Scottish Road Network Climate Change Study: UKCP09 update Autumn 2011
8 Probabilistic approach and UK context
8.1 Making use of probabilistic projections
Whilst the probabilistic nature of the UKCP09 projections provides better definition of the inherent uncertainties behind the projections, it also introduces a perhaps less welcome issue to those wishing to make decisions: namely how is the probabilistic nature of the projection to be used in decision making. In earlier studies it was common that the single value attached to the medium-high emission scenario was the value that was worked to. Users of the UKCP09 projections however need to consider how they wish to balance the probability of a change occurring against the risk that it may be exceeded and the subsequent consequences of having under adapted.
A sensible way forward is to perhaps use the 50 percentile change as the main indicator of the scale of change, but to test the sensitivities of the system to the 10 percentile and 90 percentile changes to be informed as to the implications of the actual change being within this range. (The implications may refer to the design variable being exceeded more often than planned for and the ensuing damage or disruption that that may cause; or equally that the adaptation measures may have resulted in over design to a variable that did not change as much as expected and the associated over spend).
An alternative risk averse way forward would be to design everything to be capable of accommodating the 90 percentile change. Perhaps for critical aspects this would be a prudent course though not so for less critical aspects. Or, where possible, where costs are high, designs could be followed that would permit additional adaptation to be built on later if needs be (an example from flood defence would be to build the foundations of defences so that if it became necessary the defences could be further raised at a later date without new foundations being required).
It is necessary for those considering adaptation measures to consider how to balance the probabilistic nature of the projections together with the associated risks and consequences to the issue they are managing.
8.2 Projected future climate in the context of existing UK climates
Setting the projected future Scottish climate in the context of already existing climates in the UK and if necessary beyond, may be helpful when considering whether there is a need for new or innovative adaptation techniques, or whether the issue more simply requires the adoption of existing understanding and management practice from that other geographical zone. Table 8.1 provides an approximate summary of the existing geographical climates that may be approximately similar to the projected 2080s medium, 50 percentile climate of each of the key Scottish variables considered in this study. The assessment is only intended to give a broad indication based on a high level rapid assessment and inevitably masks much of the detail relevant to each of the variables. In particular had the 90 percentile changes been considered then analogous climate beyond the UK would need to have been selected. Notwithstanding these limitations, the following table gives a useful indication of where new or innovative adaptation techniques may be required.
| Variable | Analagous current climate |
|---|---|
| Annual mean temperature |
Lowland Scotland → south east England Highland Scotland → English midlands |
| Frost days |
Lowland Scotland → coastal margin of south west England Highland Scotland → central and southern England |
| Freeze-thaw | (Not available) |
| Temperature exceeding 25 ºC |
Lowland Scotland → central southern England Highland Scotland → central northern England |
| Temperature exceeding 30 ºC |
Lowland Scotland → central southern England Highland Scotland → central northern England |
| Growing season length | (Not available) |
| Annual mean precipitation | Little change in annual precipitation is predicted |
| Seasonal mean precipitation (Winter) | The steep east to west winter precipitation gradient suggests that the likely 10% to 20% increase will give totals similar to existing Scottish locations to the west of their location. However the western Highlands are the wettest region of the UK and one of the wettest in Europe and a 10% to 20% increase will result in seasonal depths exceeding that presently experienced anywhere in the UK or Europe at the moment. This coupled with a likely reduction in freezing and snow conditions suggest that the winter precipitation that does fall in the future wont be as likely to be stored on high ground as ice\snow. It is likely that the rivers draining this area will as a result experience a greater number of flood events though it is less clear as to how this will affect the magnitude and frequency of major floods. Existing climatic analogues to this situation are not simple to identify. |
| Seasonal mean precipitation (Summer) | The opposite to the winter precipitation is true with all areas receiving less summer rainfall. |
| 2-year and 10-year extreme daily and sub-daily rainfall |
1-day extreme rainfall shows an east-west gradient (though this gradient is not as steep as for the seasonal winter totals) across the UK. The percentage increases projected for 2080s are reasonably uniform across Scotland. Therefore most parts of Scotland are likely to experience extreme daily rainfalls that are currently to be expected further to the west within Scotland. However for the western Highlands future daily extremes are likely to exceed values currently experienced anywhere else in the UK. The situation for 1-hour extreme rainfall is slightly different. An east-west gradient across Scotland does occur, but so does a UK north-south gradient with higher values in the south. It may be better to suggest that the future extreme short duration rainfall for eastern and central Scotland will be better represented by the currently existing extreme rainfall of eastern and south-east England. |
| Wettest day in a season | Similar remarks to 2-year and 20-year daily extreme rainfall apply. |
| Soil moisture deficit | Limited information to assess but it is likely that summer soil moisture deficits will in the future become larger and more similar to those currently experienced further south within the UK. Future Scottish deficits are likely to be within the current English range. |
| Groundwater recharge | Limited information to assess but it is likely that summer recharge will reduce (similar to existing drier parts of England), whilst potential winter recharge is likely to increase and the above comments on winter precipitation apply. |
| Sea-level rise | Sea level rises are projected to be less than around the English coast and particularly less significant than around the south east of the UK. |