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This paper analyses the effect of different emission metrics and metric values on timing and costs of greenhouse gas mitigation in least-cost emission pathways aimed at a forcing level of 3.5 W m⁻² in 2100. Such an assessment is currently relevant in view of UNFCCC's decision to replace the values currently used. An emission metric determines the relative weights of non-CO₂ greenhouse gases in obtaining CO₂-equivalent emissions. For the first commitment period of the Kyoto Protocol, the UNFCCC has used 100 year global warming potential (GWP) values as reported in IPCC's Second Assessment Report. For the second commitment period, the UNFCCC has decided to use 100 year GWP values from IPCC's Fourth Assessment Report. We find that such a change has only a minor impact on (the optimal timing of) global emission reductions and costs. However, using 20 year or 500 year GWPs to value non-CO₂ greenhouse gases does result in a significant change in both costs and emission reductions in our model. CO₂ reductions are favored over non-CO₂ gases when the time horizon of the GWPs is increased. Application of GWPs with time horizons longer than 100 year can increase abatement costs substantially, by about 20% for 500 year GWPs. Surprisingly, we find that implementation of a metric based on a time-dependent global temperature potential does not necessary lead to lower abatement costs. The crucial factor here is how fast non-CO₂ emissions can be reduced; if this is limited, the delay in reducing methane emissions cannot be (fully) compensated for later in the century, which increases total abatement costs.