The accord, reached at the December 2009 summit, sets a goal of limiting a rise in temperature to less than 2 degrees Celsius (3.6° F), but does not say how to achieve the goal. Rich nations also aim to give $100 billion a year in climate aid from 2020.

Major emitters -- led by China, the United States, the European Union, Russia and India -- all back the deal. Russia has given verbal support even though a U.N. website has not registered a formal letter of backing.

More than 60 countries have also submitted domestic goals for reining in climate change by 2020 under the Copenhagen Accord. A U.N. analysis indicates these pledges will only be sufficient to limit global warming to 3 degrees Celsius (5.4° F).

Requirements for emissions reduction continue through 2012 under the Kyoto Protocol.