L'Aquila, Italy  - The world's greatest powers at last began to talk the talk on climate change when they met in the earthquake-ravaged Italian town of L'Aquila on Thursday.

Now the question is whether they can walk the walk fast enough as they build up to crucial United Nations talks in Copenhagen in December.

"In the last year, it was never possible to get an agreement on the scientific evidence ... We've made huge progress, but the discussions are going to continue right up to the last minute," British Prime Minister Gordon Brown warned.

On Thursday, leaders of major world powers including the United States, China and India agreed for the first time that global warming should be kept to less than 2 degrees centigrade in order to prevent catastrophic weather changes.

That was "an indication that the leaders are taking climate change seriously," Kim Carstensen, climate change expert at environmental lobby group WWF, said.

But the meeting failed to approve any more detailed goals, such as levelling off world greenhouse gas emissions by 2020 and halving them by 2050, despite pressure from European leaders.

That lack means that the summit decisions are "not sufficient," UN Secretary General Ban Ki-Moon said bluntly.

"Much more needs to be done if governments are to seal the deal on a new climate agreement in December," the head of the United Nations said.

Political leaders at the summit hailed the adoption of the 2-degree goal as a breakthrough after years of scepticism on climate change from states such as the US and Australia.

"I know that in the past the US has sometimes fallen short of meeting our responsibilities (on climate change). Let me be clear: those days are over," US President Barack Obama, whose support was seen as key to the agreement, said.

But rather than years, those same countries now have just five months to come to a deal on all aspects of climate change in Copenhagen.

That means not just agreeing on the principles of what the world should do, but answering the far more explosive questions of what exact greenhouse-gas emission reduction targets each country should accept, and how much each one should spend in meeting them.

"It'll be about targets, it'll be about timetables, it'll be about technology and the funding of that technology," Australian Prime Minister Kevin Rudd said.

As just one example of the bridges that remain to be crossed, the world's top five emerging economies are demanding that rich states cut their emissions at least 40 per cent below 1990 levels by 2020.

The best Western offer currently on the table is the 20-per-cent cut offered by the European Union, which will rise to 30 per cent if other developed powers make similar pledges.

Japan's goal, meanwhile, is to cut emissions by 8 per cent below 1990 levels - an improvement of just 2 per cent on the pledge it made when it signed the Kyoto Protocol on climate change.

"While some of the (leading developed) nations have individually agreed to mid-term targets, they are nowhere near enough to bring the developing nations on board," Oxford University climate expert Niel Bowerman said.

Brown pointed out that the 40-per-cent demand from developing nations was an initial negotiating position, saying that "it's got to be an encouragement that we've had this sea-change in the way the world looks at this issue."

But the head of the EU's executive, Jose Manuel Barroso, who as the man who drafted the bloc's bitterly-fought 20-per-cent climate goal has more experience than most of the difficulty of reaching decisions on climate change, warned that with just five months left to reach a deal, the hard work is only beginning.

"The road to Copenhagen and a global deal still remains steep and rocky, and the last stretch is always the most difficult," Barroso said. (dpa)