EVIDENCE FROM THE TYNDALL CENTRE FOR CLIMATE
Awareness of climate change and its implications
for water security in the UK is amongst the highest in the world.
There remains considerable uncertainty about the rate and magnitude
of climate change, particularly for precipitation change, which
is vital for water resources. The water supply industry and its
regulators have developed a series of scenario-based methodologies
for responding to climate change, although these do need more
development and have yet to be seriously tested in practice. Irrigators
have generally not yet addressed the implications of climate change.
The actual impacts of climate change on supply
reliability depend not only on the change in runoff and recharge,
but also on the way water supplies are managed and regulated,
so the same change can have different impacts in different catchments.
The expected impacts on irrigation will be increased demand as
well as lower and less reliable supplies, with possible major
implications for the production of high-value potatoes and vegetables
in eastern England. Impacts on flood risk depend not only on climate
change but also on how exposure to flood changes over the coming
decades. Of the three broad methods for managing flood risk (reducing
the physical hazard, reducing exposure to flood loss and reducing
vulnerability), reducing exposure to flood loss is least sensitive
to the effects of climate change, because it depends on policy
and planning decision making.
In many instances, climate change is likely
to exacerbate existing pressures or vulnerability in water resources
management. This means that areas already experiencing flooding
events during winter and supply deficits during summer and dry
years may be affected more often and to a greater degree. Many
other driving forces will operate on the water sector during this
century and the effects of climate change will be strongly mediated
by direct and indirect human activities and development trends.
Water supply for new housing development in the southeast is a
good example of this.
FUTURE UK CLIMATE
1. The main implication of climate change
for water resources management is that it is no longer feasible
to rely on the conventional assumption that past records are a
good guide to future availability. Unfortunately we cannot predict
how climate change will affect future water resources because
of the substantial uncertainties associated with its assessment,
so a scenario approach must be adopted considering a range of
plausible futures. This is well recognised in the water supply
industry and by the regulators, and scenarios were used in the
construction of water resource plans for the third and fourth
Asset Management Plans (AMP3 and 4).
Significant uncertainty surrounds the details
of possible future climate changes; some of this uncertainty can
be reduced by further scientific research, some is inherent in
predicting future climate. The particular aspects of potential
climate change that are of most importance for UK water security
(a) Changes in evapotranspiration. The potential
for evapotranspiration will definitely increase with increasing
temperature, with negative impacts for water supply/drought.
(b) Changes in rainfall. A number of characteristics
(b.i) Average rainfall. Changes to the annual
average are uncertainfor example, UKCIP98 gave an increase,
while UKCIP02 gave a decrease. This controls whether the main
concern will be water supply or flood management.
(b.ii) Seasonality. It is more certain that
there will be increased seasonality, with drier summers and wetter
winters (the uncertainty in the smaller annual average change
results from sensitivity to whether the summer drying exceeds
the winter wetting, or vice versa). Increased seasonality in the
direction expected has negative impacts generally, and also allows
the possibility that both drought and floods could become more
frequent (within their seasons).
(b.iii) Geographic distribution. Somewhat
uncertain, given that the resolution of the geographic features
of the UK is near to the limit of what current climate models
can simulate with rigorously tested reliability, but available
scenarios indicate an increased heterogeneity, with enhanced gradients
in precipitation between the S or SE of the UK and the N or NW.
(b.iv) Variability on various time scales.
Science is still inadequate to provide the required information
about possible changes in variability, partly in terms of the
development of capable models, but more particularly in terms
of evaluating and analysing the outputs of those models. Key (all
inter-related) factors are whether the variability increases or
decreases at daily to inter-annual time scales; whether there
is a concentration of rain into fewer (or more) days of rain,
with more intense rainfall on those days; whether extreme events
(on various time scales) become more or less frequent; and whether
the sequencing of rainfall changes, altering the occurrence of
long periods of dry or wet, or sequences of dry or wet months
Increased variability is important because it
can lead to increases in both droughts and floods. Changes in
intensity could alter the balance between runoff and groundwater
recharge. There are various other effects. Scientific uncertainty
is high, though indications are that sequences, for example, of
dry months/seasons are unlikely to change over and above the changes
caused directly by changed average rainfall (because the UK relies
less on recycling of water by evaporation than, for example, continental
interiors, and hence feedbacks between previous and subsequent
months are smaller). But changes in dry and wet extremes, over
and above the changes caused directly by changed average rainfall,
are a real possibility.
2. The effects of climate change on the
adequacy of water supplies depends partly on changes in river
flows and groundwater recharge, but also on how the supplies are
managed and the current balance between supply and demand. For
example, the effect on supplies to a town (within a specific resource
zone) of a given change in river flows depends on whether supplies
are taken directly from a river or from a reservoir, and the volume
of abstractions relative to river flows. A particular change in
flows or recharge will therefore have a different effect on the
security of supplies in different catchments or supply zones.
Therefore, some water companies currently have a healthy supply-demand
balance (eg, in the north), whereas others have a water deficit
during dry years (eg, in the southeast). This differentiation
is due to a range of factors such as population growth, leakage,
rainfall patterns, etc. It does emphasise that there is a geographic
dimension, in terms of water resource zones, to the adequacy of
present water supplies.
3. Under the UKCIP02 scenarios, it is well
established that river flows are likely to decrease during spring,
summer and autumn across much of the UK, and show only slight
increases in winter. Groundwater recharge is unlikely to increase
substantially despite increases in winter rainfall, because the
recharge season will be reduced. The actual amounts of change,
however, are very uncertainlargely due to uncertainties
in how rainfall may change in the futureand different climate
change scenarios yield larger increases in winter runoff and smaller
changes in summer runoff. In many areas the rate of water extraction
from aquifers far outstrips the rate of natural recharge and is
4. The guidance on climate change provided
to water-supply companies by the regulators for AMP4 is arguably
the most explicit anywhere in the world: there is a clear methodology
for estimating the broad-scale effects of climate change on supply
yields, and also on demand, as well as guidance on what to do
if the impacts are estimated to be felt by defined time horizons.
5. Underneath this broad framework, however,
the water industry and its regulators are facing a number of practical
methodological problems with coping with climate change. Central
to these is the assessment of the likelihood of different impacts
of climate change. The industry uses a risk-based approach to
estimate "headroom" allowances for its estimated reliable
yields, taking into account a range of uncertainties, but climate
change uncertainty is not yet included in a very realistic way:
only the range between UKCIP scenarios is considered, and it is
known that the UKCIP scenarios do not span the full range of possible
climate-change effects. The industry recognises this limitation,
and has commissioned research with the Tyndall Centre to incorporate
climate-change uncertainty into the headroom methodology in a
more sophisticated manner. The industry is able to cope with this
information, and indeed in many senses is pushing science forward.
6. Whilst OFWAT accepts in principle the
need for investment to maintain supplies in the face of climate
change, this principle has yet to be tested in practice. Such
a test will be the first challenge for the new methods for incorporating
uncertain climate change into operational water planning. In some
instances there may be a mismatch of planning horizons between
OFWAT, the economic regulator (five years) and the EA (30 years).
This can create difficulties for water companies which, for example,
have to make big investments over long timescales and lead-in
times (as for a reservoir, 20 years) which are likely to be affected
by climate change.
7. Irrigators are arguably more exposed
to climate-change impacts on water resources because most rely
on a single source; individual irrigators do not have the same
opportunities as water companies to switch during droughts between
surface and groundwater, or between abstraction points. Many catchments
are already considered over-licensed or even over-abstracted,
and many licences are under threat even without climate change.
Further reductions in surface flow and reliability would have
8. Agricultural abstractors will be particularly
affected by decisions on how environmental flows are protected
under climate change. The Agency's present methodology sets Hands
Off Flows (HOFs) to protect the minimum hydrograph, with the balance
available for abstraction. If HOFs are simply held constant whilst
flows decline, the abstractors will be squeezed disproportionately.
The necessary debate over how and when environmental flows should
be changed with climate change has not yet occurred.
9. Simultaneously, climate change is likely
to increase irrigation water demand. Warmer summers will increase
demand for salad crops, and all irrigated crops will transpire
more water. Higher atmospheric carbon dioxide concentrations may
offset some of the increase, and also help produce higher yields
(and hence use water more efficiently), Overall, increases in
water demand of around 30% by the 2020s and 30% by the 2050s have
been forecast, over and above changes (mostly increases) due to
socio-economic change, but there are major uncertainties involved.
10. Given higher demand and scarcer supplies,
irrigators will have to adapt. Sustainable water resources and
adaptation options in the rural sector (agriculture and leisure)
are being studied in a current Tyndall Centre project. Winter
storage reservoirs, rainfall harvesting, better equipment and
better scheduling will all have a role. Irrigation will be further
concentrated on the highest value crops (irrigation of low value
crops has already stopped). Some crops may have to be grown further
north and west. Water management policies that leave flexibility
to abstractors will allow more adaptation options; the present
moves towards licence trading are welcome in this context.
11. Climate change is likely to increase
flood risk, both inland and along the coast, although the change
in flood frequencies, particularly inland, is highly uncertain
and will vary from catchment to catchment. The impact of climate
change will also depend on the rate of change of exposure to the
12. In general terms, there are three main
ways of managing flooding. The first is to construct physical
flood defences (dykes and embankments, for example), to prevent
water from the sea or river flooding land, and this has been the
traditional approach to flood management in the UK. Climate change
affects this approach by altering the standard of service provided
by existing defences and making it difficult to design future
defences (although Defra allows a defined allowance to be considered).
The second approach is to reduce exposure to flood loss by preventing
development in flood-prone areas, making sure that individual
properties are flood-proofed, and by implementing flood-warning
schemes. Future flood losses will be very dependent on how successful
these measures are at reducing exposure. Measures to reduce exposure
are relatively insensitive to the precise characteristics of climate
change. The third group of measures manage the flood risk by reducing
vulnerability to loss. In the UK (almost uniquely) this is largely
done through the provision of flood insurance, although poorer
flood victims are much more likely to be uninsured than wealthier
victims. Climate change affects the financial viability of measures
to reduce vulnerability by changing the frequency and magnitude
13. Measures to reduce current and future
exposure to flooding are therefore likely to be the least sensitive
to the effects of climate change.
14. Climate change affects in-stream and
riverine ecosystems primarily through changes in river flow regimes
(the volume and timing of flow through the year) and water temperature.
In practice it will be impossible to offset the effects of increases
in water temperature, but it may be possible to introduce river
management regimes which seek to maintain current flow regimes
and hence ecosystems. However, this is potentially unsustainable
in the long term, and would magnify impacts on abstractors in
the medium term. A more appropriate policy would therefore be
to seek to encourage ecosystems and species to migrate along the
river corridor and to ensure that actions taken to meet other
impacts of climate changesuch as increased abstractiondo
not exaggerate the effects of climate change on the water environment.
Further research into practical application of these adaptation
options will be required as ecosystems change.
Prepared by Professor Nigel Arnell, Tyndall
Centre and University of Southampton, Dr Keith Weatherhead, Tyndall
Centre and Cranfield University, Mr Suraje Dessai, Tyndall Centre,
University of East Anglia, Dr Tim Osborn, Climatic Research Unit,
University of East Anglia, and Dr Declan Conway, Tyndall Centre
and School of Development Studies, University of East Anglia.