Leaders of the G8 leading industrial countries have agreed to try to limit global warming to just 2C (3.6F) above pre-industrial levels by 2050.
The summit, in the Italian city of L'Aquila, also set targets for carbon emissions considered necessary to achieve the goal.
G8 nations are to cut carbon emissions by 80% by 2050.
But analysts say there are several questions over the announcement, and no indication of how targets will be met.
They say the main problem is that no interim targets have been agreed, and developing nations appear unprepared to accept big cuts to their emissions until developed nations pledge more financial assistance.
Difficult talks still lie ahead as negotiators try to firm up the ambitious goals, correspondents say.
G8 KEY ISSUES/TIMETABLEAlso, the cut in carbon emissions is only a target and will need the co-operation of rapidly industrialising such as China and India.
Our environment analyst Roger Harrabin says: "This is another example of how the politics of climate change is lagging behind the science. For the first time the G8 has accepted there are scientific limits to the amount of greenhouse gases we can emit - the Bush administration wasn't willing to accept that.
"But scientists insist that the rich nations should cut between 25-50% by 2020 to stabilise the climate - and that's a step too far for the G8. The rich nations will now ask the emerging economies to stem the growth of their own emissions - but India will accuse the West of failing to the action that it knows is necessary."
UK Prime Minister Gordon Brown said he was confident 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.
"I hope tomorrow when we meet other countries we'll follow that through and this is a very significant development, the first time it's ever been done," he said.
Security summit
The summit agenda also includes the global economic downturn, food security, terrorism, and nuclear proliferation.
On Wednesday, the G8 leaders issued a statement reaffirming that they were "deeply concerned" by Iran's nuclear programme.
They also condemned North Korea's recent nuclear test and missile launches.
HAVE YOUR SAY"We have all of the technology available to make this happen tomorrow - these are realistic goals, the bigger question is will it be enough to save the human race"
US President Barack Obama announced he would call a summit on nuclear security in Washington next March.
The G8 - Canada, France, Germany, Italy, Japan, Russia, UK and the US - are joined at the summit by leaders or representatives from the G5 group of emerging economies - Brazil, China, India, Mexico and South Africa.
Chinese president Hu Jintao flew back to Beijing amid continuing unrest in the western region of Xinjiang.
On the global economy, an official statement noted "some signs of stabilisation" but that the outlook remained uncertain, with "significant risks".
"We will take, individually and collectively, the necessary steps to return the global economy to a strong, stable and sustainable growth path," the statement said.
Mr Brown said G8 leaders recognised that the path out of recession was not yet secure, citing the recent rise in oil prices to $75 a barrel and fears of rising unemployment.
Earthquake zone
The area where the three-day summit is being held is still suffering aftershocks from the April quake and an evacuation plan is in place in case a serious tremor should hit.
African leaders will join the summit on Friday to push for a new initiative to fund farming in the developing world and tackle global hunger.
Italian Prime Minister Silvio Berlusconi moved the summit from Sardinia to L'Aquila to show solidarity with the victims of April's earthquake.
He took Mrs Merkel on a tour of Onna, a village almost flattened by the quake, and later Mr Obama was given a tour of L'Aquila to survey the damage.
On Wednesday, dozens of protesters occupied four coal power plants in different regions of Italy, demanding tougher measures in fighting climate change, Greenpeace said.
In Rome on Tuesday police said they had arrested 36 people after masked protesters blocked roads, threw objects and set fire to tyres.
The BBC's Bridget Kendall in L'Aquila says the big question hovering over the summit is whether the whole concept of G8 has been outdated and if a bigger gathering - a G20 - is needed to tackle today's problems.
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