The United Nations Kyoto Protocol, which aims to curb the global warming process, comes into force on Wednesday with most of the industrialized countries ratifying it while the United States and Australia holding out.

The protocol will have legal force for its participants from Feb. 16 after meeting twin conditions -- backing from at least 55 countries and support from nations representing at least 55 percent of developed countries' carbon dioxide emissions.

It passed the second hurdle in November 2004 when ratified by Russia and now has backing from nations representing 61.6 percent of emissions. The United States, the world's biggest polluter, has pulled out, saying Kyoto is too expensive and wrongly omits developing nations.

The treaty was agreed at a 1997 UN conference in Kyoto, Japan by 159 countries that are members of the 1992 UN Framework Convention on Climate Change (UNFCCC). However, it took almost four more years of negotiations to complete its rulebook and then nearly three more years to get the deal ratified so that it could take effect.

A total of 141 nations have ratified the pact, according to UN data.

It commits the industrialized countries who have ratified it to reduce the amount of six greenhouse gases (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6).) by 5.2 percent of 1990 levels during the five-year period 2008-2012.

These greenhouse gases are trapping the Sun's heat, causing the Earth's surface to warm and thus changing the planet's delicately-balanced climate system.

Only 39 countries -- relatively developed ones -- have target levels for the 2008-2012 period, adhering to the principle set under the UNFCCC that richer countries should take the lead.

Each country negotiated different targets, with Russia aiming for stabilization at 1990 levels and the European Union trying for an eight percent cut. Developing countries don't have to cut back. Signatories have some flexibility in how they attain these emissions reductions.

Under a 2001 deal made by environment ministers in Germany, countries overshooting their targets in 2012 will have to make both the promised cuts and 30 percent more in a second period from2013. They rejected the idea of a financial penalty.