The Kyoto pact is meant to rein in rising temperatures that many scientists say will cause more storms, droughts and floods and raise world sea levels.
Climate shifts could disrupt farming and wipe out thousands of species of animals and plants. The 141-nation protocol, which will force countries to cap emissions of gases, enters into force years after it was agreed to in 1997.
"For the climate, Kyoto's only a small step," said Paal Prestrud, head of the Centre for International Climate and Environmental Research in Oslo. "But entry into force is a big political victory" for the idea of capping world emissions.
Many climate experts predict catastrophic changes within decades unless action is taken now. But US President George Bush pulled out of Kyoto in 2001, saying it was too costly and wrongly exempted poor nations.
The US, the biggest world polluter, accounted for 23.1% of the world's emissions in 2000 - about four times the second biggest polluter Russia.
Among developed nations, only Australia backed the US view, leaving Washington with fewer allies for its climate policy than for the 2003 Iraq war.
Even so, many countries bound by Kyoto are running far above its overall goal of curbing rich nations' emissions of heat-trapping gases such as carbon dioxide from cars, factories and power plants by 5.2% below 1990 levels by 2008-12.
Among Kyoto states, Spain and Portugal were 40.5% above 1990 levels in 2002, according to UN data.
Monaco, Ireland, Greece, New Zealand and Canada are all further over 1990 levels than the US, whose emissions are 13.1% over the Kyoto benchmark year. Most former communist nations are below target.
Environmentalists accuse many European Union nations of giving 12,000 industrial sites - including power plants, steel mills and oil firms - quotas for emissions that are too lax.
"In many cases they're letting off industry far too easily," said Steve Sawyer, climate policy director for Greenpeace.
The quotas are the basis for a new market where polluters exceeding their targets can buy emission allocations from those falling below. Carbon dioxide now trades at about 7.20 euros per tonne.
But unless the amount of quotas is cut sharply in coming years, Sawyer said the EU burden for meeting Kyoto goals would fall on consumers via higher taxes rather than by squeezing industries to shift to renewable energy such as solar or wind power.
"For the climate, Kyoto's only a small step ... but entry into force is a big political victory"
Head of the Centre for International Climate and Environmental Research in Oslo
Celebrations for the 16 February launch will range from meetings in the Japanese city of Kyoto where the pact was forged to prayers in churches for its success.
The Californian city of Berkeley will endorse Kyoto in a Valentine to the Planet.
The UN Environment Programme says climate change is the biggest long-term threat to humanity.
But opponents of Kyoto dismiss it as wasted money built on scaremongering science. "Put bluntly, the Kyoto protocol is one of the least good investments the world can make," said Bjorn Lomborg, Danish author of The Sceptical Environmentalist.
He said Kyoto would cost about $150 billion a year to promote a shift to renewable energies and that the cash could be better spent on combating Aids, malaria and malnutrition or on promoting free trade.
Even Kyoto's backers concede it will brake the rise in temperatures by just 0.15 Celsius by 2100, a pinprick compared to the 1.4 to 5.8 degree rise projected by a scientific panel which advises the UN.
Everyone agrees that the longer-term fate of Kyoto will hinge on whether Washington agrees to cap emissions after 2012, shifting from its current policy of curbing growth of emissions.
US participation is probably the key to whether nations such as China and India will take part from 2012.