Leaders of the world's established and emerging economic powerhouses will be under pressure at the G8 summit to give a badly-needed jolt to the quest for a new global climate pact.

Negotiators struggling to seal an agreement in Denmark by the end of this year say the three-day summit is a golden opportunity to haul the process out of a rut.

"It's time to make a difference," said Yvo de Boer, executive secretary of the UN Framework Convention on Climate Change, where the post-2012 treaty is being brokered.

"It's one of those moments in history where leadership will be remembered, but failure will also be remembered."

Together, their economies account for a whopping 80 per cent of the heat-trapping "greenhouse gas" emissions driving dangerous climate change.

Climate change has been an automatic part of the G8 since the Gleneagles summit in Scotland in 2005.

But the results of three successive summits and lengthy communiques have been meagre, mainly due to a rearguard action by former US president George W. Bush, fighting demands for tough emissions curbs.

With little more than five months left to the talks in Copenhagen many hopes are riding on Barack Obama, who has ripped up Mr Bush's climate strategies.

The G8 has prepared a draft communique calling on global emissions to peak by 2020 and then be "substantially reduced" to peg the rise in global temperatures to two degrees Celsius over pre-industrial levels.

Alden Meyer of the Union of Concerned Scientists, a US non-governmental organisation, said the 2 C objective was big news.

According to a German source, a push by Berlin for the G8 to agree a common goal for 2020 has so far met with a lukewarm reception, even from fellow Europeans. The EU is targeting an eventual 20 per cent fall on emissions, compared to 1990.

For its part, the US has set itself a target of a 17 per cent reduction by 2020 over 2005 levels, which represents only a four per cent fall on 1990, a goal which the EU regards as insufficient.

The G8 countries are reluctant to commit themselves to specific goals until they see strong promises on how emerging powers will avoid becoming the carbon culprits of tomorrow, he said.