The European Union's executive welcomed the communique, saying it kept negotiations on track for a global deal in 2009.
"This is a strong signal to citizens around the world," European Commission President Jose Manuel Barroso said, adding that the EU's benchmark for success had been achieved.
But critics outside the rich nations' club slammed the deal.
Environmental campaign group WWF said the leaders had ducked their responsibilities.
"The G8 are responsible for 62 percent of the carbon dioxide accumulated in the Earth's atmosphere, which makes them the main culprit of climate change and the biggest part of the problem," WWF said shortly after the communique was issued.
"WWF finds it pathetic that they still duck their historic responsibility...," the campaign group said in a statement.
Last year, the G8 - Japan, Britain, Canada, Germany, France, Italy, Russia and the United States - agreed to "seriously consider" a goal of halving global emissions by mid-century.
The European Union and Japan had been pressing for this year's summit to go beyond that, and Brussels wanted clear interim targets as well.
But U.S. President George W. Bush has insisted that Washington cannot agree to binding targets unless big polluters such as China and India rein in their emissions as well.
South African Environment Minister Marthinus van Schalkwyk said he feared this year's communique was actually a step backward.
"While the statement may appear as a movement forward, we are concerned that it may, in effect, be a regression from what is required to make a meaningful contribution to meeting the challenges of climate change," van Schalkwyk said.
The statement puts the focus of fighting global warming on U.N.-led talks to create a new framework for when the Kyoto Protocol expires in 2012. The U.N. talks are set to conclude in Copenhagen in December 2009.
Global warming ties into other big themes such as soaring food and fuel prices being discussed at the three-day summit at a plush mountain-top hotel on the northern Japanese island of Hokkaido, where 21,000 police have been mobilized.
In another statement released on the second day of the summit, the leaders expressed strong concern about sky-high food and oil prices, which they said posed risks for a global economy under serious financial strain.
The group also made a thinly veiled call for China to let the yuan's tightly controlled exchange rate appreciate to help reduce global financial imbalances.
"In some emerging economies with large and growing current account surpluses, it is crucial that their effective exchange rates move so that necessary adjustment will occur," the G8 said in the statement.
The leaders also agreed to bring major oil producers and consumers together in a new forum to discuss energy security. One diplomat said it would also be a venue to talk about output and prices.
The price of food and of oil, which hit a record high of $145.85 a barrel recently, is taking a particularly heavy toll on the world's poor.
A World Bank study issued recently said up to 105 million more people could drop below the poverty line due to the leap in food prices, including 30 million in Africa.
To help cushion the blow, officials said the G8 would unveil a series of measures to help Africa, especially its farmers, and would affirm its commitment to double aid to $50 billion by 2010, with half to go to the world's poorest continent.
The summit has become a magnet for protesters and although Japan has been effective at cracking down on any demonstrations - helped by the remote location of the summit - a few thousand have managed to hold small protests several km (miles) away.
A group of demonstrators marched to the sound of music and drums on July 8, holding signs saying "Smash the G8 summit" and "Free G8 political prisoners".
Tomoyuki Sueoka, a 25-year-old graduate student, said: "G8 nations do not have the right to decide the policies of the world. This is not democratic. They talk about poverty and food shortages but they are simply talking about business."