The world's richest nations agreed last night to cut their carbon emissions by 80 per cent by 2050 in a dramatic attempt to secure a new global deal to combat climate change.

Leaders of the G8 group of countries also agreed to set a limit of C on global temperature rises, the first time they have imposed such a ceiling. In return, they urged developing countries including China and India to cut their emissions by 50 per cent over the same period.

President Barack Obama cleared the way for what Gordon Brown called an "historic agreement" at the G8 summit in Italy by signing the US up to a firm emissions target for the first time – a complete contrast to the intransigence of his predecessor, George Bush. The G8 move is designed to revitalise United Nations-led talks on a global "son of Kyoto" agreement, which reach a climax in Copenhagen in December.

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In talks in L'Aquila today, President Obama will try to persuade nine non-G8 nations, including China and India, to "jump together" with the G8 countries by agreeing to halve their emissions by 2050.

But an immediate breakthrough in today's 17-nation negotiations is unlikely. The Chinese President, Hu Jintao, returned home from Italy abruptly after ethnic tensions increased in China's western Xinjiang territory. The developing nations want firm guarantees of subsidies from the rich nations' club to help them meet the cost of converting their industries to low-carbon technology. They also want the G8 members to be more specific about their interim targets for reducing emissions by 2020.

Another potential stumbling block is the baseline on which the G8's emissions cuts will be calculated. Their declaration left this unclear, prompting critics to describe it as a fudge. Britain backs a 1990 start date but the US favours a later one – meaning it would have to make a smaller reduction. However, officials hope the G8's gesture will draw developing nations into serious and ultimately successful negotiations by December. One said: "There's a long way to go yet. There will be setbacks along the way. But we now have a decent chance of getting there."

Mr Brown said: "For the first time the G8 has agreed [on] what I believe are vital decisions that take us on the road to Copenhagen and change the way we look at energy policy in the future. We have agreed for the first time that average global temperatures must rise by no more than C. That is an historic agreement. We have agreed as the G8 that we want to cut our emissions by 80 per cent by 2050 and we believe that this will allow the world to reduce its emissions by 50 per cent."

The Prime Minister has proposed a $100bn global fund to ease the path to a deal by helping developing countries become more energy efficient. There was no agreement on that last night, while some non-G8 members want a bigger fund.

José Manuel Barroso, the European Commission president, said: "We are not yet where we would like to be but I think things are [moving] in the right direction for Copenhagen."

But the breakthrough failed to satisfy green groups. While welcoming the G8's move, they criticised the group for failing to produce targets for 2020.

Antonio Hill, a spokesman for the charity Oxfam, said: "The G8 might have agreed to avoid cooking the planet by more than C, but they made no attempt to turn down the heat any time soon. 2050 is too far off to matter – poor people are being hit today. We must see emissions cuts of at least 40 per cent by 2020 and G8 money to help the poorest countries cope with climate chaos."

Tobias Muenchmeyer, Greenpeace International's political adviser, said: "While agreeing to keep temperature rise to below C without a clear plan, money or targets on how to do this, the G8 leaders will not have helped to break the deadlock in the UN climate negotiations."

Mr Brown scored a victory over the summit host Silvio Berlusconi by securing a shake-up of the G8's system of aid to the world's poorest nations to stop them backsliding on their promises. With Italy and France unlikely to deliver on pledges they made at the Gleneagles summit four years ago, Mr Brown and Mr Obama joined forces to try to prevent a repeat of the failure.

From now on, the G8 club will publish annual progress reports on the aid given by its members. A review next year, by when the Gleneagles promises were due to be kept, will lead to a "Gleaneagles 2" process so that the G8 can "catch up" by 2015, when the landmark Millennium Development Goals are due to be met. The Italian Prime Minister resisted Mr Brown's move for greater accountability over G8 aid commitments, but Japan and Canada joined the US to ensure that Mr Berlusconi was outmanoeuvred.