"This isn't some marginal environmental issue that can be ignored or played down," said EU Environment Commissioner Margot Wallstroem.
But the Bush administration, seen worldwide as friendly to big oil, says the decision to pull out of the international treaty is "unequivocal."
In a new diplomatic initiative, the European Union said Friday it would send a mission to Russia, China, Iran and Japan to gauge support for the pact.
Newspapers across the globe slammed the Kyoto pullout, calling it a short-sighted and selfish kowtow to big energy on the part of the White House:
German Chancellor Gerhard Schroeder, after meeting Mr. Bush in the White House Thursday, said, "We agreed on practically everything except ... the Kyoto protocol."
Nevertheless, Mr. Bush "does not support the Kyoto treaty. It is not in the United States' economic best interest," said spokesman Ari Fleischer.
Environmentalists say the United States has just 6 percent of the world's population yet produces more than a quarter of the globe's greenhouse gases.
The U.N. pact was agreed to in the Japanese city of Kyoto by former President Clinton and leaders of other industrialized countries, but it has not been ratified by the U.S. Senate.
Calling Mr. Bush's announcement "exceptionally serious," British Environment Minister Michael Meacher also said it could have broad repercussions.
"(Global warming) is the most dangerous and fearful challenge to humanity over the next 100 years," Meacher said.
While stressing it was too soon to discuss "tactics to punsh the United States," Wallstroem said she will go to Washington next week with an EU delegation to seek clarification of the Bush administration's position.
"I don't think this is the time to start to threaten, but we must be clear about the political implications," she said.
Others already began suggesting steps to be taken, ranging from an e-mail blitz of the White House to pickets outside Exxon, Texaco or Chevron gas stations.
The Greens group in the European Parliament called for a consumer boycott of U.S. oil companies, while a conservative British lawmaker suggested the EU could hold up resolution of trans-Atlantic trade disputes.
Meacher said Europe has "a lot of leverage," but added that sanctions were not the answer.
"I certainly don't think we should despair or try to ostracize the U.S. as a pariah," he said. "This is not the end of the story. There is clearly a power struggle going on in Washington and we have to keep hammering on."
Bush administration officials announced Wednesday that they would not implement the 1997 Kyoto Protocol, under which countries areed to legally binding targets for curbing greenhouse gas emissions.
Opposition in the U.S. Congress has focused on the lack of immediate targets for developing countries, including giants like India and China.
European officials said they believed the Bush administration was also driven by worries over the worsening U.S. economy.
"It is not acceptable that national economic worries mean that the world cannot act against a global threat," Danish Minister of Energy and Environment Svend Auken said, visibly angry.
Signs of trouble emerged earlier this year, when Mr. Bush asked for more time to study the arguments for alternative fuel strategies and renewable technologies after the collapse of talks in the Netherlands last November.
Fresh talks were scheduled for May in Bonn, Germany, but were then postponed until July.
Europe is not opposed to Washington's insistence that measures adopted to reduce greenhouse gas emissions be "cost-effective," Wallstroem said.
"At the same time, it's important not to allow them to dictate the whole process," she added. "We are ready to move, but we don't want to let the Americans off the hook. ... We want to see that they actually meet their commitments."
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