§ 32. Mrs. Gorman
To ask the Secretary of State for the Environment, Transport and the Regions what factors underlie the use of tonnes of carbon as a measure for greenhouse gas emissions from motor vehicles. 
§ Ms Glenda Jackson
Under international guidelines, greenhouse gas emissions are measured in terms of their global warming potential. Conversion to a common unit of measurement (whether in tonnes of CO2 equivalent or tonnes of carbon equivalent) allows comparisons to be made between countries, between various policies and measures and between different greenhouse gases.