How deep should the deep cuts be? Optimal CO₂ emissions over time under uncertainty

At the recent UN climate negotiations in Bali, most countries sought an agreement to cut industrialized countries' emissions of CO₂ by between 25% and 40% by 2020. The final agreed wording at Bali was that 'deep cuts' in emissions were required, without further quantification. The PAGE2002 probabilistic integrated assessment model is deployed to investigate how deep those deep cuts should be. We calculate optimal CO₂ emissions over time under uncertainty using the same assumptions and assessment model as in the <italic>Stern Review</italic>, where optimal emissions are those which minimize the mean net present value of the sum of climate change impacts and adaptation and abatement costs. The optimal global CO₂ emissions under these assumptions require much deeper cuts, of 55% by 2020 and 75% by 2060 compared with emissions in 2000. If the <italic>Stern Review</italic> is to be believed, emissions cuts of 25-40% by 2020 will be insufficient to achieve global climate objectives.