RUSUTSU, JAPAN - President Bush and leaders of the world's richest nations pledged Tuesday to "move toward a low-carbon society" by cutting greenhouse gas emissions in half by 2050, the latest step in a long evolution by a president who for years played down the threat of global warming.

The declaration by the Group of Eight -- the United States, Japan, Germany, Britain, France, Italy, Canada and Russia -- is the first time that the Bush White House has publicly backed an explicit long-term target for eliminating the gases that scientists have said are warming the planet. But it failed to set a similar goal for cutting emissions over the next decade and drew sharp criticism from environmentalists, who called it a missed opportunity.

In exchange for agreeing to the "50 by 2050" language, Bush got what he has sought as his price for joining an international accord: a statement from the rest of the Group of Eight that developing nations such as China and India, which have declined to accept mandatory caps on carbon emissions, must be included in any climate change treaty.

European leaders, who have long pressed Bush to take a more aggressive stance on global warming, said the declaration could enhance efforts to reach a binding agreement to reduce emissions when negotiators meet in Copenhagen, Denmark, next year under U.N. auspices.

"This is a strong signal to citizens around the world," the president of the European Commission, Jose Manuel Barroso, told reporters at a news conference. "The science is clear; the economic case for action is stronger than ever. Now we need to go the extra mile to secure an ambitious global deal in Copenhagen."

The leaders of the eight industrialized countries, who gathered on the northern Japanese island of Hokkaido for their annual meeting, spent months debating the language of Tuesday's communique in lower-level negotiations. Critics said that it was short on specifics and that both developed and developing countries would need to make much sharper cuts in emissions to head off the worst effects of global warming.

The statement left unclear, for instance, whether the cuts made by 2050 would be pegged to current emissions levels or to 1990 levels, as many advocates had hoped.

The United States emitted about 20 percent more carbon dioxide in 2007 than it did in 1990.

"It is one step forward from the U.S. point of view, because President Bush has agreed that the United States, for the first time, must be bound by an international treaty," said Philip E. Clapp, director of the Pew Environmental Group, who is in Japan monitoring the negotiations. "But the emissions reduction goal is extremely weak. The language in the communique is almost meaningless."

The White House painted the document as a victory.

"The G-8 is giving a lot, but the G-8 is also suggesting that others need to be part of that equation," said James L. Connaughton, Bush's top adviser on environmental matters.