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DecisionSupport 2Box 2. Comparing Mitigation and AdaptationCost-effectiveness analysis has become the primary appraisal method for mitigation. However,adaptation is very different to mitigation, for the following reasons:Mitigation involves a single, common metric for benefits, i.e. tonnes of GHG emissions. This metricrelates to a global burden, so a reduction in a tonne of GHG emissions is treated the same,irrespective of the technology, sector or location. This means a tonne of CO 2abated from roadtransport in an urban area has the same benefit (and unit of effectiveness) as a tonne abated fromthe electricity sector in a rural location. This allows equivalent cross-economy analysis of optionsusing €/tCO 2.In contrast, adaptation is a response to a local, regional or national level impact, rather than to aglobal burden, and involves different types of risks in different sectors. There is therefore nocommon single metric which allows cross-sectoral cost-effectiveness analysis. Furthermore, thereare often many risks even within a single sector, which can make even sectoral CEA studieschallenging: for example, adaptation to sea level rise (SLR) can involve protecting people from floodrisk, reducing coastal erosion, conserving coastal ecosystems, etc. all of which involve differentmetrics. Finally, the analysis of impacts in adaptation (rather than burdens in mitigation) means thattechnology, location and time period are important: for adaptation, it makes a different how, whereand when cost-effectiveness is assessed.undertaken through the use of marginalabatement cost (MAC) curves. The approach canalso identify the largest benefits possible with theavailable resources, and can even be used tohelp set targets, by selecting the point wherecost-effectiveness falls significantly (i.e. wherethere are disproportionately high costs for lowbenefits).Cost-effectiveness analysis has been widelyused in European and Member State policyappraisal, and was previously the main approachused for air quality policy (Watkiss et al, 2007).Air quality concentration or deposition targetlevels were set on a scientific basis, and thecosts of alternative ways of achieving thesetargets (or progressing towards them) wereassessed using CEA.It has also been used in risk-based floodprotection assessment, particularly for coastalzones (e.g. RIVM, 2004) assessing the costeffectivenessof achieving flood protectiontargets (defined as a level of acceptable risk,such as protection against a 1 in 10000 yearreturn period).Most recently, cost-effectiveness analysis hasbecome the main appraisal technique used forclimate change mitigation, as it allows thecomparison and ranking of greenhouse gas(GHG) abatement options, using the costeffectivenessmetric of cost per tonne of GHGabated (€/tCO 2). There has also beenwidespread use of marginal abatement costcurves for mitigation. These show the relativeranking of options in order of cost-effectiveness,and can be used for identifying the least costway of achieving emission reductions includingcross-economy targets (e.g. CCC, 2008).Because of the widespread use of CEA forclimate change mitigation, many commentatorshave also highlighted its potential use for climatechange adaptation. However, the application ofCEA to adaptation involves a number of majordifferences, see Box 2.Cost-effectiveness analysis involves a series ofcommon methodological steps.• Establish the effectiveness criteria.• Collate a list of options.• Collect cost data for each option – noting thisinvolves the full costs over the lifetime of theoption, including capital and operating costs –and thus requires all values to be expressedon a common economic basis (in equivalent2