Major polluting nations reached on Thursday what US President Barack Obama called an "historic consensus" on battling climate change, but only by steering clear of tough pledges to cut greenhouse gases.
The G8 industrialised nations and the nine most important emerging powers agreed during the summit in Italy to limit global climate warming to no more than two degrees Celsius from pre-industrial era levels.
"We also agree that developed countries, like my own, have a historic responsibility to take the lead. We have the much larger carbon footprint per capita," Obama said at an expanded G8 summit in Italy.
"And I know that in the past the United States has sometimes fallen short of meeting our responsibilities, so let me be clear, those days are over."
Despite the agreement, they shied away from committing as a group to specific cuts in greenhouse gas emissions by the middle of the century in order to keep climate warming under control.
UN chief Ban Ki-moon also said the summit represented a lost opportunity to make progress in the build-up to a major climate conference taking place in Copenhagen in December. "The outcomes are not sufficient," he said.
Obama said, however, that he did not expect an instant meeting of minds.
"And while we don't expect to solve the problem in one meeting, or one summit, I believe we have made important strides forward," he said.
"I don't think I have to emphasize that climate change is one of the defining challenges of our time. The science is clear and conclusive and impacts can no longer be ignored.
"Ice sheets are melting. Sea levels are rising. Our oceans are becoming more acidic, and we've already seen its effects on weather patterns, our food and water sources, our health and our habitats," he said.
Leaders of the major industrial powers are facing growing pressure to commit to specific goals for slashing greenhouse gases with the clock ticking down to a key December summit in Copenhagen to fix international targets.
Swedish Prime Minister Frederik Reinfeldt, whose country holds the European Union's rotating presidency, hailed the agreement on the two-degree target as a "major breakthrough."
"But we need to engage more talks with the major economies and also others to enable (us) to meet the two degree target," he said.
Danish Prime Minister Lars Lokke Rasmussen, who will host the Copenhagen summit, said: "We need to translate this target into short term and mid term targets when it comes to reductions.
"Therefore we have a lot of work to do from now to Copenhagen," he added.
The G8 leaders called on rich, developed nations as a group to take steps to cut their greenhouse gases by 80 percent by 2050 so that overall global emissions are halved by then.
However, the broader group of leaders, including many fast industrialising nations such as China and India, dropped any reference to such targets, pledging instead in a joint statement only "to identify a global goal for substantially reducing global emissions by 2050."
"If we are to have a fair chance of staying below two degrees Celsius we must cut global emissions by at least 50 percent compared to 1990 by 2050. This is what science tells us," said European Commission chief Jose Manuel Barroso.
Analyst Niel Bowerman, who heads the Climatico think tank, said that developing nations may be holding out to commit to such specific cuts in carbon emissions in order to "have some cards to play at Copenhagen."