38 Current methodologies for assessment of the effects of policies and measures on greenhouse gas emissions are policy specific with no standard guidance available. There are some models available that can be used to assess the effects of particular types of proposals on emissions (e.g. National Road Traffic Forecasts).21
39 The impact of a new policy, project or programme on emissions should be expressed in terms of carbon savings, or in terms of additional emissions, measured in million tonnes of carbon-dioxide equivalent (MtC02).
40 In cases where quantification of the climate change effect is impractical, an assessment of whether the policy is likely to increase or decrease emissions, combined with a qualitative assessment of the significance of this change, should be included in the appraisal.
41 Once the emissions impact of a proposal has been quantified, current research informs the calculation of illustrative values for the social damage cost of carbon.22 This can then be used to estimate the monetary value of the impacts.
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21 Contact Defra for further advice on assessing the effects of a proposal on emissions.
22 A Government Economic Service working paper 'Estimating the Social Cost of Carbon Emissions' suggests illustrative values for the social damage cost of carbon that can be used to estimate the monetary value of impacts once they have been quantified. A copy of this working paper is available on the Treasury's website http://www.hm-treasury.gov.uk Defra can provide an associated guidance note on how to use these values in policy appraisal