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All climate change projections are subject to uncertainty. The degree to which temperature or rainfall, for example, might change in the future cannot be predicted exactly. This page gives more detail about the where uncertainties in UKCP09 come from. For further discussion of uncertainty in the UKCP09 projections please see the Handling uncertainty in UKCP09 page.

Uncertainty in the UKCP09 projections comes from four principal sources:

1 - Modelling uncertainty

Modelling uncertainty arises from our incomplete understanding of the climate system and the inability of climate models to represent the real system perfectly.

The treatment of modelling uncertainty in UKCP09 is a significant advance from previous UK climate scenarios (such as UKCIP02), which did not account for modelling uncertainties in their projections. This model uncertainty arises partly from uncertainty in model parameters, and partly from uncertainty in model structure. UKCP09 accounts for modelling uncertainty by:

· Using different variants of the Met Office Hadley Centre climate model, distinguished by different values for parameters defining key atmospheric and ocean processes; this is known as a Perturbed physics ensemble (PPE), and captures parametric uncertainty.

· Using a number of different international global climate models: this captures structural uncertainty.

For more information on the way in which modelling uncertainty is addressed in UKCP09 see Chapters 2 and 3 of the climate change projections report

2 - Natural climate variability

Climate varies naturally from year to year and decade to decade due to internal dynamical and physical processes in the climate system. Climate change is superimposed onto this variability, and can potentially modify some of its characteristics. The effect of natural variability alongside changes in future climate relative to the 1961-1990 baseline are included in the UKCP09 projections.

See Chapters 2 and 3 of the climate change projections report for more information.

3 - Emissions uncertainty

This arises from uncertainty in the future man-made emissions of greenhouse gases and aerosols.

Emissions uncertainty is explored by presenting probabilistic projections of future climate change separately for three emissions scenarios: low (IPCC SRES: B1), medium (IPCC SRES: A1B), and high (IPCC SRES: A1FI).

4 - Contributions to uncertainty from downscaling

UKCP09 presents climate projections at the 25km level in order to make them more relevant for decision-makers. This scale is finer than the 300km resolution of the global climate models used to simulate climate change, and is obtained by 'downscaling' the global models to the 25km scale. The downscaling process accounts for the local effects of features such as mountains and coastlines and their associated uncertainties, increasing the total uncertainty accounted for in the climate projections at any particular location. The contribution of downscaling to the overall uncertainty depends on the variable, location and season in question, but is generally modest.

For more information on uncertainty see Chapters 1, 2 and 3 of the climate change projections report.