What are the sources of uncertainty considered in UKCP09?

There are three sources of uncertainty that are explicitly considered in UKCP09:

1) natural internal climate variability,
2) uncertainty in the climate models, and
3) uncertainty in future emissions.

In detail

There are various sources of uncertainty in the UKCP09 climate projections, however, three principle sources of climate science uncertainty are explicitly considered: 1) natural internal climatic variability, 2) uncertainty in the climate models, and 3) uncertainty in future emissions. In UKCP09, the Met Office Hadley Centre climate model HadSM3 (which is HadCM3 but with a simplified ocean model – the slab model) is used to address some of these issues, whilst acknowledging that it is itself, a source of uncertainty.

• Natural climatic variability: this source of uncertainty is incorporated in the probabilistic projections of UKCP09 by running a series of model simulations using HadSM3, with different initial starting conditions for the climate model parameters.

• Modelling uncertainty: arises from an incomplete understanding of the physical processes of the climate system, and, ability to model or represent the physical processes adequately, leading to differences between different climate models. In UKCP09, uncertainty arising from incomplete understanding of the physical processes is considered, by using HadSM3 in a series of perturbed physics ensembles (PPE), to generate a 300 member ensemble of possible future global climate. This ensemble is then used to train a climate model emulator, with which an even greater range of variation in HadSM3 model parameters is explored and quantified. Each member of the ensemble is assessed as to its quality, with respect to how well it models recent climatic observations and evolution. This assessment of model quality is used to develop a weighting scheme, whereby the most accurate model simulations receive a greater weighting than less accurate ones, in establishing the probability of a particular outcome. To account for uncertainty that arises from the ability of one model to provide an accurate representation of the real world, thirteen additional IPCC models, are used to generate a multi-model ensemble (MME). The uncertainty from the PPE and MME are then combined with downscaling and timescaling uncertainty in a statistical framework, to arrive at a final assignment of probability to each future climate projection.

• Emissions uncertainty: climate projections are provided in UKCP09 based on three different assumptions of greenhouse gas emission scenarios: Low, Medium and High, which respectively correspond to B1, A1B, and A1FI, of the IPCC special report on emissions scenarios (SRES).