Why does UKCP09 use 30-year time periods?

Climate is conventionally defined as the average weather conditions over a 30-year period. Consequently, the results of climate models (such as UKCP09) are typically reported using 30-year time-periods to minimise the uncertainty produced by natural climate variability on annual and decadal timescales.

In detail

Examples of natural variability are the inter-annual variations in temperature and precipitation patterns that are caused by large-scale climate processes such as El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation.

The presence of natural climate variability means that future changes cannot be given for individual future years or decades, as in any given year natural climate variation may act to enhance or lessen the effect of climate change. Consequently, the UKCP09 climate scenarios are averaged across 30-year periods, meaning they describe the typical changes in climate that are associated with each future 30-year time-slice as a whole.