Online climate change projections report 1.2.1 Climate change over land areas
Temporal averaging. For most variables changes are given as averages over three periods: month, season and year, except as shown in the last column of Table 1.1 (overleaf).
Spatial averaging. The resolution of the projections is 25 km over the land area of the UK, including islands large enough to be seen at this resolution (Figure 1.2(a)). Due to the probabilistic nature of the projections, it is not possible for probabilities of change over several individual grid squares to be simply averaged by the user in order to obtain of change over the total area of the grid squares. For this reason, we also provide probabilities of change for two different sets of aggregated areas over land, each decided upon following consultation.
The first of these aggregated areas (Figure 1.2(b)) encompasses the 16 regions made up of:
- the nine administrative regions of England
- Northern Ireland
- Scotland, subdivided into its three climatological regions
- the Isle of Man
- the Channel Islands (represented by a single 25 km grid square)
For simplicity, these are all referred to as administrative regions.
The second set of aggregated areas are river basins, shown in Figure 1.2(c). These are based on the 13 Water Framework Directive River Basin Districts in England, Wales and Northern Ireland. In Scotland, these are based on the 10 Advisory Group Boundaries.
Time periods. Changes are given averaged over each of seven future overlapping 30 year time periods, stepped forward by a decade, starting with 2010–2039 (specifically 1 December 2009 to 30 November 2039). These future time periods are referred to for simplicity by their middle decade, starting from the 2020s (2010–2039) and ending with the 2080s (2070–2099).
User surveys showed overwhelming support for retaining the same baseline period as used in UKCIP02, and hence all changes are expressed relative to a modelled baseline 30-year period of 1961–1990 (specifically 1 December 1960 to 30 November 1990). The future time periods are illustrated in Figure 1.3.
Emission scenarios. Changes are given corresponding to three future emissions scenarios — Low, Medium and High.
In the case of mean sea-level pressure, precipitation, relative humidity, temperature (mean, maximum and minimum) and cloud amount, UKCP09 also makes available probabilistic projections over land of future climate in addition to those of the change in climate. This is done by combining probabilistic projections of climate change with the corresponding baseline (1961–1990) climate taken from observations. This is preferable to directly taking the climate model output for future years as it reduces the effect of biases in the model’s simulation of the baseline climate, but obviously can not account for any errors in the projected climate change response.