Quantifying future climate change

Quantitative projections of future climate are in increasing demand from the scientific community, policymakers and other stakeholders. Climate models of varying complexity are used to make projections, but approximations and inadequacies or 'errors' in models mean that those projections are uncertain, sometimes exploring a very wide range of possible futures. Techniques for quantifying the uncertainties are described here in terms of a common framework whereby models are used to explore relationships between past climate and climate change and future projections. Model parameters may be varied to produce a range of different simulations of past climate that are then compared with observations using 'metrics'. If the model parameters can be constrained to a tighter range as a result of observational comparisons, projections can also be constrained to a tighter range. The strengths and weaknesses of different implementations are discussed.