CER is a Kyoto Protocol unit equal to 1 metric tonne of CO2-equivalent.
CERs are issued for emission reductions from CDM project activities.
Two special types of CERs called temporary certified emission reduction (tCERs) and long-term certified emission reductions (lCERs) are issued for emission removals from afforestation and reforestation CDM projects.
(UNFCCC glossary, 17 August 2007)
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