CMIP5 (Taylor, Stouffer, and Meehl 2012
GCMs and earth system models (ESMs) that have been submitted to the World Climate Research Programme. A subset of 35 models was used in this study (based on
availability at the time of writing)
The magnitudes of projected changes in precipitation are shown in Figure 2.8 for the 35 CMIP5 models (orange lines) the 11
crosses) and the seven credible
values vary dramatically from model to model and from summer to winter. Whilst the pattern is noisy, the majority of the models projects a move toward wetter winters and drier summers, a result that is consistent with the projections of UKCP09
2009) and UKCIP02 (Hulme et al. 2002 similar irrespective of the model s
Figure 3.8 Projected changes (% difference monthly precipitation totals for 2070
(orange lines), 11 UKCP09 regional models (black crosses) and the seven (red dots).
21 Credible models based on the K
from CMIP5 climate models
and Meehl 2012) represents the current state
GCMs and earth system models (ESMs) that have been submitted to the World Climate Research Programme. A subset of 35 models was used in this study (based on
availability at the time of writing).
The magnitudes of projected changes in precipitation are shown in Figure 2.8 for the 35 CMIP5 models (orange lines) the 11-member Met Office regional climate model (black crosses) and the seven credible21 CMIP5 models identified above (red circles). The values vary dramatically from model to model and from summer to winter. Whilst the pattern is noisy, the majority of the models projects a move toward wetter winters and drier summers, a result that is consistent with the projections of UKCP09
Hulme et al. 2002). It is notable that degree of spread is largely similar irrespective of the model subset.
8 Projected changes (% difference from the 1900-1999 baseline
monthly precipitation totals for 2070-2099 by month for each of the 35 CMIP5 models (orange lines), 11 UKCP09 regional models (black crosses) and the seven
Credible models based on the K-S test described earlier in the section (Table 3.1)
33 | P a g e represents the current state-of-the-art in GCMs and earth system models (ESMs) that have been submitted to the World Climate Research Programme. A subset of 35 models was used in this study (based on
The magnitudes of projected changes in precipitation are shown in Figure 2.8 for the 35 member Met Office regional climate model (black CMIP5 models identified above (red circles). The values vary dramatically from model to model and from summer to winter. Whilst the pattern is noisy, the majority of the models projects a move toward wetter winters and drier summers, a result that is consistent with the projections of UKCP09 (Jenkins et al.
. It is notable that degree of spread is largely
1999 baseline) in expected 2099 by month for each of the 35 CMIP5 models (orange lines), 11 UKCP09 regional models (black crosses) and the seven credible models
S test described earlier in the section (Table 3.1)
Since credible models (albeit only marginally credible) of low precipitation exist for the EWP region for the reference period (1900
projections for the future (2070
changes in the average monthly precipitation totals lead to a mixed pattern of changes in the precipitation anomalies accumulated over longer time scales. In general, wetter winters tend to ameliorate the effects of summer dro
longest sequences of below normal rainfall. Thus, the risk of multi
be thought to decrease. However, the risk of a dry winter in a particular year or series of years, whilst reduced, still remains, and
dry summer, a severe long
can be seen in Figure 3.9 which compares the distribution of dry run lengths (consecutive negative precipitation anomalies f
reference period and the projected future. The shape of the distribution shifts to favour the probability of short period droughts whilst the risk of long period events remains.
Figure 3.9 Comparison of distributio precipitation anomalies for 6
the projected future (by 2100)
lengths (drought durations) under the pre
for the climate in 2100. Panel (b) shows the same data as cumulative distributions with a 95% confidence interval (shaded).
Since credible models (albeit only marginally credible) of low precipitation exist for the EWP region for the reference period (1900-1999) it is reasonable to examine their projections for the future (2070-2099) under the H++ scenario. The mixed pattern of changes in the average monthly precipitation totals lead to a mixed pattern of changes in the precipitation anomalies accumulated over longer time scales. In general, wetter winters tend to ameliorate the effects of summer droughts and serve to break up the longest sequences of below normal rainfall. Thus, the risk of multi-annual droughts might be thought to decrease. However, the risk of a dry winter in a particular year or series of years, whilst reduced, still remains, and when a particular occurrence is coupled with a dry summer, a severe long-period drought can still emerge. Such a mixture of effects
9 which compares the distribution of dry run lengths
(consecutive negative precipitation anomalies for 6-monthly accumulations) between the reference period and the projected future. The shape of the distribution shifts to favour the probability of short period droughts whilst the risk of long period events remains.
distributions of dry run lengths (consecutive negative
precipitation anomalies for 6-monthly accumulations) between the reference period and (by 2100). The grey bars in panel (a) show the histogram of run lengths (drought durations) under the present climate. The red bars are model estimates for the climate in 2100. Panel (b) shows the same data as cumulative distributions with a 95% confidence interval (shaded).
34 | P a g e Since credible models (albeit only marginally credible) of low precipitation exist for the
1999) it is reasonable to examine their cenario. The mixed pattern of changes in the average monthly precipitation totals lead to a mixed pattern of changes in the precipitation anomalies accumulated over longer time scales. In general, wetter
ughts and serve to break up the annual droughts might be thought to decrease. However, the risk of a dry winter in a particular year or series of
when a particular occurrence is coupled with a period drought can still emerge. Such a mixture of effects 9 which compares the distribution of dry run lengths
monthly accumulations) between the reference period and the projected future. The shape of the distribution shifts to favour the probability of short period droughts whilst the risk of long period events remains.
of dry run lengths (consecutive negative
ly accumulations) between the reference period and The grey bars in panel (a) show the histogram of run
sent climate. The red bars are model estimates for the climate in 2100. Panel (b) shows the same data as cumulative distributions with a
35 | P a g e A similar pattern is seen in the changes in probability of low rainfall over short and long durations for England and Wales between the baseline and future periods; that is with the largest changes for 6 month durations, while the possibility of longer drought
remains. These are presented for summer and winter droughts in Figures 3.10 and 3.11 respectively. The figures indicate credible ranges on the probabilities expected by 2100.
The changes in probability are computed on a cell by cell basis. Minimal (optimistic) estimates are computed by applying the minimum shift (in terms of a move toward drier conditions) from the 7 credible models to the lower bound of the 95% confidence interval of the present day probabilities (estimated from the full observed EWP time series).
Likewise, maximal (pessimistic) estimates are computed by applying the maximum shift from the 7 models to the upper bound of 95% confidence interval of the present day probabilities (i.e. by shifting the probabilities shown in Figure 3.3). Comparison of the baseline figures to the minimal and maximal future figures provides information on the possible changes in future periods of low rainfall. For example for England and Wales 6 month summer rainfall there was 1.3% chance of a 50% rainfall deficit for the baseline period (Figure 3.4 lower pane), which changes to a 0.2% to 13.4% chance of a 50%
rainfall deficit in future periods (Figure 3.10). For England and Wales winter rainfall there is a 1% chance of 30% rainfall deficit over 30 months for the baseline period (Figure 3.4, upper pane), which becomes less likely changing to a zero to 1% chance in future (Figure 3.11).
In the context of developing H++ scenarios for short and longer droughts, these results suggest:
• Future summer meteorological droughts in England and Wales could be more or less severe; the largest changes suggest the possibility of significant
increases in the probabilities of severe 6 month duration summer droughts.
The chance of encountering deficits of up to 60% of the expected precipitation (under the current climate) increases from 0% to 5%.
• No significant change in winter droughts; however, the possibility remains of some longer dry periods lasting several years similar to the most severe long droughts on record.
The current generation of global climate models are not capable of synthesising realistic droughts for regions as small as Scotland and Northern Ireland and little can be inferred about the change in risk over these regions.
36 | P a g e Figure 3.10 Upper (top panel) and lower (bottom panel) estimates of summer drought probability for the England and Wales precipitation region (EWP) credible by 2100.
37 | P a g e Figure 3.11 Upper (top panel) and lower (bottom panel) estimates of winter drought probability for the England and Wales precipitation region (EWP) credible by 2100.
38 | P a g e
3.5 Physical limits
Thermodynamic arguments favour moister air in a warmer world and increased rainfall intensities (Allan 2011) (Section 5) however for this to be realised the moisture must be delivered and precipitated out. In general for the UK, large scale low pressure (cyclonic) systems deliver new water into the hydrological system which is in turn recycled through local convection. Drought can be initiated either by a reduction in delivery (fewer
cyclones) and/or the suppression of precipitation (more anticyclones). Mid latitude cyclones and anticyclones are an inherent feature of our climate system resulting from the rotation of the Earth and its orientation to the sun (Carlson 1991). The path of cyclones across the north Atlantic and hence their incidence over the UK is biased toward a particular path and results in the emergence of what is known as the north Atlantic storm track. Analysis suggests that the position of the storm track is dependent on ocean-atmosphere coupling (Woollings et al. 2012). The dynamics which control the position of the storm track are complicated and poorly understood (Woollings 2010).
However, under anthropogenic greenhouse-gas forcing, there is some evidence for the strengthening and eastward extension of the storm track towards Europe which may favour enhanced precipitation (Woollings et al. 2012) and an increased number of cyclones in winter incident upon central Europe (Zappa et al. 2013) (Section 4). This enhancement is counter balanced by the tendency of more warmer conditions to favour the development of larger scale anticyclonic systems (~2% larger for a warming of 4ºC) (James 1951, Holton 2004). There is also evidence that high temperatures, a common feature of anticyclones in summer, can dry the soil which in turn reduces the amount of latent cooling and can thus drive temperatures even higher and soil moisture lower (Fischer et al. 2007). This in turn reduces the moisture available for local recycling.
Physical considerations thus reveal competing influences which are consistent with the empirical findings from the climate models analysed.
Spatial coherency
A detailed analysis of the spatial coherency of UK droughts is provided by Rahiz and New (2012). They report a complex picture dependent on drought severity, duration and timing. This is consistent with previous analysis by the UK Environment Agency at the European scale (Hannaford et al. 2009). In general, drought over the UK is associated with blocked atmospheric flow across the North Atlantic Ocean and/or Eurasian land mass. The associated high pressure (anticyclonic) features that tend to suppress rainfall have a typical area that is several times that of the UK. Thus, whilst not all UK droughts
39 | P a g e are spatially coherent, since the high pressure centre may not be located directly over the UK, the underlying physics suggest that spatial coherency is always a possibility.
Thus, in this section we have used the physical limits concept as a sense check of the results and to provide some explanation of the model behaviour.
3.6 Other evidence
Palaeo analogue / evidence
Analysis of European tree-ring data from the last 2500 years (Buntgen et al. 2011) suggest that earlier hydro-climatic changes have at times exceeded recent variations.
Particularly alarming is the 200 year long period of reduced precipitation around 500 AD.
During this period precipitation was reduced by 15% to 50% of the long-term average (range defined by ±1 standard deviation) for a continuous period of 50 years. This period of time coincided with the demise of the Western Roman Empire and the turmoil of the Migration Period (ibid). The severity of this low rainfall period (15%-50% deficits) is similar to what is proposed for a H++ low rainfall (10%-60% over specific time periods) but clearly its longer duration is significant and is a scenario that has not been
considered as part of H++. The lack of specific paleo data for the UK precludes any further analysis here but suggests an area for further research.
Industry data
The water industry use information on meteorological droughts for the design of water infrastructure, supply-demand planning and drought planning. In general the industry uses long term records (1920-present day) to understand drought risks and several companies have also considered more severe long duration droughts from the late 19th century. For strategic planning climate change scenarios are used to perturb the historical data making historical droughts in summer more severe but not changing the duration or spatial extent of droughts. For drought planning companies consider the drought situation and plan ahead using historical analogues – “what if the drought develops like 1976”, or simple percentage deficits of rainfall, for example a 20%
reduction in rainfall over 12 months. The biggest concerns for UK water companies are related to long multi-season droughts with durations of 18 months to 3 or more years.
The water resources impacts of H++ have been considered in a separate ASC project (HR Wallingford, 2015).
40 | P a g e