Online Briefing report 5.4 Changes in precipitation
The of changes in annual mean precipitation (Figure 13) are within a few percent of zero everywhere. In winter, precipitation increases are in the range +10 to +30% over the majority of the country. Increases are smaller than this in some parts of the country, generally on higher ground. In summer, there is a general south to north gradient, from decreases of almost 40% in SW England to almost no change in Shetland.
Note that the changes at 10, 50 and 90% probability levels not only have different magnitudes, but can also be in different directions (that is, can become wetter or drier). Thus summer precipitation (the lowest 3 maps in Figure 13) is projected to decrease almost everywhere in the UK at the 10 and 50% probability levels, but increase almost everywhere at the 90% probability level. In other words, using a specific area as an example, it is very unlikely that Northern Ireland in summer will dry by more than 30–40%, and very unlikely that it will be more than 0–10% wetter, with a central estimate of 10–20% drier.
Central estimates of changes in precipitation on the wettest day of the winter by the 2080s with Medium emissions range from zero in parts of Scotland to +29% in parts of England. Corresponding changes in precipitation on the wettest day of the summer range from –9% in parts of southern England to +25% in parts of Scotland.
In Figure 14, we show changes in annual mean precipitation averaged over river basins.