Leaders of the Group of Eight leading industrial countries have agreed to try to limit global warming to just 2 degrees celsius above pre-industrial levels by 2050.
The summit, in the Italian city of L'Aquila, also set tough new targets for carbon emissions considered necessary to achieve the goal.
Developed nations are to cut carbon emissions by 80% by 2050, to allow a global 50% reduction by the same date.
Analysts say there is no indication of how the targets, or costs, will be met.
Developed nations have been criticised for ducking interim goals, and difficult talks still lie ahead as negotiators try to firm up the ambitious goals, the BBC reports.
The cut in carbon emissions is only a target and will need the co-operation of rapidly industrialising such as China and India.
The BBC reports the baseline for the cuts could be later than 1990. That could allow some countries more modest cuts, as emissions in most rose after that date.
British Prime Minister Gordon Brown said he was confident that non-G8 countries would back the commitments when climate change was discussed on Thursday.
Mr Brown said the G8 deal paved the way for a global agreement at the UN conference in Copenhagen in December.
The G8 comprises Canada, France, Germany, Italy, Japan, Russia, Britain and the United States.
Nuclear deadline for Iran
France says G8 members will give Iran until September to accept negotiations over its nuclear ambitions or else face tougher sanctions.
President Nicolas Sarkozy said on Wednesday G8 leaders would review the situation at a G20 meeting in Pittsburgh on 24 to 25 September. He said decisions would have to be taken if there was no progress by then.
Western countries believe Iran is trying to build an atomic bomb. Tehran says it wants to generate electricity for domestic use and has rejected all overtures for talks.
In a separate statement, the G8 said it was committed to finding a diplomatic solution to Iran's nuclear programme.
A nuclear security summit is to be held in Washington in March. United States President Barack Obama says nuclear terrorism is the most immediate global threat.