"It is now urgent that the parties to Kyoto get serious about negotiating the successor agreement. They need to come together on an agreement that is bold," Leape said in an interview at the WWF headquarters.

"This is not a time where we can afford a hiatus in the effort," he said.

Diplomats at a United Nations ministerial meeting on climate change next month need to pledge commitment to a new deal, and set a schedule for its completion by the end of 2008 to ensure a seamless shift, said Leape, who will attend the Nairobi talks.

Last year the ministers agreed to work out the post-2012 rules as soon as possible but set no deadline.

Many experts say 2008 is too soon and that a deal may have to wait until after US President George W. Bush, a Kyoto opponent, leaves office in January 2009.

Leape described calls from German Chancellor Angela Merkel for the European Union to cut carbon emissions to up to 30 percent below 1990 levels by 2020, and from California Gov. Arnold Schwarzenegger to reduce his state's emissions to 80 percent below the 1990 benchmark by 2050, as the right magnitude for a successor agreement to Kyoto.

"These two leadership statements give you a sense of the trajectory we need to be getting on, and that is the challenge for these talks," Leape said. "These are the kind of targets these negotiations should be aimed at."

STRICTER PACT

Certainty that a broader, stricter pact will follow Kyoto would encourage companies currently building power plants -- that will be running when the new deal comes into effect -- to invoke energy-saving and emissions-cutting plans now, he said.

"Then at least industry knows what it is dealing with and can act accordingly," Leape said.

Most scientists think the planet's average temperature will continue to rise through the 21st century due to atmospheric accumulations of heat-trapping gases such as carbon dioxide, formed from burning fossil fuels for power and transport.

Keeping global warming below a 2 degree Celsius (3.6 F) rise -- a threshold above which many environmentalists foresee "dangerous" impacts such as extreme weather, animal and plant species loss and widespread water shortages -- will require immediate emissions cuts as well as long-term planning through a new climate change accord, Leape said.

"If we are going to succeed in avoiding catastrophic climate change we have to begin to see a decline in global carbon emissions in 10 to 15 years. If we are going to see that happen, we need very aggressive action in the industrialised countries now," Leape said.

While some have questioned the feasibility of capping temperature rises below 2 degrees, Leape said: "It is possible."

"What we are calling on Europe to lead in, and other countries to do, is to step out and make the sort of bold commitments that are required," he said.