Memorandum submitted by the Met Office
CLIMATE CHANGE AND WATER SECURITY
Note that research at the Hadley Centre, on
which this submission of evidence is based, is largely funded
by the Global Atmosphere Division of Defra, with additional resources
from the Ministry of Defence and the European Commission.
SUMMARY
The Terms of Reference of this inquiry are concerned
with the future adequacy of water supplies and with flood management.
Central to both of these issues is the effect that climate change
will have, and may already be having, on precipitation. This submission
from the Met Office summarises the changes that have recently
happened to precipitation climate, and those which are predicted
to take place in the future. It demonstrates that good progress
is being made in predicting detailed patterns of change over the
UK. Nevertheless, substantial uncertainties remain which are a
hindrance to estimating impacts and hence to efficient adaptation.
The challenge for the next few years is not only to reduce these
uncertainties but also to quantify them. The probabilistic predictions
which will become available over the next few years will allow
planners to incorporate the effects of climate change into formal
risk assessments and hence plan adaptation more cost-effectively.
1. RECENT CLIMATE
CHANGE AND
ITS CAUSES
1.1 Substantial changes in climate have
been observed over the past century, and particularly over the
past 3 or 4 decades. Since the 1970s, global temperatures have
risen rapidly by about 0.5ºC. Research at the Met Office
Hadley Centre has demonstrated that this rise is extremely difficult
to explain other than by invoking human activities, particularly
the enhancement of the greenhouse effect by fossil fuel burning.
More recently, we have been able to attribute recent warming at
a continental scale (for example, over Europe) to human activities.
And, although this attribution cannot yet be made at the scale
of the UK, it would not be unreasonable to link human activities
to the warming of almost 1ºC in the Central England Temperature
that has been observed over the past few decades.
1.2 Changes in precipitation have also been
observed. England and Wales precipitation has increased in winter
months and decreased in summer months, and the proportion of winter
precipitation which falls in heavy events has changed, too, over
the past 40 years; with increases in winter months and decreases
in summer. Such changes are in line with model predictions of
the effect of human activities although, once again, we cannot
formally attribute them to that cause. The observed changes in
rainfall over the UK are larger than we would expect from the
current level of warming, possibly due to a contribution from
changes in circulation patterns (winds) which may or may not be
associated with human-made changes in greenhouse gases.
2. PREDICTIONS
OF FUTURE
CHANGE IN
CLIMATE
2.1 We expect to see climate changing in
the future as emissions of greenhouse gases, particularly carbon
dioxide, increase from fossil fuel burning. In order to estimate
the change we use a global climate model which represents the
climate system in the atmosphere, land, ocean and cryosphere;
in our case the Hadley Centre HadCM3 model which is recognised
to be world-leading. We drive this with scenarios of future emissions
of greenhouse gases which have been developed by the Special Report
on Emission Scenarios of the Intergovernmental Panel on Climate
Change; these depend upon assumed changes in population, energy
use, technology etc and range (in the case of carbon dioxide)
from relatively little change in emissions over the next 100 years
to a quadrupling of emissions. Under the lowest of these scenarios
we predict a temperature rise over land by 2100 of almost 3ºC,
under the highest scenario this becomes about 7ºC.
2.2 The climate model also predicts patterns
of change, in climate quantities such as rainfall, at a coarse
resolution of about 300 km. In order to obtain more detail in
the predictions over the UK, we "downscale" the global
results to a resolution of 50 or 25 km using a regional climate
model (PRECIS). Results from this were used to generate new climate
change scenarios for the UK Climate Impacts Programme (referred
to as UKCIP02), launched by Mrs Beckett in April 2002. The results
predict that, by the 2080s, under the highest emissions scenario,
rainfall over Southern England will be reduced by more than 50%
in summer and over most of England it will increase by 25% or
more in winter (Fig 1). For other emissions scenarios, and in
earlier future periods, changes will be correspondingly less,
as they will also be over more northerly and westerly parts of
the UK.
2.3 As well as changes in amounts of rainfall,
we predict that the frequency of intense rainfall events will
also change, such that in wintertime over most parts of the UK
these will at least double. This has obvious implications for
the frequency with which river and urban flooding could occur,
and for the intensity of the 1-in-100 year flood. Research in
collaboration with CEH Wallingford will translate the climate
scenarios we have developed into river flows and river levels,
so that change in flood risk can be estimated. In summertime the
opposite will be the case.
3. UNCERTAINTIES
IN PREDICTIONS
3.1 The UKCIP02 scenarios are probably the
best available for any country, and are being extensively used
to study the impacts of climate change on various sectors and
regions of the UK. However, as is clearly pointed out in the report,
but not always heeded, the scenarios contain substantial uncertainties
which should be taken into account when impacts are assessed.
3.2 The first uncertainty is due to the
lack of certainty in future emissions; this is handled in UKCIP02
by showing scenarios for the full range of possible future emissions.
In the event, because of the great inertia of the climate system,
climate change up to the middle of the century doesn't depend
much on emissions to that point, because most of the early-century
change is already built into the climate system by current emissions
and those over the past few decades. At the end of the century,
climate change is very dependent on which emissions path the world
follows.
3.3 The second uncertainty is due to the
climate models themselves, both the global one required to estimate
broad-scale changes and the regional one used to downscale these
to a resolution useful for impacts assessment. The models incorporate
our best representation of the climate system, and in particular
the interactions between different components in it (feedbacks)
but this understanding is limited (largely by lack of observations
in the real world atmosphere, oceans, etc) and the models themselves
cannot even represent this limited understanding (in part because
of the limitations of supercomputing power and hence the resolution
of the global model).
The change in annual global average rainfall
predicted by the 11 models shown in the last IPCC report ranges
from 1% to 9%. On the scale of the whole UK, the situation is
worse: in wintertime the increase ranges from 1% to 60%. And for
East Anglia it ranges from a small decrease of a few percent to
an increase of over 50%. Although the Hadley Centre model has
been extensively validated and is a world-leader, and its predictions
fall roughly in the middle of the range from all models, we cannot
formally claim that its predictions are any more or less likely
to be correct than any of the others.
3.4 Faced with this uncertainty, the flood
defence planner is put in an unenviable position; if he plans
on the basis of the least sensitive model his defence costs will
be small or even zero but he runs the risk of major flooding if
the most sensitive model turns out to be close to reality; if
he plans for the worst he could waste billions of pounds on protection
which turns out not to be needed. At present we cannot even rank
the various model predictions in terms of credibility.
4. PROBABILISTIC
PREDICTIONS
4.1 What planners really wants is not several
deterministic predictions of unknown relative quality, but a probabilistic
prediction of change, which tells them the probability of different
levels of change which can be factored into formal risk assessments.
So, for example, this might say that the probability of winter
rainfall in SE England increasing by 10% or more by 2050 is 90%,
the probability of it by 30% or more is 15%, etc (values given
are illustrative). For the first time, this is now possible through
the use of model ensembles. The technique involves taking a climate
model and changing within it one of the representations of processes
in the atmosphere, ocean, etc to make a new variation of the climate
model. This is then run to predict changes (for example, in SE
England winter rainfall by 2050). A further process in the model
is changed and a different prediction ensues. By doing this a
large number of times, rating the credibility of each of the models
by reference to its performance at simulating the current climate,
and combining all the predictions, we can form a probability distribution
of change. In the Hadley Centre this has been started using 50
variations of the climate model, and Fig 2 shows the initial probability
predictions that have been made. These are not yet useable, as
they only include an initial selection of representation uncertainties,
but they will develop over the next 2 or 3 years to the point
where they do provide useable information to planners. Hence one
valid option for planners would be to delay decisions until they
have better information to work with.
Fig 2. Preliminary probability predictions of
% changes in South-East England precipitation in summer (left)
and winter (right) by the 2080s under a Medium-High emissions
scenario.
4.2 Up to now, these probabilistic scenarios
have been generated using global climate models, which have a
poor resolution (300km) and do a poor job of estimating changes
in extremes. Both these limitations are overcome by using regional
climate models (RCMs) as mentioned earlier, and the goal is to
have high-resolution probabilistic predictions by the time of
the next set of climate change scenarios for UKCIP in about 2007.
5. NATURAL VARIABILITY
5.1 The last main uncertainty in prediction
future climate is natural variability. We know that, because interactions
between the atmosphere and oceans makes the climate system a chaotic
one, climate varies widely from one year to the next unconnected
with any human influence. The UK, because it is at the end of
Atlantic storm tracks and at the interface between continent and
ocean, has a very variable climate, and for precipitation this
variability is likely to be greater than any underlying human-made
change over the next few decades. Until recently it was taken
for granted that the variability was unpredictable, but recent
research is indicating that some useable predictability exists,
and useable forecasts over the next decade ("decadal"
forecasts) will be possible in the future. This is partly due
to developments in modelling, and partly due to the increasing
availability of good observational data from the oceans (which,
because of their inertia, determine the state of the climate over
the next few years). Hence, any planning for flood defences or
water availability should also take account of decadal forecasts
once they become available.
6. SEA LEVEL
RISE AND
STORM SURGES
6.1 Global warming due to human activities
will cause the sea level to rise, mainly due to the expansion
of ocean waters and the melting of land ice. There is a wide range
of uncertainty in the global average rise over the next 100 years,
ranging from 10cm to 90cm. The rise will not be uniform across
the globe, and local effects add to this uncertainty. Flooding
mainly occurs when storm surges, produced by a combination of
high winds and low pressures, hit vulnerable coastlines. Surge
heights for which coastal defence will have to cater will change
both by rising sea levels and changes in storminess; the latter
is notoriously difficult to predict. Nonetheless, modelling done
for the UKCIP02 report showed that even a models rise in sea level,
of 30cm by the 2080s, coupled with changes in land levels, would
change the height of the typical "50-year" storm surge
by up to a metre or more in some parts of the coast, with other
parts seeing 30 or 40cm. Again, uncertainties in sea level predictions
are substantial, arising from both the climate model and the storm
surge model. Probability predictions will be made, on a somewhat
longer timescale than those of precipitation and river flows.
The Met Office
April 2004
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