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 have 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 latest declaration was issued by the Major Economies Forum, of 16 developed and developing nations - the G8, G5, Australia, South Korea and Indonesia - plus the European Union.
The group accounts for about 80% of the world's total greenhouse gas emissions.
Barack 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.
President 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."
Early agreement 'unlikely'
RK 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 World Service Newshour programme.
G8 KEY ISSUES/TIMETABLE
THURSDAY: Climate Change
Brazil, China, India, Mexico, South Africa, Egypt join talks
1230 GMT - Junior G8
1300 GMT - Major Economies Forum meeting
0630 GMT - crisis' impact on Africa with African leaders attending
0830 GMT - food security
1100 GMT - final news conference
G8 members: Britain, Canada, France, Germany, Italy, Japan, Russia, US
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