Why bother using the projections if they are going to be replaced in time?

Users need to understand clearly that, if they choose to adapt to a climate change corresponding to a specific probability level (e.g. 90%), this is likely to change in future projections. The UKCP09 methodology is designed to capture known uncertainties in the climate system built into the current generation of climate models, this is the most comprehensive approach to doing so to date. The UKCP09 projections can make a useful contribution to assessing risks posed by future climate; they are appropriate for decisions on adaptation to long-term climate change which need to be taken on the basis of current knowledge. Unless our understanding of the climate system changes in a major way, users who are incorporating the probabilities given in UKCP09 into their decision making are likely to find that their decisions are robust to changes in the next generation of projections.

In detail

Although modellers have improved many aspects of their models over the past decade or so, the current range of changes over the UK is not significantly narrower than that shown in UKCIP02. In practice, the prospects for better projections will depend on which aspects of future climate users are most interested in. The width of the PDFs in UKCP09 are substantial even for the next few decades, due mainly to natural variability, and grow larger through the century due to uncertainties in climate feedbacks. It may be possible to reduce short-term uncertainties with higher resolution models which may simulate better (for example) the North Atlantic storm track, and by starting model experiments with the recently-observed state of the ocean. However, this may not improve projections of (say) changes in surface temperature a hundred years ahead; at these lead times improved projections would come from more faithful representations of climate feedbacks and the carbon cycle in models.