BARACK Obama and other leaders have backed historic new targets for tackling global warming in an agreement designed to pave the way for a world deal later this year.

For the first time, America and the other seven richest economies agreed to the goal of keeping the world’s average temperature from rising more than 2C.

They also agreed to cut greenhouse gas emissions by 80 per cent by 2050 as they strove for a worldwide deal at Copenhagen in December.

The moves were designed to put the squeeze on the world’s developing nations, most of whose leaders will join the G8 for a debate chaired by Mr Obama today.

There were signs last night that the G13 – the eight joined by China, India, Mexico, South Africa and Brazil – would also sign up to the 2C limit.

Climate Change Minister Penny Wong, who took part in a marathon meeting of world environment ministers in Rome yesterday, ahead of a meeting of leaders of 17 major economies today, welcomed the G8’s agreement.

“We welcome the G8 statement today which says quite clearly climate change is a global challenge, it commits these economies to working together and recognises the science that we need to restrain global warming to around two degrees if we are to avoid dangerous climate change,” Senator Wong told the ABC this morning.

However, Senator Wong said there was still a long way to go to reach agreement on binding targets before December’s global climate change conference in Copenhagen.

Hopes of an international deal remain on a knife edge because earlier yesterday China and India declined to support the objective of halving their greenhouse gas emissions by 2050, and Hu Jintao, the Chinese President, flew home to deal with escalating problems in his own country.

While there are signals that India may be prepared to move, G8 leaders do not expect agreement from the developing countries to halve emissions.

The G8 agreement was also immediately undermined by Russian President Dmitry Medvedev’s top economic aide, who dismissed the target for developed countries to cut emissions by 80 per cent by 2050 as unacceptable.

“For us the 80 per cent figure is unacceptable and likely unattainable,” Arkady Dvorkovich said.

“We won’t sacrifice economic growth for the sake of emission reduction.”

Mr Dvorkovich declined to reveal Russia’s precise targets, saying they ranged from 20 per cent to 60 per cent by 2050.

“We still have the time to agree our positions before Copenhagan,” Mr Dvorkovich said.

Even so, the G8 deal was being hailed by leaders.

British Prime Minister Gordon Brown said the agreement was historic.

“Today in Italy we have laid the foundations for a Copenhagen deal that is ambitious, fair and effective. The change from where we were two, three, four years ago is significant. The world has now agreed that the scientific evidence on climate change is compelling,” he said.

The agreement marks a significant step in efforts to limit greenhouse gases, which are blamed for the world’s rising temperature. The G8 previously had not been able to agree on that temperature limit as a political goal.

It remains only a target, however, and it is far from clear that it will be met, especially as China, India and other rapidly industrialising nations generate and consume more energy from coal and other sources.

Climate change experts say that the 2C threshold would not eliminate the risk of runaway climate change, but would reduce it. Even a slight increase in average temperatures could wreak havoc on farmers around the globe.

Mr Brown also welcomed an agreement from the G8 that “significant risks” remained in the world economy and it was too early to start preparing to exit from growth plans at the moment.

Some leaders, such as Angela Merkel, the German Chancellor, have been calling for early spending cuts to reduce deficits but has been opposed by Britain and America.

Mr Brown pointed to a sentence in the communique saying that exit strategies should only be put into effect “once the recovery is assured”.

At a press briefing he said that the G8 had decided to take all necessary steps individually and collectively to deliver global growth. He had been saying that the G8 needed to sound a second wake-up call on the world economy. “That wake-up call is being heard loud and clear,” he said.

Welcoming the agreement on climate change – made possible by America’s change in stance since Mr Obama succeeded George Bush as president – Mr Brown said: “For the first time the G8 has agreed what I believe are vital decisions that take us on the road to Copenhagen and change the way we look at energy policy.

“We have agreed for the first time that average global temperatures must rise by no more than 2C. We have agreed as G8 that we want to cut our emissions by 80 per cent by 2050 and we believe this will allow the world to reduce its emissions by 50 per cent.”

In chairing the major economies forum today, Mr Obama is signalling that he wants to play a big role in the run-up to Copenhagen. The G8 also called on Iran to allow foreign diplomats and visitors to be allowed to conduct their business unhindered. After the violence following the contested election result a number of British Embassy staff were arrested.

The G8 said embassies in Iran should be allowed to go about their normal business in accordance with the Vienna Convention.