EXECUTIVE
SUMMARY
Climate change due to human activities is expected
to increase the risk of flooding in England and Wales, in two
distinct ways. Firstly, by the 2050s the rise in sea level consequent
on global warming is predicted to increase the frequency of dangerous
high water levels from once a century to, typically, once a decade.
If storminess were to increase, and there is no consensus view
that this will happen, the situation would be further exacerbated.
Secondly, it is predicted that days with heavy rainfall will become
typically three or four times more common, and this will increase
the risk of inland flooding in certain areas.
Following a brief introduciton to climate change
and how it is predicted, more detail is given on the expected
changes relevant to flood risk, based on The Met Office's world-leading
climate model. Nevertheless, such predictions are very uncertain
due to our incomplete understanding of the climate system. The
maintenance of research effort should enable them to be refined
over the coming years.
Most of the research described in this submission
has been carried out within the Climate Prediction Programme contract
from the Department of the Environment, Transport and the Regions,
and the Public Meteorological Service Research and Development
Programme, funded by the Ministry of Defence.
1. Climate change due to human activities
Climate varies naturally from year to year and
decade to decade. It also changes over periods of tens of thousands
of years, ranging from ice ages to interglacials, due to small
variations in the earth's orbit around the sun. However, it is
increasingly accepted that human activities are also likely to
change global climate substantially over the next century and
beyond, and that this process may have started already. Although
there is a range of activities which could change climate, the
main effect will come from the emission of greenhouse gases, such
as carbon dioxide from fossil fuel burning. These gases can remain
in the atmosphere for decades or centuries; they act to trap some
of the heat from the earth (which would otherwise be lost to space)
and hence raise the temperature at the earth's surface. This mechanism
is referred to as the greenhouse effect; it has been known about
for well over a century, and is scientifically well established.
The natural greenhouse effect (due to naturally occurring water
vapour and carbon dioxide in the atmosphere) keeps the earth over
30C warmer that it would otherwise be, and hence inhabitable.
Concern arises from the prospect that human-made additions to
the natural concentrations of greenhouse gases will lead to a
change in climate which would threaten sustainability in many
parts of the world.
2. Making estimates of future climate change
Predicting future climate involves several complex
stages. Firstly, projections are made of future human-made emissions
of the greenhouse gases; these come from models which take account
of such factors as growth in population, energy demand and technological
change. Next, models of the carbon cycle are used to calculate
how much of the human-made emissions of carbon dioxide will be
taken up by the ocean and the land biosphere, and hence the amount
retained in the atmosphere. Finally a climate model is used to
calculate the effect of human-made changes of greenhouse gas concentrations.
A climate model is a mathematical representation of the real climate
system; atmosphere, land, oceans and cryosphere (snow and ice).
The model incorporates equations describing the known physical
laws as they apply to the climate system and these are solved
at a number of points forming a grid over the globe; over the
UK the grid points are about 300 kilometres apart. The ocean is
important because it will absorb some of the heat from the atmosphere
and act as a delaying mechanism. It also contains strong currents
which transport vast quantities of heat between the equator and
the poles.
One of the main uncertainties in climate predictions
is the nature of "feedbacks", ie processes which follow
a warming and which can act to amplify or reduce the sensitivity
of climate to increases in greenhouse gas concentrations. The
melting of sea-ice, for example, will reduce the amount of sunlight
reflected, and thus enhance the warming in high latitudes.
Climate can also be affected by a number of
other agents in addition to greenhouse gases. Small sulphate particles
("aerosols") from industrial sulphur dioxide emissions
have a cooling effect on climate. However, the size of this effect
is very uncertain and, although calculations have been made, it
should be noted that the results described in this submission
concentrate on the effect of greenhouse gases only.
In the UK, The Met. Office's Hadley Centre has
developed, over the past 20 years, one of the most sophisticated
climate models in the world, and results of experiments with this
model form the basis for this submission. Nevertheless, substantial
uncertainties are attached to every stage of the process of climate
prediction: projection of emissions, conversion to atmospheric
concentrations, and the sensitivity of climate to changes in greenhouse
gas and aerosol concentrations.
3. Predictions of climate change
The climate model is run for many hundreds of
(simulated) years, starting from the middle of the last century
when any human influence would have been negligible. From 1860
to 1995, observed changes in greenhouse gases are used in the
model to estimate changes in climate to date. From 1995 to 2100,
a "non-intervention" scenario of future greenhouse gas
emissions, derived by the Intergovernmental Panel on Climate Change
(IPCC), is used.
Predictions from the Hadley Centre climate model
are shown in Figure 1. Global average temperatures are expected
to rise by about 3C, and global average rainfall to increase
by about 5 per cent, over the next century. These changes will
not be uniform across the globe; for example, temperature rises
will be greatest at higher latitudes in winter and smaller over
the oceans. Rainfall is predicted to increase in most areas, but
decrease in some areas; changes are greatest in tropical regions.
4. Has climate change already begun?
One of the current most pressing questions,
both scientifically and politically, is: Have human activities
already begun to alter global climate? We address this by asking
in turn, firstly: Has climate changed?, secondly: Has this change
been unusual? and, thirdly: Is the change attributable to human
activities?
Measurements since 1860, analysed by the Hadley
Centre and the University of East Anglia, shown in Figure 2, demonstrate
that global temperatures have risen by about 0.6C since the
last century. 1997 was the warmest year on record, and the first
five months of 1998 have been warmer than any similar period.
Hence there has indeed been a real change in global climate. Based
on computer simulations and limited observational evidence (for
example, indications from tree rings) we conclude that the observed
temperature in recent decades has gone beyond that expected from
natural variability. To attribute this rise to specific causes,
we compare climate model simulations of patterns of global temperature
change expected from human-made increases in greenhouse gases
and other factors ("fingerprints") with detailed observations,
both across the surface of the earth and through the depth of
the atmosphere. These comparisons increasingly lead us to believe
that human activities have played at least a part in the warming
seen in recent decades, and may well be the main cause.
5. Changes in sea level
As the atmosphere warms due to increasing greenhouse
gases, heat will penetrate into the ocean water and cause it to
expand. It is this simple thermal expansion of the oceans which
will provide the main contribution to sea level rise. There are
two other contributors to sea level rise. Firstly, higher temperatures
on land will cause glaciers and snowfields to melt; the meltwater
will run off into the oceans. Secondly, there will be changes
to the layers of ice several kilometres deep which cover Greenland
and Antarctica. Note that neither Arctic sea-ice nor Antarctic
ice-shelves (such as those which have broken away recently) will
make any contribution to sea level rise as they melt, since they
are already floating. Based on a careful analysis of tide-gauge
records, global sea level is estimated to have risen by between
10 centimetres and 25 centimetres since pre-industrial times.
It is likely that much of this rise has been related to the rise
in global temperature seen over the same period.
The contributions to future sea level rise from
thermal expansion, from glacier melt and from changes in the Greenland
ice-sheet have been calculated by the Hadley Centre in cooperation
with the University of Utrecht; these are shown in Figure 3. (The
contribution from Antarctica is expected to be small, and may
even be negative, as snowfall there is expected to increase in
a warmer world). The total rise in sea level due to human activities
is predicted to be just over 50 centimetres between now and the
end of the next century, about half of this coming from thermal
expansion. The uncertainty in this prediction is large; if human-made
emissions and the sensitivity of global climate are both at the
high end of the range of estimates, the sea level rise would be
about 100 centimetres. If both factors are at the low end, the
rise could be as small as 15 centimetres. In this submission we
assume a central estimate for these factors. Note that the Kyoto
Protocol agreed in December 1997, even if fully ratified, will
reduce the best estimate of sea level rise by about 2 centimetres
at 2100; ie only a few per cent of the total rise expected.
The rise in sea level is not expected to be
uniform across the globe; because of variations in temperature,
salinity and ocean currents, some parts of the ocean are expected
to rise by perhaps 50 per cent more than the global average, and
others some 50 per cent less. Seas around the British Isles show
a predicted rise about 10 per cent greater than the global average.
As the greenhouse heating penetrates deeper
and deeper into the ocean, thermal expansion will continue. Even
if the increase in atmospheric greenhouse gas concentrations were
to be completely halted (by 60 per cent cutbacks in emissions)
this heat penetration, and hence the thermal expansion, would
continue. This is illustrated in Figure 4 in which carbon dioxide
concentrations have been increased in the model by 1 per cent
per annum for 70 years, after which no further increase was made.
The thermal expansion which has taken place in the 500 years after
greenhouse gas concentrations are stabilised is seen to be many
times greater than that during the 70-year period of rising concentrations.
This stored-up, irreversible, commitment has obvious relevance
to policy on emissions.
In addition to sea-level rise due to climate
change, a change in the level of the sea relative to coastal land
will also occur due to natural vertical movements of the land
"rebounding" after the retreat of ice-sheets, which
weighed down the earth's crust, at the end of the last ice age.
In general, the southern UK is sinking and the northern UK is
rising. The extent of this change by the middle of the next century
is estimated (by Dr Ian Shennan, University of Durham); in Table
1 this has been added to the climate change sea level rise, to
calculate the net change in sea level for the decade of the 2050s
relative to today's (1998). Note that current MAFF guidelines
assume 6 millimetres/year in south and east England, 5 millimetres/year
in South West England and Wales, and 4 millimetres/year in North
England; differences from the scenario below are small compared
to overall uncertainties.
Table 1. Changes
in sea level (centimetres) relative to coastal land, for the decade
of the 2050s relative to today's.
