US President George W. Bush has renounced the backing for the pact given by his predecessor Bill Clinton. Washington says Kyoto was based on dubious science and would hurt the economy.
That has angered Europeans, some of whom accuse Mr Bush, a former Texas oilman, of putting the business interests of the world's biggest polluter ahead of saving the planet.
Mr Koizumi's cautious pessimism brought angry responses from environmental campaigners who say delay can only increase the threat that global warming would melt polar ice-caps and flood coastlines and islands.
Greenpeace climate change campaign head Bill Hare said Japan's unwillingness to go ahead without the US had cast a "huge black cloud over the conference."
Green groups, concentrating their efforts on shaming Japan into honouring the deal which was agreed in its ancient capital, want Japanese Environment Minister Yoriko Kawaguchi to agree to a deal that would allow countries to ratify the Kyoto Protocol with or without the US.
But much of the high-level political arm twisting - as well as potentially violent protests - is likely to be reserved for this weekend's Group of Eight summit of leading industrial powers at Genoa, Italy. German Chancellor Gerhard Schroeder, French President Jacques Chirac and British Prime Minister Tony Blair are likely to press Mr Bush and Mr Koizumi on the issue.
Some European officials still hope Japan can be persuaded to back European Union efforts to get a majority of key states to ratify the pact and then try to increase pressure on Washington to back it.
"The negotiations will be very difficult, and it could be that the whole enterprise collapses," said German Environment Minister Juergen Trittin. "Japan helped with the birth of Kyoto but it also could contribute to burying Bonn."
The talks will focus on detailed issues such as deadlines for cutting gas emissions, mechanisms for trading emissions allowances among countries and formulas for offsetting gas emissions against forests, which can turn carbon dioxide back to oxygen.
Until these details are agreed, most countries are reluctant to be bound by the deal. Only one country with an emissions target, Romania, has so far ratified Kyoto.
In order to be binding, the pact must be ratified by at least 55 countries which also account for 55 per cent of the industrialised world's greenhouse gas emissions. That means that without the United States, which accounts for 36.1 per cent of emissions, most of the other big industrial nations must sign up together.
- REUTERS
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Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
Executive summary: Climate change impacts on NZ
IPCC Summary: Climate Change 2001