
A CER is a certificate which is issued every time the United Nations prevents one tonne of CO2 equivalent being emitted through carbon projects registered with the Clean Development Mechanism (CDM). CDM Projects include for example replacing coal or fossil fuel fired electricity with clean technology, or methane capture from landfill sites.

CERs have emerged as a strong carbon currency accepted in most existing and emerging national and regional emissions trading schemes.
CER prices are increasingly important in the EU and global carbon markets. CERs can be substituted for European Union Allowances (EUAs) by European emitters to meet obligations under the EU ETS.

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http://www.impactglobalemissionsolutions.com/certified-emission-reduction