The global carbon market more than doubled in value in 2007 to $64 billion, but that masked slow growth in actual greenhouse gas emissions cuts, according to the World Bank's carbon finance unit.

Climate change policies which limit the production of planet-heating gases in rich countries are driving booming global demand for emissions permits.

One way industrialised nations can buy carbon offsets is by funding greenhouse gas emissions cuts in developing nations, through a U.N.-led scheme under the Kyoto Protocol, but growth in value is outstripping emissions cuts.

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