"Many countries moved on this issue," Merkel said on Thursday, adding the accord gave impetus to negotiations beginning in Bali in December to find a successor to the UN-backed Kyoto protocol on capping greenhouse gases that expires in 2012.
"The G8 summit is nothing but media attention and wasted money"
Politics, Cambridge, UK
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Merkel said that talks on a post-Kyoto pact would take place within the framework of the UN.
The deal is the most serious commitment to action on climate change by the US, the world's largest global warmer.
Washington had initially resisted attempts by Merkel to set a goal for cuts needed to combat a warming of the Earth's surface, which many scientists say will cause sea levels to rise, and more droughts and floods.
But she secured a partial victory by securing an inclusion of the target in a document put out to summit leaders.
Environmentalists, however, have condemned the G8's failure to agree on specific, binding goals.
Neil Adger of the UK's Tyndall Centre for Climate Change Research, said: "Agreeing on a numerical target is a significant first step, and not taking that first step is going to condemn us to a lot of pain and suffering in terms of the impacts of climate change."
While Daniel Mittler of Greenpeace said that Bush had not agreed to anything worthwhile.
"He has agreed to seriously consider [the goal of cutting gas emissions by 50 per cent by 2050], which I think we can translate as, 'Bush will watch while the rest of the world acts'".
"Bush has rejected binding emission cuts, which is what is really necessary."
Merkel is hosting three days of talks at a Baltic coast resort with counterparts from Britain, Canada, France, Italy, Japan, Russia and the US.
Vladimir Putin, the Russian president, and George Bush, the US president, have also begun talks on the sidelines of the G8 summit amid a dispute over a US missile defence system.
Russia has been angered by the US plan for a missile defence system in Central Europe, while Bush has accused Putin's government of seeking to roll back democratic reforms.
|Merkel, centre, is hosting the talks [EPA]|
On Thursday, Bush said that the missile shield dispute was "not something we should hyperventilate about".
Russia believes it is the sole target of the US missile shield, which would be in Poland and the Czech Republic, and Putin has threatened to aim Russian missiles at European targets if the deployment goes ahead.
Bush said he was "looking forward" to meeting Putin despite the dispute.
"I will explain to him once again that a missile defence shield is aimed at a rogue regime that may try to hold Russia and or Europe hostage.
"It is important for Russia and the Russians to understand that I believe the Cold War ended, that Russia is not an enemy of the United States, that there's a lot of areas where we can work together."