The Group of Eight countries - the United States, Japan, Russia, Germany, France, Britain, Canada and Italy - also called on all major countries such as China and India to join in the effort to stem the potentially dangerous rise in world temperatures.
"This global challenge can only be met by a global response, in particular, by the contributions from all major economies," the G-8 said in a joint, five-page communique on climate.
The G-8 last year at a summit in Germany pledged to seriously consider the same target, and this year's Japanese hosts had hoped to solidify that commitment at the current meeting in Toyako, northern Japan.
The G-8 has been under pressure to voice commitments by wealthy nations to push forward stalled U.N.-led talks on forging a new accord to battle global warming by the end of next year to succeed the troubled Kyoto Protocol when its first phase expires in 2012.
The United States hailed the agreement as substantial progress, and a top European Union official called it a "new, shared vision" by wealthy nations on climate.
Tuesday's statement, however, addressed total world emissions rather than just those produced by wealthy countries, and critics attacked it for failing to go much beyond the G-8 statement last year. The communique also did not set a base year from which emissions would be cut.
"So little progress after a whole year of minister meetings and negotiations is not only a wasted opportunity, it falls dangerously short of what is needed to protect people and nature from climate change," said Kim Carstensen, Director WWF Global Climate Initiative.
Environmentalists have argued that the 50 percent reduction target was insufficient, and have clamored for ambitious midterm targets for rich countries to cut emissions by 2020. Japan itself has set a national target for cutting emissions by between 60 percent and 80 percent by 2050, but has not yet set a midterm goal.
"To be meaningful and credible, a long term goal must have a base year, it must be underpinned by ambitious midterm targets and actions," said Marthinus van Schalkwyk, South African Minister of Environmental Affairs and Tourism. "As it is expressed in the G8 statement, the long term goal is an empty slogan."
Shorter-term targets have been much more difficult to reach consensus on, since they would require much quicker action than long-term goals. The United States, for instance, has argued that meeting a Europe-supported goal of reducing emissions by between 25 percent and 40 percent by 2020 is unrealistic.
In a nod to such disagreements, Japanese Prime Minister Yasuo Fukuda - the summit host - said the G-8 countries would set individual targets, and he did not mention a range. The statement also said that the issue would be discussed in talks on Monday among the 17-member Major Economies Meeting, a U.S.-led group working on climate change.
"The G-8 will implement aggressive midterm total emission reduction targets on a country-by-country basis," Fukuda said.
The agreement also urged nations to set high goals for energy efficiency, promote clean energy and technologies, and mobilize financing to help poor nations cut their own emissions and grapple with the effects of warming.
Scientists say urgent action is needed to make greenhouse gas emissions peak in the next 10 to 15 years, and then to steeply fall to limit the increase in global temperatures to under 2 degrees Celsius (3.6 degrees Fahrenheit). Temperatures beyond that could trigger the worst effects of warming, such as melting ice sheets and extreme weather.
The U.N.-led climate talks have been plagued by divisions. Quickly developing nations have urged wealthy countries to take the first, toughest steps. The United States, Japan and others, meanwhile, say they want to hear what up-and-coming economies like China are willing to do.
The Europeans have pushed harder for rich countries to reinvigorate talks by making unilateral commitments. Germany, for instance, has pledged to cut emissions by 20 percent by 2020, and by 30 percent if other countries join the effort.
The United States said Tuesday's pact fit with its stance that all major economies need to participate in reducing emissions.
"It has always been the case that a long term goal is one that must be shared. So the G-8 has offered today is a G-8 view of what that goal could be and should be but that can only occur with the agreement of all the other parties," said Jim Connaughton, chairman of the White House's Council on Environmental Quality.
European Commission President Jose Manuel Barroso said the agreement would support the U.N.-led effort.
"This is a strong signal to citizens around the world," he said in a statement, calling for a renewed push behind the U.N. talks, which aim to conclude a new pact at a meeting in Copenhagen, Denmark, in December 2009.
Meanwhile, Russian President Dmitry Medvedev said Tuesday that his first meeting with U.S President George W. Bush since taking office brought no progress toward bridging deep disagreements between the former Cold War foes.
Deeply wary of creeping Western clout in former Soviet republics and satellite states, Russia adamantly opposes the Bush administration's plans to deploy missile defense installations in Central Europe and its support for bids by Georgia and Ukraine to join NATO.