Online Briefing report 4 What climate change projections does UKCP09 provide?
The variables available over land, and over marine regions, are shown in Table 1. Precipitation is a total of precipitation of all types — rain, snow and hail — and is given as a rate, in millimetres per day; however, when discussing , seasonal or average changes we refer to it for convenience as simply precipitation. Mean daily maximum (minimum) temperature is sometimes shortened to maximum (minimum) temperature, again for convenience.
In order to be statistically robust, the changes in extreme values (such as the 99th percentile of daily precipitation rate) are calculated as percentiles of 30 yr of daily data in a season. In Table 1 more user-friendly (albeit less accurate) names for these variables follow in brackets.
Variables include two measures of temperature extremes (high and low percentiles) and one precipitation extreme. For most variables, changes are given for three temporal averaging periods: month, season and year. Additional projections at daily and hourly resolution, consistent with the probabilistic projections, are available from a weather generator, described later in this report.
The spatial resolution of the projections over land areas is 25 km (Figure 7a), including islands large enough to be seen at this resolution. Because it is not possible for users to infer projections for larger regions by combining those from a number of individual 25 km squares, we also provide probabilities of change for two different sets of regions. The first of these (Figure 7b) is referred to for simplicity as administrative regions and encompasses the countries of Wales, Northern Ireland and Scotland (the latter subdivided into three climate regions), the nine administrative regions of England, together with the Isle of Man and a single grid square for the Channel Islands. The second set of regions is composed of river basins, based on those within the Water Framework Directive; these are shown in Figure 7(c). Lastly, probabilistic projections of change over the oceans surrounding the UK are not available at 25 km resolution from UKCP09, but instead are averaged over nine (Figure 7d). The names of the marine regions have been chosen specifically for the convenience of this report, and hence may not be geographically or politically correct.
Projections are given averaged over each of seven future overlapping 30-yr time periods, stepped forward by a decade, starting with 2010–2039. The use of 30-yr time periods reduces the effect of uncertainty due to natural internal variability. 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). All changes are expressed relative to a modelled 30-yr baseline period of 1961–1990. Note that, by 2009, a significant proportion of the time between the baseline period and future time periods has already elapsed, so the changes should not be referred to as “from today’s climate”.
For some variables, UKCP09 also makes available probabilistic projections of future climate over land areas, also at 25 km resolution, 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. For marine regions, only climate change projections are available, and not projections of future climate.
As explained in Section 3, projections are given corresponding to three future emissions scenarios — Low, Medium and High. In UKCIP02 four emissions scenarios were used; two of them (Low and High) are the same as the corresponding scenarios in UKCP09. Factors such as inertia in the climate system mean that climate change over the first two or three decades from now is relatively insensitive to emissions. However, after the 2040s, projections based on different emissions scenarios increasingly diverge.