Developed and developing nations have agreed that global temperatures should not rise more than two degrees Celsius above 1900 levels, a G8 summit declaration in L'Aquila, Italy, said.
That is the level above which, the United Nations says, the Earth's climate system would become dangerously unstable.
US President Barack Obama said the countries had made important strides in dealing with climate change.
But the G8 failed to persuade developing countries to accept targets of cutting emissions by 50 per cent by 2050.
According to the BBC, on Wednesday, the G8 agreed its own members would work towards 80 per cent cuts by the same date.
UN Secretary General Ban Ki-moon said the G8 had not done enough and should also set 2020 targets.
He said that while the G8's Wednesday agreement was welcome, its leaders also needed to establish a strong and ambitious mid-term target for emissions cuts.
The second day of the summit, in the Italian city of L'Aquila, opened its discussions to take in the so-called G5 nations - Brazil, China, India, Mexico and South Africa. Egypt is a special invitee.
In other developments, the world's biggest economies also agreed to work to reach a global trade deal by 2010.
Leaders of major developed and developing nations have agreed not to resort to competitive currency devaluations
In a joint statement, President Obama and UK Prime Minister Gordon Brown said the detention of British embassy staff by the Iranian authorities was unacceptable
The Major Economies Forum, of 16 developed and developing nations - the G8, G5, Australia, South Korea and Indonesia - plus the European Union, issued the latest declaration.
The group accounts for about 80 per cent of the world's total greenhouse gas emissions.
Obama said developed nations had a "historic responsibility to take the lead."
"We recognise the scientific view that the increase in global average temperature above pre-industrial levels ought not to exceed 2C," it said.
It added that the economies would work towards a global goal for substantially reducing emissions by 2050 between now and December, when the UN holds talks in Copenhagen on a successor to the Kyoto treaty.
Obama, who chaired the meeting, said the countries had had a candid and open discussion about the growing threat of climate change and what must be done both individually and collectively to address it.
"I believe we've made some important strides forward as we move towards Copenhagen," he said.
"I don't think I have to emphasise that climate change is one of the defining challenges of our time. The science is clear and conclusive and the impacts can no longer be ignored."
Nobel Laureate R K Pachauri, who chairs the Inter-governmental Panel on Climate Change, praised the declaration's mention of the 2C limit but said more details were needed.
"It certainly doesn't give you a roadmap on how you should get there but at least they've defined the destination," he told the BBC.
Obama added that the United States, as a major polluter, had not met its responsibilities in the past, but those days were over.
