Poznan, Poland - With talks on global warming stalled, the world's eyes are on Barack Obama to break the impasse after he takes over as US president.

In some ways, it's easy. Under President George W Bush, the United States was vilified for its stand on climate change. Obama has promised US leadership in the complex global give-and-take, so he'll be widely welcomed as the anti-Bush.

Winning US congressional approval for his plans will be the crucial challenge.

Even before that, an activist president in the White House - Obama arrives there on January 20 - means other rich countries will face pressure to join in the solution.

If the long-time biggest emitter of greenhouse gases - a nation of petrol-guzzling cars and extravagant energy use - gets serious about cutting fossil-fuel pollution, others have fewer excuses.

Apart from the United States, economic powers like Japan, Canada and Australia have set no specific targets for cutting emissions of heat-trapping gases over the next decade or so. Neither has Russia.

Under Obama, "countries can't hide any more behind the United States," said Stephan Singer, chief climate analyst at the Worldwide Fund for Nature (WWF).

That could help unlock a key conflict at UN climate talks: poorer nations have little incentive to agree to curbs in their fast-rising emissions until industrialized countries set their own goals for 2020.

That dispute is just one of many playing out through December 12 at a UN climate conference in Poznan, Poland. Delegates hope to set the stage for intense bargaining over a global emissions-cutting deal due by next December.

Even the European Union may feel a bit more heat once Obama gets in gear. Developing countries say EU climate goals set out last year are too modest for one of the world's top polluters and biggest economic powers.

"The EU will no longer be seen as the self-proclaimed leader because there is no one else who leads," Singer said. "We need this competition."

After eight years of Bush, Obama has radically shifted the climate debate by pledging to scale back US emissions of gases blamed for global warming to 1990 levels by 2020.

Given that US emissions rose 14.7 per cent from 1990 to 2006, it's a tough task. Already, environmental activists are urging Obama to do more.

WWF's Singer said Obama's near-term plan is a bold step after "the criminal inaction by the Bush administration." But it's "not enough," he said, after the past rise in US emissions.

Although Obama's Democratic Party will have a majority in both houses of Congress, passage of his climate plans is sure to take time - quite possibly beyond the December 2009 deadline for a UN agreement at Copenhagen, Denmark.

"We have to negotiate on the basis of a domestic consensus, and that's now emerging," said Elliot Diringer of the Washington-based Pew Global Centre for Climate Change.

But it's "very unlikely" that Congress will settle on a plan for mandatory cuts in US emissions before 2010, he said.

"Things won't happen overnight," Diringer said. "Which limits what may be achievable in Copenhagen."