AMSTERDAM, Netherlands -- American states and private companies are adopting laudable initiatives on global warming, but the United States will lag behind Europe for years to come, the new U.N. climate chief said Friday.

In an interview four days after taking office as head of the U.N.'s climate change secretariat, Yvo de Boer said the Bush administration no longer doubts climate change is occurring and has begun to take action to control it -- even though it still rejects an international treaty ratified by most other industrial countries.

"The time when the United States called climate change into question is behind us. I don't hear that happening anymore," De Boer said in a telephone interview from his office in Bonn, Germany.

But the real action is outside Washington in controlling pollution that contributes to the warming of the Earth.

"There are some really important initiatives being taken across the country," De Boer said.

He praised legislation sealed last month by California that set mandatory controls on the emission of heat-trapping greenhouse gases, and said other states are not far behind.

Eleven states were discussing ways to cooperate in controlling emissions, he said. "And what also is very promising is that there is an increasing interest among the private sector to come to grips with climate change," while still remaining competitive.

Despite these moves, the Americans cannot catch up with the Europeans in cutting the emissions of carbon dioxide and other greenhouse gases because the U.S. has no mandatory regime, De Boer said.

"I don't think they'll be on par with Europe in the next four or five years," he said. But he hoped that within that time "they will be part of an international agreement that is working toward longer term action."

President Bush, voicing skepticism over scientific reports on global warming, opted out of the Kyoto Protocol, a treaty that set mandatory targets for industrial nations to reduce gases produced by cars, power plants and many industries to levels lower than what they were in 1990.

Many of the 35 countries that endorsed the protocol are behind in meeting their targets, De Boer said.

But he expects the overall goal will be met by the 2012 deadline, provided they follow a dual-track policy of reducing their own emissions and of gaining credits by transferring technologies and sponsoring efficient energy projects in developing nations.

"When you sign up to an international treaty it's usually something that you do seriously. I don't get the sense from any country that they have the intention to back away from their commitments," he said. "But fortunately they do still have a number of years to meet those targets because in a number of cases it's not going to be easy."

Climate change already has begun and cannot be reversed, he said. Sea levels are rising, glaciers are melting and droughts are occurring more frequently, alongside more violent storms.

"There is no optimistic scenario that can make all of that go away," he said. "The thing that I personally find most frightening is the longer that we take to act, the bigger the uncertainties to come."

The U.N. Framework Convention on Climate Change, which De Boer heads, is the forum for governments to set agendas, gather information on how human activity affects the climate, and to negotiate agreements. Most U.N. member countries will send delegates to the next international meeting -- in Nairobi, Kenya in November -- to plan what happens after the Kyoto Protocol expires in 2012.