TOYAKO, Japan (AP) - President Bush hailed the move by G-8 leaders to coalesce behind a strategy for a global climate-change accord, saying Wednesday "significant progress" was made. But environmentalists and the U.N.'s top climate official disputed his claims.

"I don't find the outcome very significant," Yvo de Boer, who head the United Nations-led global negotiations to forge a new climate change treaty, told The Associated Press in telephone interview from his home in the Netherlands.

De Boer said the summit's vague pledge to work toward slashing greenhouse gas emissions by 50 percent by 2050 mentioned no baseline, did not appear to be legally binding and was open to vastly different interpretations. He praised China's President Hu Jintao for acknowledging that developing countries must act on climate change even if Beijing rejects specific national targets.

Environmentalists also argued the goal of cutting greenhouse gases by 50 percent did not go far enough and amounted to political window-dressing.

"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, who called the G-8 statement an "empty slogan."

It was Bush's final summit with leaders of the world's richest democracies, and he gave reporters a sunny view of its accomplishments before flying home to Washington.

His main demand on a climate change accord is that eight poor but enery-guzzling nations be included in some requirements along with the major industrialized democracies that make up the Group of Eight. "That's what took place today," Bush said, referring to the embrace of this idea by fellow G-8 leaders.

The G-8 nations are the U.S., Britain, Germany, France, Italy, Canada, Japan and Russia.

But the five key developing nations at an expanded meeting on climate on the sidelines of the summit - China, India, Brazil, Mexico and South Africa - issued a statement rejecting the notion that all share in the 50-percent reduction goal. "It is essential that developed countries take the lead in achieving ambitious and absolute greenhouse gas emissions reductions," said the statement.

"We're not in complete convergence yet," acknowledged Jim Connaughton, one of Bush's top environmental advisers.

It was, nevertheless, the first time that the G-8 heads of state sat down to talk about global warming at the same table with the eight emerging economies that, with them, are responsible for spewing 80 percent of the carbon dioxide and other greenhouse gases into the atmosphere.

Bush heartily backed the broad emissions-reduction goal stated by his summit partners. This position represents quite a progression for a president who in his first term disputed scientists' assertions about global warming.

"We made clear, and the other nations agreed, that they must also participate in an ambitious goal," Bush said, "with an interim goal, with interim plans to enable the world to successfully address climate change. And we made significant progress toward a comprehensive approach."

In a statement that Bush read to reporters here, he reiterated his position that further progress will likely hinge on further development of clean energy technologies. Developing nations, he said, will need assistance so they can become "good stewards of the environment."

The president praised his fellow summit leaders for their work, not only on climate change but also on advancing the so-called Doha Round of negotiations on opening markets to free trade and on their cooperation with U.S. efforts to help poor nations combat disease and food shortages.

The president headed home with a mixed scorecard.

He ran into opposition to talk of trade sanctions against President Robert Mugabe's Zimbabwe for an election that Bush has labeled a "sham" balloting. And he made no headway in resolving differences with Russia over U.S. plans to put a missile defense system in Eastern Europe.

Russian President Dmitry Medvedev said in a separate talk with reporters that that American defense system "deeply distresses" Moscow and he accused Washington of engaging in "halfhearted negotiations that have come to nothing."

Bush held one-on-one talks with several other world leaders while in Japan for four days, including China's President Hu Jintao, whom he assured he was excited about going to the Beijing Olympics later this summer. Hu told Bush he was grateful that he hadn't politicized the event because of China's crackdown in Tibet.

In an early morning meeting with Indian Prime Minister Manmohan Singh, Bush defended a languishing deal his administration negotiated to sell India nuclear fuel and technology. The deal, which would reverse three decades of U.S. policy by allowing the sale of atomic fuel and technology to India, faces significant opposition on both sides.

Bush took no questions from reporters at the closing of the meeting. Nor did he address criticisms that emerged about the G-8's climate positions.

He said he and his summit partners had "served both our interests as Americans, and we've served the interests of the world."

Bush was instrumental in broadening the global warming discussions beyond the G-8 membership. But he won't be in office long enough to see the next chapter of the contentious climate change debate play out.

The discussion on global warming is a run-up to U.N.-led efforts to craft a new climate change accord at a meeting in Copenhagen, Denmark, in December 2009. That new accord would succeed the Kyoto Protocol that starts to expire in 2012.

AP writer Arthur Max contributed to this report from Amsterdam