With greater certainty in anthropogenic influence on observed changes in climate there is increasing pressure for agreements to control emissions of greenhouse gases (). While it is difficult to assess the appropriate level of mitigation, it has been argued that flexibility in meeting emission targets offers significant economic savings. Such flexibility can be exercised in terms of timing of mitigation (i.e. delay) or geographic location of the intervention (e.g. permit trading and Joint-Implementation). Much of this insight is based on standard models of technical change in energy supply and demand. However, standard model formulations rarely consider: (i) a link between the pattern of technical change and policy interventions; (ii) economies of learning; and (iii) technical progress in discovery and recovery of oil and gas. While there is evidence to support the importance of these factors in historic patterns of technical progress, the data necessary to calibrate internally consistent economic models of these phenomena have not been available. In this paper simple representations of endogenous and induced technical change have been used to explore the sensitivity of mitigation cost estimates to how technical change is represented in energy economics models. The scenarios involve control of CO₂ emissions to limit its concentration to no more than 550 ppm(v), starting in the year 2000, and delayed to 2025. This sensitivity analysis has revealed four robust insights: (i) If endogenous technical change is assumed, expected business as usual emissions are higher than otherwise estimated — nevertheless, while 25% greater CO₂ control is required for meeting the CO₂ concentration target, the cost of mitigation is 40% lower; (ii) If technical progress in oil and gas discovery and recovery is assumed, energy use and CO₂ emissions increase by 75% and 65%, respectively above the standard estimates; (iii) If the economies of learning exhibited in various manufacturing sectors are repeated in development of non-fossil technologies and abatement of CO₂ emissions, the costs of abatement can be 50% lower than those assessed using standard models; and (iv) In this sequential learning framework, delay in abatement towards a 550 ppm(v) CO₂ concentration target leads to expected net economic loss in seven of nine model structures studied. Only when the model structure permits new oil and gas discoveries while keeping other features of standard models does delay offer economic gain with greater than 60% confidence.