All climate change projections are subject to uncertainties. The degree to which temperature or rainfall, for example, might change in the future cannot be predicted exactly. This page gives more detail about sources of uncertainty in the 11-member RCM ensemble.
1. Modelling uncertainty
This arises from incomplete understanding of Earth's system processes and the consequent incomplete representation of them in climate models. Some aspects of modelling uncertainty are represented in the RCM ensemble by running 11 variants of the regional climate model, which have different values for a set of model parameters controlling a range of surface and atmospheric processes.
This is a significant advance from previous climate projections (such as UKCIP02, which had no representation of modelling uncertainty). However, the RCM ensemble does not include as complete an estimate of uncertainty as that available in the probabilistic projections of UKCP09. In particular, the probabilistic projections include results from models other than the Hadley Centre model to estimate structural uncertainty, and also account for uncertainties in carbon cycle feedbacks.
2. Natural climate variability
The evolutions of climate from 1951-2100 in the 11 variants differ partly due to the systematic effects of differences in the model parameters, and partly due to the random effects of different evolutions of natural climate variability.
3. Emissions uncertainty
This arises because future global greenhouse gas emissions are uncertain; they depend on assumptions relating to future developments in various socio-economic factors, such as population growth, technological development, energy mix and so on. Only one emissions scenario was used to force the 11 RCM simulations, so the effects of emissions uncertainty are not included.