There have been deep divisions over how to curb global emissions
World leaders say they will aim to set a global target of cutting carbon emissions by at least 50% by 2050 in an effort to tackle global warming.
It strengthens last year's G8 pledge to "seriously consider" the cuts.
But the US has refused to set any interim targets for cutting emissions - and environmentalists have criticised the progress at talks as "pathetic".
Five of the world's biggest emerging economies said the G8 should increase its targets to more than 80% by 2050.
China, India, Mexico, Brazil and South Africa - who will join talks on Wednesday - also urged developed countries to commit to an interim target of a 25-40% cut below 1990 levels by 2020.
Climate change has been one of the stickiest issues tackled at the summit in Japan, with divisions over what targets should be set and what would be expected of developing countries.
The BBC website's environment correspondent, Richard Black, says the joint statement, in fact, is exactly what leaders of nearly 200 countries signed up to in the original UN climate change convention agreed at the 1992 Earth Summit.
He says that if re-stating a 16-year-old commitment is progress, then this is clearly a success.
It leaves many ends untied - including the failure to specify a baseline date.
The EU wanted the G8 to confirm that the 50% cut would be measured from 1990 levels of CO2 - as agreed under the Kyoto climate protocol.
But when the question was raised in a press conference Japanese Prime Minister Yasuo Fukuda said the cuts would be measured from "current levels".
Our correspondent says this is significant in several ways, not least because a 50% cut from now is worth far less than a 50% cut from 1990 levels.
The G8 statement repeats last year's "vision" to reach the target of cutting emissions by at least 50% by 2050 - but this time adds that the effort must be global.
It also acknowledges that to make progress, G8 countries have to take the lead through ambitious interim goals and national plans.
Our correspondent says the US has moved a certain amount by agreeing that a long-term concrete target is desirable.
But, he says, the deal shows much less ambition than many climate scientists and environmental groups would want, in particular by avoiding setting short-term targets.
The statement also implies that G8 members will adopt the 50% figure only if major developing economies agree to some concrete action, and it is by no means certain that they will, our correspondent adds.
South Africa's government earlier rejected the G8 agreement as a "regression", criticising the lack of firm targets to achieve sufficient cuts in emissions.
The global environmental group WWF said the target date of 2050 was insufficient and called the lack of progress "pathetic".
In their assessment of the global economy released on Tuesday, the G8 leaders expressed serious concerns at the threat posed to the global economy by soaring oil prices.
The price of crude oil has doubled since the last G8 summit, with highs of more than $146 (£72) a barrel.
But they said they remained positive about the long-term resilience of their economies, so long as countries resisted the introduction of trade barriers.
The G8 has issued statements on several key issues:
- Aid and development: Commitment to fulfil earlier pledge to raise annual aid levels by $50bn by 2010, of which $25bn is intended for Africa
- Global food prices: Call for countries with sufficient food stocks to release reserves to others struggling to cope with rising costs
- Biofuels: Pledge to ensure biofuel policies are compatible with food security
A statement is also expected on the disputed elections in Zimbabwe, which President George W Bush on Monday described as a sham.
In their first face-to-face meeting, on the summit's sidelines, Russian President Dmitry Medvedev told Britain's Prime Minister, Gordon Brown, that he wanted to normalise ties, which are at their lowest ebb since the Cold War.
Relations soured after Russia refused to extradite the chief suspect in the poisoning of the former Russian intelligence officer, Alexander Litvinenko, in London two years ago.
The meeting is taking place in Toyako, on the Japanese island of Hokkaido.
Leaders from the G8 nations - Britain, Canada, France, Germany, Italy, Japan, Russia and the United States - are being joined by counterparts from some 15 other countries.