"On February 16, 2005, the Kyoto Protocol to the United Nations Framework Convention on Climate Change (UNFCCC) entered into force, committing those nations that have ratified it to specified mandatory reductions in greenhouse gas emissions below 1990 levels. The nations that have ratified the Protocol now represent 61.6% of the 1990 emissions baseline, with 55% being the amount that must be accounted for by industrialized countries that have ratified in order for the Protocol to enter into force. The United States is a party to the UNFCCC, but not to the Kyoto Protocol. As of August 31, 2005, some 155 nations or regional economic integration organizations had ratified or acceded to the Protocol."
CRS Issue Brief for Congress, IB89005