoto's reduction targets for greenhouse gases to long-term global targets on how much of the gases the atmosphere should contain.

Carbon dioxide levels have risen about 30 percent since the start of the 18th century to almost 380 parts per million.

"We need a degree of agreement on where to stabilize concentrations," he said. "We have to try to come up with an understanding of where we are heading in the next 30-40 years."

Pachauri leads work to produce a 2007 U.N. climate report based on research by more than 2,000 scientists, updating a 2001 assessment that concluded there was "new and stronger evidence" that human activities were to blame for rising temperatures.

"My hope is that this (2007 report) will be able to fill gaps, reduce uncertainties and produce a much stronger message," said Pachauri, who is based in New Delhi.

Under Kyoto, developed nations have agreed to cut emissions of carbon dioxide by at least five percent below 1990 levels by 2008-12 by restraining use of coal, oil and natural gas and shifting to renewable energies like wind and solar energy.