The White House has contended that complying with the treaty's requirement could cost millions of jobs, many of them to places like India and China, both signers of Kyoto but exempted from any limits on greenhouse gases.

"We are still learning about the science of climate change," White House spokesman Scott McClellan said Tuesday. In the meantime, McClellan said, "We have made an unprecedented commitment to reduce the growth of greenhouse gas emissions in a way that continues to grow our economy."

Elsewhere, officials made solemn pledges Tuesday to fulfill Japan's requirement under the treaty to cut emissions of carbon dioxide and other so-called greenhouse gases by 6 percent below 1990 levels by 2012.

"Although the hurdle is high, we ask the Japanese people, including industries, for their cooperation," said Environment Minister Yuriko Koike.

The concerns are many. The Japanese government says many industries will need quick action to meet the goals, studies show much of the country is behind on implementation, and critics say Japan lacks a coherent climate-change policy.

Japan had an elaborate celebration planned for the enactment of the agreement on Wednesday at the convention hall where it was negotiated in December 1997.

A series of speeches and a panel discussion was planned with environmental officials, experts and activists, as well as 2004 Nobel Peace Prize laureate Wangari Maathai of Kenya. The festivities were featuring messages from U.N. Secretary-General Kofi Annan and environmental officials around the world.

As the agreement comes into force, Japan is scrambling to put together a strategy to make sure it meets its obligations. Some officials are pondering a "carbon tax" to punish polluters — a move opposed by business — while others favor expansion of nuclear power and promotion of energy-saving technologies.

Tetsunari Ida, executive director of Tokyo's International Sustainable Energy Policy Institute, said the effort was suffering from a lack of coordination between the Environmental Ministry and the Ministry of Economy, Trade and Industry, or METI.

"Those two ministries are taking two separate climate change strategies," Ida said.

A METI report this month showed that 11 of 30 top industries — steel and power among them — risked failing to meet targets without quick action. Thirteen others had already cleared preliminary goals and were expected to meet the goals, the report said.

One area where Japan has been especially active is carbon trading — a system under which governments have allocated carbon dioxide quotas to industrial facilities. Those which emit less gas than allowed can sell the "credit" to other companies who emit too much.

Makoto Katagiri, whose Natsource Japan is acting as a credit broker between Japanese and foreign companies, estimated in a study for the World Bank that Japan bought 41 percent of the carbon credits on the international market last year.

"From this figure, you can imagine how serious the Japanese companies (are)," Katagiri said.