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lation (NAO “ that has a strong in¬‚uence on the character of the winters
in north west Europe) and the El Ni˜ o events mentioned in Chapter 5
n
are examples of such regimes. Important components of climate change
in response to the forcing due to the increase in greenhouse gases can be
expected to be in the form of changes in the intensity or the frequency of
established climate patterns illustrated by these regimes.14 There is little
consistency at the present time between models regarding projections of
many of these patterns. However, recent trends in the tropical Paci¬c for
the surface temperature to become more El Ni˜ o-like (see Table 4.1 on
n
128 Climate change in the twenty-¬rst century and beyond



page 62“3), with the warming in the eastern tropical Paci¬c more than
that in the western tropical Paci¬c and with a corresponding eastward
shift of precipitation, are projected to continue by many models. There
is also evidence that warming associated with increasing greenhouse gas
concentrations will cause an intensi¬cation of the Asian summer mon-
soon and an increase of variability in its precipitation. The in¬‚uence of
increased greenhouse gases on these major climate regimes, especially
the El Ni˜ o, is an important and urgent area of research.
n
A complication in the interpretation of patterns of climate change
arises because of the differing in¬‚uence of atmospheric aerosols as com-
pared with that of greenhouse gases. Although in the projections based
on the SRES scenarios the in¬‚uence of aerosols is much less than in those
based on the IS 92 scenarios published by the IPCC in its 1995 Report,15
their projected radiative forcing is still signi¬cant. When considering
global average temperature and its impact on, for instance, sea level rise
(see Chapter 7) it is appropriate in the projections to use the values of
globally averaged radiative forcing. The negative radiative forcing from
sulphate aerosol, for instance, then becomes an offset to the positive
forcing from the increase in greenhouse gases. However, because the
effects of aerosol forcing are far from uniform over the globe (Figure
3.7), it is not correct, when considering climate change and its regional
characteristics, to consider the effects of increasing aerosol as a simple
offset to those of the increase in greenhouse gases. The large variations in
regional forcing due to aerosols produce substantial regional variations
in the climate response. Detailed regional information from the best cli-
mate models needs to be employed to assess the climate change under
different assumptions about the increases in both greenhouse gases and
aerosols.


Changes in climate extremes
The last section looked at the likely regional patterns of climate change.
Can anything be said about likely changes in the frequency or intensity of
climate extremes in the future? It is, after all, not the changes in average
climate that are generally noticeable, but the extremes of climate “ the
droughts, the ¬‚oods, the storms and the extremes of temperature in very
cold or very warm periods “ which provide the largest impact on our
lives (see Chapter 1).
The most obvious change we can expect in extremes is a large
increase in the number of extremely warm days (Figure 6.6) coupled
with a decrease in the number of extremely cold days. A number of
model projections show a generally decreased daily variability of surface
air temperature in winter and increased daily variability in summer in
Changes in climate extremes 129



Figure 6.6 Schematic
Increase in mean
diagrams showing the
Probability of occurrence



(a) effects on extreme
More temperatures when (a) the
hot
Previous mean increases leading to
weather
climate
more record hot weather,
More
Less record hot (b) the variance increases
cold weather
New and (c) when both the
weather climate
mean and the variance
increase, leading to much
Cold Average Hot more record hot weather.

Increase in variance
Probability of occurrence




(b) Previous
climate
More
more hot
cold weather
More
weather
record More
New
cold record hot
climate
weather weather



Cold Average Hot


Increase in mean and variance
Probability of occurrence




(c) Much more
Previous hot
climate weather

More
record hot
Less
weather
change
for
cold New
weather climate

Cold Average Hot



Northern Hemisphere land areas, suggesting that the situation in Figure
6.6(c) could apply in these areas. An example of this can be found in the
box in Chapter 7 on page 177.
However, the changes that are likely to lead to most impact are those
connected with the hydrological cycle. In the last section it was explained
that in a warmer world with increased greenhouse gases, average pre-
cipitation increases and the hydrological cycle becomes more intense.16
Consider what might occur in regions of increased rainfall. Often in such
regions with the more intense hydrological cycle the larger amounts of
rainfall will come from increased convective activity: more really heavy
showers and more intense thunderstorms. The result of a study with an
Australian climate model of the effect on rainfall amounts of doubling
130 Climate change in the twenty-¬rst century and beyond



Figure 6.7 Changes in the
frequency of occurrence of
different daily rainfall amounts
with doubled carbon dioxide
as estimated by a CSIRO
model in Australia.




the carbon dioxide concentration is shown in Figure 6.7. The number of
days with large rainfall amounts (greater than 25 mm day’1 ) doubled.
The probability of conditions leading to ¬‚oods would also at least have
doubled. Similar results (fewer rainy days, higher maximum daily rain-
falls for a given mean rainfall rate) have been obtained from many other
climate models. For instance, a recent modelling study (Figure 6.8) has
shown that if atmospheric carbon dioxide concentration is doubled from
its pre-industrial value, the probability of extreme seasonal precipitation
in winter is likely to increase substantially over large areas of central and
northern Europe and likely to decrease over parts of the Mediterranean
and north Africa. The increase in parts of central Europe is such that
the return period of extreme rainfall events would decrease by about a
factor of ¬ve (e.g. from ¬fty years to ten years). Similar results have been
obtained in a study of major river basins around the world.17
Note also from Figure 6.7 that the number of days with lighter rain-
fall events (less than 6 mm/day) is expected to decrease in the globally
warmed world. This is because, with the more intense hydrological cy-
cle, a greater proportion of the rainfall will fall in the more intense events
and, furthermore, in regions of convection the areas of downdraught be-
come drier as the areas of updraught become more moist. In many areas
with relatively low rainfall, the rainfall will tend to become less “ which
has implications for the likelihood of drought.
Take, for instance, the likelihood of drought in regions where the
average summer rainfall falls by perhaps twenty per cent as is likely to
occur, for instance, in southern Europe (Figure 6.5(b)). The likely result
of such a drop in rainfall is not that the number of rainy days will remain
the same, with less rain falling each time; it is more likely that there
will be substantially fewer rainy days and considerably more chance of
Changes in climate extremes 131



Greenhouse/control
2

2
2
5
7

5

7
5
5

5
3
5
5
2
3
2
2
2
1
1




1 2 3 5 7

Figure 6.8 The changing probability of extreme season precipitation in Europe
in winter as estimated from an ensemble of nineteen runs with a climate model
starting from slightly different initial conditions. The ¬gure shows the ratio of
probabilities of extreme precipitation events in the years sixty-one to eighty of
eighty-year runs that assumed an increase of carbon dioxide concentration of
one per cent per year (hence doubling in about seventy years) compared with
control runs with no change in carbon dioxide.


prolonged periods of no rainfall at all (see Figure 6.9). In other words,
much more likelihood of drought. Further, the higher temperatures will
lead to increased evaporation reducing the amount of moisture available
at the surface “ thus adding to the drought conditions. The proportional
increase in the likelihood of drought is much greater than the proportional
decrease in average rainfall.
Thus in the warmer world of increased greenhouse gases, different
places will experience more frequent droughts and ¬‚oods “ we noted in
Chapter 1 that these are the climate extremes which cause the greatest
problems and we will be considering the impacts in more detail in the
next chapter.
What about other climate extremes, intense storms, for instance?
How about hurricanes and typhoons, the violent rotating cyclones that
are found over the tropical oceans and which cause such devastation
when they hit land? The energy for such storms largely comes from
the latent heat of the water which has been evaporated from the warm
132 Climate change in the twenty-¬rst century and beyond




ocean surface and which condenses in the clouds within the storm, re-
Figure 6.9 Daily rainfall for
Italy for a three-year period leasing energy. It might be expected that warmer sea temperatures would
simulated by a climate model, mean more energy release, leading to more frequent and intense storms.
(a) for the current climate
However, ocean temperature is not the only parameter controlling the
situation and (b) for the
genesis of tropical storms; the nature of the overall atmospheric ¬‚ow is
climate if the equivalent
also important. Further, although based on limited data, observed vari-
carbon dioxide concentration
ations in the intensity and frequency of tropical cyclones show no clear
increased by a factor of four
from its pre-industrial value trends in the last half of the twentieth century. Models can take all the
(predicted for instance to
relevant factors into account but, because of the relatively large size
occur before 2100 under the
of their grid, they are unable to simulate very reliably the detail of rela-
A2 scenario).
tively small disturbances such as tropical cyclones. There is no consistent
Regional climate models 133



evidence from model projections of changes in the frequency of tropi-
cal cyclones or their areas of formation. However, model projections18
and theoretical studies suggest that, if carbon dioxide concentration is
doubled, the peak wind intensities will tend to increase by perhaps ¬ve or
ten per cent and the mean and peak precipitation intensities by twenty to
thirty per cent.
Regarding storms at mid latitudes, the various factors that control
their incidence are complex. Two factors tend to an increased intensity of
storms. The ¬rst, as with tropical storms, is that higher temperatures, es-
pecially of the ocean surface, tend to lead to more energy being available.
The second factor is that the larger temperature contrast between land
and sea, especially in the Northern Hemisphere, tends to generate steeper
temperature gradients, which in turn generate stronger ¬‚ow and greater
likelihood of instability. The region around the Atlantic seaboard of
Europe is one area where such increased storminess might be expected,
a result that some model projections have shown. However, such a picture
may well be too simple; other models suggest changes in storm tracks
that result in very different changes in some regions and there is little
overall consistency between model projections.
For some other extremes such as very small-scale phenomena (e.g.
tornadoes, thunderstorms, hail and lightning) that cannot be simulated in
global models, although they may have important impacts, there is cur-
rently insuf¬cient information to assess recent trends, and understanding
is inadequate to make ¬rm projections.
Table 6.2 summarises the state of knowledge regarding the likely fu-
ture incidence of extreme events. Although general indications of trends
can be given, there have been few projections with quantitative estimates
of likely changes in the frequency or intensity of extreme events. In many
research centres, work is under way on more detailed studies of the in-
¬‚uence of increased greenhouse gases on extreme events and changes in
climate variability.


Regional climate models
Most of the likely changes that we have presented have been on the scale
of continents. Can more speci¬c information be provided about change
for smaller regions? In Chapter 5 we referred to the limitation of global
circulation models (GCM) in the simulation of changes on the regional
scale19 arising from the coarse size of their horizontal grid “ typically 300
km or more. Also in Chapter 5 we introduced the regional climate model
(RCM) that typically possesses a resolution of 50 km and can be ˜nested™
in a global circulation model. Examples are shown in Figures 6.10 and
7.8 of the improvement achieved by RCMs in the simulation of extremes
134 Climate change in the twenty-¬rst century and beyond



Table 6.2 Estimates of con¬dence in observed and projected changes in extreme weather and
climate events

Con¬dence Changes in Con¬dence in
in observed changes phenomenon projected changes
(latter half of the twentieth century) (during the twenty-¬rst century)
Likelyb Higher maximum temperatures and Very likely
more hot days over nearly all land
areas
Very likely Higher minimum temperatures, fewer Very likely
cold days and frost days over nearly
all land areas
Very likely Reduced diurnal temperature range Very likely
over most land areas
Increase of heat indexa over land areas
Likely, over many areas Very likely, over most areas
Likely, over many Northern More intense precipitation events Very likely, over most areas
Hemisphere mid to high latitude
land areas
Likely, in a few areas Increased summer continental drying Likely, over most mid-latitude
and associated risk of drought continental interior (lack of
projections consistent in other areas)
Not observed in the few analyses Increase in tropical cyclone peak wind Likely, over some areas
available intensities
Insuf¬cient data for assessment Increase in tropical cyclone mean and Likely, over some areas
peak precipitation intensities

a Heat index is a combination of temperature and humidity that measures effects on human comfort.
b See Note 1 of Chapter 4 for explanation of likely, very likely, etc.

Source: Table SPM-1 from Summary for policymakers. In Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der
Linden, P. J., Dai, X., Maskell, K., Johnson, C. A. (eds.) Climate Change 2001: The Scienti¬c Basis. Contribution of Working
Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University
Press.




and in providing regional detail that in many cases (especially for precip-
itation) shows substantial disagreement with the averages provided by
a GCM.
Regional models are providing a powerful tool for the investigation of
detail in patterns of climate change. In the next chapter the importance
of such detail will be very apparent in studies that assess the impacts
of climate change. However, it is important to realise that, even if the
models were perfect, because much greater natural variability is apparent
in local climate than in climate averaged over continental or larger scales,
projections on the local and regional scale are bound to be more uncertain
than those on larger scales.
Longer-term climate change 135



Figure 6.10 Example of
100
simulations showing the
Observations
GCM probability of winter days
RCM
over the Alps with different
10
daily rainfall thresholds, as
observed, simulated by a




Probability (%)
300-km resolution GCM
1
and by a 50-km resolution
RCM. The RCM shows
much better agreement
0.1
with observations
especially for higher
thresholds.
0.01
<0.1 >10 >20 >30 >50
Thresholds (mm day ’1)




Longer-term climate change
Most of the projections of future climate that have been published cover
the Twenty-¬rst century. For instance, the curves plotted in Figure 6.2 ex-
tend to the year 2100. They illustrate what is likely to occur if fossil fuels
continue to provide most of the world™s energy needs during that period.
From the beginning of the industrial revolution until 2000 the burning
of fossil fuels released approximately 600 Gt of carbon in the form of
carbon dioxide into the atmosphere. Under the SRES A1B scenario it is
projected that a further 1500 Gt will be released by the year 2100. As
Chapter 11 will show, the reserves of fossil fuels in total are suf¬cient to
enable their rate of use to continue to grow well beyond the year 2100.
If that were to happen the global average temperature would continue
to rise and could, in the twenty-second century, reach very high levels,
perhaps up to 10 —¦ C higher than today (see Chapter 9). The associated
changes in climate would be correspondingly large and could well be
irreversible.
A further longer-term effect that may become important during this
century is that of positive feedbacks on the carbon cycle due to climate
change. This was mentioned in Chapter 3 (see box on page 40) and the
+30% uncertainty in 2100 in the atmospheric concentrations of carbon
dioxide shown in Figure 6.2 was introduced to allow for it. Taking this
effect into account would add nearly a further degree Celsius to the
projected increase in global average temperature in 2100 at the top end
of the range shown in Figure 6.4. Some of the further implications of
this feedback will be considered in Chapter 10 on page 255“259.
Especially when considering the longer term, there is also the possi-
bility of surprises “ changes in the climate system that are unexpected.
136 Climate change in the twenty-¬rst century and beyond



The discovery of the ˜ozone hole™ is an example of a change in the at-
mosphere due to human activities, which was a scienti¬c ˜surprise™. By
their very nature such ˜surprises™ cannot, of course, be foreseen. How-
ever, there are various parts of the system which are, as yet, not well
understood, where such possibilities might be looked for20 ; for instance,
in the deep ocean circulation (see box in Chapter 5, page 99) or in the
stability of the major ice sheets. In the next section we shall look in more
detail at the ¬rst of these possibilities; the second will be addressed
in the section of the next chapter entitled ˜How much will sea level
rise?™.



Changes in the ocean thermohaline circulation
The ocean thermohaline circulation (THC) was introduced in the box
on page 99 in Chapter 5, where Figure 5.18 illustrates the deep ocean
currents that transport heat and freshwater between all the world™s oceans.
Also mentioned in the box was the in¬‚uence on the THC in past epochs
of the input of large amounts of fresh water from ice melt to the region in
the north Atlantic between Greenland and Scandinavia where the main
source region for the THC is located.
With climate change due to increasing greenhouse gases we have
seen that the precipitation will increase substantially especially at high
latitudes (Figure 6.5(b)), leading to additional fresh water input to the
oceans. The dense salty water in the north Atlantic source region for the
THC will become less salty and therefore less dense. As a result the THC
will weaken and less heat will ¬‚ow northward from tropical regions to the
north Atlantic. All coupled ocean“atmosphere GCMs show this occur-
ring; an example is shown in Figure 6.11(a), which indicates a weakening
of about twenty per cent by 2100. Although there is disagreement be-
tween the models as to the extent of weakening, all model projections
of the pattern of temperature change under increasing greenhouse gases
show less warming in the region of the north Atlantic (Figure 6.5(a)) “
but none show actual cooling occurring in this region during the twenty-
¬rst century. In the longer term, some models show the THC actually
cutting off completely after two or three centuries of increasing green-
house gases. Figure 6.11(b) illustrates the effect that cut-off would have
on the pattern of surface temperature around the globe. Note the severe
cooling that would occur in the north Atlantic and north west Europe
and the small compensating warming in the Southern hemisphere. In-
tense research is being pursued “ both observations and modelling “ to
elucidate further likely changes in the thermohaline circulation and their
possible impact.
Other factors that might in¬‚uence climate change 137



Figure 6.11 (a) Change
(a) 22
in the strength of the
Circulation strength (Sv)

thermohaline circulation
(THC) in the north Atlantic
20
as simulated by the Hadley
Centre climate model for
different SRES scenarios.
No change
18
The unit of circulation is
SRES A1FI
the Sverdrup, 106 m3 s’1 .
SRES A2
SRES B1 (b) Changes in the surface
16
SRES B2 air temperature, relative to
the present day, twenty
years after the hypothetical
14
collapse of the
1850 1900 1950 2000 2050 2100
thermohaline circulation as
(b) 90° N
2 simulated by the Hadley
1




Centre climate model. To
45° N obtain the temperature
distribution for a real
situation, the changes due

to increased greenhouse
0




gases at the time of
45° S collapse would have to be
added to these changes
resulting from THC
90° S
collapse.
180° 90° W 0° 90° E 180°

+1 0 ’1 ’2 ’3 ’4 ’5
Temperature change (°C)




Other factors that might in¬‚uence climate change
So far climate change due to human activities has been considered. Are
there other factors, external to the climate system, which might induce
change? Chapter 4 showed that it was variations in the incoming solar
energy as a result of changes in the Earth™s orbit which triggered the ice
ages and the major climate changes of the past. These variations are, of
course, still going on; what in¬‚uence are they having now?
Over the past 10 000 years, because of these orbital changes, the solar
radiation incident at 60 —¦ N in July has decreased by about 35 W m’2 ,
which is quite a large amount. But over one hundred years the change is
only at most a few tenths of a watt per square metre, which is much less
than the changes due to the increases in greenhouse gases (remember
that doubling carbon dioxide alters the thermal radiation, globally
averaged, by about 4 W m’2 “ see Chapter 2). Looking to the future
and the effect of the Earth™s orbital variations, over at least the next
138 Climate change in the twenty-¬rst century and beyond




Does the Sun™s output change?

century when very few sunspots were recorded.21
Some scientists have suggested that all climate vari-
ations, even short-term ones, might be the result of Studies of the recent measurements of solar out-
changes in the Sun™s energy output. Such sugges- put correlated with other indicators of solar activ-
tions are bound to be somewhat speculative because ity, when extrapolated to this earlier period, suggest
the only direct measurements of solar output that are that the Sun was a little less bright in the seventeenth
century, perhaps by about 0.4% or about 1 W m’2 in
available are those since 1978, from satellites out-
side the disturbing effects of the Earth™s atmosphere. the average solar energy incident on the Earth™s sur-
These measurements indicate a very constant solar face. This reduction in solar energy may have been
output, changing by about 0.1% between a maxi- a cause of the cooler period at that time known as
mum and a minimum in the cycle of solar magnetic the ˜Little Ice Age™. Careful studies have estimated
activity indicated by the number of sunspots. that since 1850 the maximum variations in the solar
It is known from astronomical records and from energy incident on the Earth™s surface are unlikely
to be greater than about 0.5 W m’2 (Figure 6.12).
measurements of radioactive carbon in the atmo-
sphere that this solar sunspot activity has, from time This is about the same as the change in the energy
to time over the past few thousand years, shown regime at the Earth™s surface due to about a ten
large variations. Of particular interest is the period years™ increase in greenhouse gases at the current
known as the Maunder Minimum in the seventeenth rate.
Radiative forcing (W m’2)




Figure 6.12 Radiative forcing due to variations in the energy input from the Sun as estimated by Lean
et al. (1995) and by Hoyt and Schatten (1993).
Notes 139



50 000 years the solar radiation incident in summer on the polar regions
will be unusually constant so that the present interglacial is expected to
last for an exceptionally long period.22 Suggestions therefore that the
current increase of greenhouse gases might delay the onset of the next
ice age are unfounded.
These orbital changes only alter the distribution of incoming solar
energy over the Earth™s surface; the total amount of energy reaching
the Earth is hardly affected by them. Of more immediate interest are
suggestions that the actual energy output of the Sun might change with
time. As we mentioned in Chapter 3 (see Figure 3.8) and as is described
in the box above, such changes, if they occur, are estimated to be much
smaller than changes in the energy regime at the Earth™s surface due to
the increase in greenhouse gases.
There have also been suggestions of plausible indirect mechanisms
whereby effects on the Sun might in¬‚uence climate on Earth. Changes
in solar ultraviolet radiation will in¬‚uence atmospheric ozone and hence
might affect climate. There is a possibility that the galactic cosmic
ray ¬‚ux, modi¬ed by the varying Sun™s magnetic ¬eld, could in¬‚u-
ence cloudiness and hence climate. From studies of these possible
mechanisms there is as yet insuf¬cient evidence of signi¬cant climate
effects.23
Another in¬‚uence on climate comes from volcanic eruptions. Their
effects, lasting typically a few years, are relatively short-term compared
with the much longer-term effects of the increase of greenhouse gases.
The recent large volcanic eruption of Mount Pinatubo in the Philippines
which occurred in June 1991 has already been mentioned (Figure 5.21).
Estimates of the change in the net amount of radiation (solar and thermal)
at the top of the atmosphere resulting from this eruption are of about
0.5 W m’2 . This perturbation lasts for about two or three years while
the major part of the dust settles out of the atmosphere; the longer-term
change in radiation forcing, due to the minute particles of dust which
last in the stratosphere for somewhat longer, is much smaller.
To summarise this chapter:

r The increase in greenhouse gases is by far the largest of the factors
which can lead to climate change during the twenty-¬rst century.
r The likely climate changes for a range of scenarios of greenhouse gas
emissions have been described in terms of global average temperature
and in terms of regional change of temperature and precipitation and
the occurrence of extremes.
r The rate of change is likely to be larger than any the Earth has seen at
any time during the past 10 000 years.
140 Climate change in the twenty-¬rst century and beyond



r The changes that are likely to have the greatest impact will be changes
in the frequencies, intensities and locations of climate extremes, espe-
cially droughts and ¬‚oods.
r Suf¬cient fossil fuel reserves are available to provide for continuing
growth in fossil fuel emissions of carbon dioxide well into the twenty-
second century. If this occurred the climate change could be very large
indeed and have unpredictable features or ˜surprises™.

The next chapter will look at the impact of such changes on sea level,
on water, on food supplies and on human health. Later chapters of the
book will then suggest what action might be taken to slow down and
eventually to terminate the rate of change.


Questions
1 Suggest, for Figure 6.6, an appropriate temperature scale for a place you
know. De¬ne what might be meant by a very hot day and estimate the per-
centage increase in the probability of such days if the average temperature
increases by 1, 2 and 4 —¦ C.
2 From Figure 6.9, compare the maximum length of periods in the summer
with less than 1 mm, 2 mm, 5 mm of total rainfall under normal climate
conditions and under conditions with increased carbon dioxide.
3 It is stated in the text describing extremes that in convective regions, with
global warming, as the updraughts become more moist the downdraughts
tend to be drier. Why is this?
4 Look at the assumptions underlying the full range of SRES emission sce-
narios in the IPCC 2001 Report. Would you want to argue that some of the
scenarios are more likely to occur than the others? Which (if any) would you
designate as the most likely scenario?
5 It is sometimes suggested that northwest Europe could become colder in the
future while most of the rest of the world becomes warmer. What could cause
this and how likely do you think it is to occur?
6 How important do you consider it is to emphasise the possibility of ˜surprises™
when presenting projections of likely future climate change?
7 Estimate the effect on the projected carbon dioxide concentrations for 2100
shown in Figure 6.2, the projected radiative forcing for 2100 shown in Figure
6.4(a) and the projected temperatures for 2100 shown in Figure 6.4(b), of
assuming the climate feedback on the carbon cycle illustrated in Figure 3.5
(note: ¬rst turn the accumulated atmospheric carbon in Figure 3.5 into an
atmospheric concentration).



Notes for Chapter 6
1 Nakicenovic, N. et al. 2000. Special Report on Emission Scenarios (SRES).
A Special Report of the IPCC. Cambridge: Cambridge University Press.
Notes 141



2 For details of IS 92a see Leggett, J., Pepper, W. J., Swart, R. J. 1992. Emission
scenarios for the IPCC: an update. In Houghton, J. T., Callender, B. A.,
Varney, S. K. (eds.) Climate Change 1992: the Supplementary Report to
the IPCC Assessments. Cambridge: Cambridge University Press, pp. 69“
95. Small modi¬cations have been made to the IS 92a scenario to take into
account developments in the Montreal Protocol.
3 This summary is based on the Summary of SRES in the Summary for pol-
icymakers. In Houghton, Climate Change 2001, p. 18.
4 The +30% amounting to an addition of between 200 and 300 ppm to the
carbon dioxide concentration in 2100 (see box on carbon feedbacks on
page 40).
5 The World Energy Council Report Energy for Tomorrow™s World. 1993. Lon-
don: World Energy Council. In its most likely scenario this report projects
that global sulphur emissions in 2020 will be almost the same as in 1990,
although with a different distribution (more over Asia but less over Europe
and North America). An extension of this study (Global Energy Perspec-
tives to 2050 and Beyond. London: World Energy Council, 1995) projects at
2050 global sulphur emissions that are little more than half the 1990 levels.
6 Detailed values listed Ramaswamy, V et al. 2001. Technical summary. In
.
Houghton, Climate Change 2001, Chapter 6.
7 Note that because the response of global average temperature to the in-
crease of carbon dioxide is logarithmic in the carbon dioxide concentration,
the increase of global average temperature for doubling of carbon dioxide
concentration is the same whatever the concentration that forms the base for
the doubling, e.g. doubling from 280 ppm or from 360 ppm produces the
same rise in global temperature. For a discussion of ˜climate sensitivity™ see
Cubasch, U., Meehl, G. A. et al. 2001. In Houghton, Climate Change 2001,
Chapter 9.
8 See Houghton, Climate Change 2001.
9 See Harvey, D. D. 1997. An introduction to simple climate models used in
the IPCC Second Assessment Report. In IPCC Technical Paper 2. Geneva:
IPCC.
10 Note that the uncertainty ranges in Figure 6.4 do not include those that arise
from lack of knowledge concerning climate feedbacks on the carbon cycle
(see box in Chapter 3 on page 40).
11 The assumption that greenhouse gases may be treated as equivalent to each
other is a good one for many purposes. However, because of the differences
in their radiative properties, accurate modelling of their effect should treat
them separately. More details of this problem are given in Gates, W. L.
et al. 1992. Climate modelling, climate prediction and model validation. In
Houghton, J. T., Callander, B. A., Varney, S. K. (eds.) Climate Change 1992:
the Supplementary Report to the IPCC Scienti¬c Assessments. Cambridge:
Cambridge University Press, pp. 171“5.
12 Related through the Clausius Clapeyron equation, e“1 de/dT = L/RT2 ,
where e is the saturation vapour pressure at temperature T , L the latent
heat of evaporation and R the gas constant.
142 Climate change in the twenty-¬rst century and beyond



13 Allen, M. R., Ingram, W. J. 2002. Nature, 419, pp. 224“32.
14 For more on this see Palmer, T. N. 1993. Weather, 48, pp. 314“25; and
Palmer, T. N. 1999. Journal of Climate, 12, pp. 575“91.
15 Kattenberg, A. et al. 1996. Climate models “ projections of future climate. In
Houghton, J. T., Meira Filho, L. G., Callander, B. A., Harris, N., Kattenberg,
A., Maskell, K. (eds.) Climate Change 1995: the Science of Climate Change.
Cambridge: Cambridge University Press, Chapter 6.
16 For a more detailed discussion of the effect of global warming on the
hydrological cycle, see Allen, M. R., Ingram, W. J. 2002. Nature, 419, pp.
224“32.
17 Milly, P. C. D. et al. 2002. Increasing risk of great ¬‚oods in a changing
climate. Nature, 415, pp. 514“17.
18 More information in Georgi, F., Hewitson, B. et al., Regional climate
information: evaluation and projections. Chapter 10 in Houghton, Climate
Change 2001, Chapter 10.
19 For de¬nition of continental and regional scales see Note 23 in Chapter 5.
20 See also Table 7.4.
21 Studies of other stars are providing further information, see Nesme-Ribes,
E. et al. 1996. Scienti¬c American, August, pp. 31“6.
22 Berger, A., Loutre, M. F. 2002. Science, 297, pp. 1287“8.
23 For a review and assessment of these mechanisms see Ramaswamy, V et al. .
2001. In Houghton, Climate Change 2001, Chapter 6, Section 6.11.2.
Chapter 7
The impacts of climate change




The last two chapters have detailed the climate change that we can
expect during the twenty-¬rst century because of human activities in
terms of temperature and rainfall. To be useful to human communities,
these details need to be turned into descriptions of the impact of climate
change on human resources and activities. The questions to which we
want answers are: how much will sea level rise and what effect will that
have?; how much will water resources be affected?; what will be the
impact on agriculture and food supply?; will natural ecosystems suffer
damage and how will human health be affected? This chapter considers
these questions.1



A complex network of changes
In outlining the character of the likely climate change in different regions
of the world, the last chapter showed that it is likely to vary a great
deal from place to place. For instance, in some regions precipitation will
increase, in other regions it will decrease. Not only is there a large amount
of variability in the character of the likely change, there is also variability
in the sensitivity (for de¬nition see box below) of different systems to
climate change. Different ecosystems, for instance, will respond very
differently to changes in temperature or precipitation.
There will be a few impacts of the likely climate change that will
be positive so far as humans are concerned. For instance, in parts of
Siberia or northern Canada increased temperature will tend to lengthen
the growing season with the possibility in these regions of growing
a greater variety of crops. In some places, increased carbon dioxide

143
144 The impacts of climate change



will aid the growth of some types of plants leading to increased crop
yields.



Sensitivity, adaptive capacity and vulnerability: some
de¬nitions2
Sensitivity is the degree to which a system is affected, either adversely
or bene¬cially, by climate-related stimuli. These encompass all the ele-
ments of climate change, including mean climate characteristics, climate
variability, and the frequency and magnitude of extremes. This may be
direct (e.g. a change in crop yield in response to a change in the mean,
range or variability of temperature) or indirect (e.g. damage caused by
an increase in the frequency of coastal ¬‚ooding due to sea level rise).
Adaptive capacity is the ability of a system to adjust to climate
change (including climate variability and extremes), to moderate po-
tential damage, to take advantage of opportunities or to cope with the
consequences.
Vulnerability is the degree to which a system is susceptible to, or
unable to cope with, adverse effects of climate change, including climate
variability and extremes. Vulnerability is a function of the character,
magnitude and rate of climate change and also the extent to which a
system is exposed, its sensitivity and its adaptive capacity.
Both the magnitude and the rate of climate change are important in
determining the sensitivity, adaptability and vulnerability of a system.



However, because, over centuries, human communities have adapted
their lives and activities to the present climate, most changes in climate
will tend to produce an adverse impact. If the changes occur rapidly, quick
and possibly costly adaptation to a new climate will be required by the
affected community. An alternative might be for the affected community
to migrate to a region where less adaptation would be needed “ a solution
which has become increasingly dif¬cult or, in some cases, impossible in
the modern crowded world.
As we consider the questions posed at the start of this chapter, it will
become clear that the answers are far from simple. It is relatively easy
to consider the effects of a particular change (in say, sea level or water
resources) supposing nothing else changes. But other factors will change.
Some adaptation, for both ecosystems and human communities, may be
relatively easy to achieve; in other cases, adaptation may be dif¬cult, very
costly or even impossible. In assessing the effects of global warming
and how serious they are, allowance must be made for response and
adaptation. The likely costs of adaptation also need to be put alongside
the costs of the losses or impacts connected with global warming.
How much will sea level rise? 145



Sensitivity, adaptive capacity and vulnerability (see box above)
vary a great deal from place to place and from country to country.
In particular, developing countries, especially the least developed
countries, have less capacity to adapt than developed countries, which
contributes to the relative high vulnerability to damaging effects of
climate change in developing countries.
The assessment of the impacts of global warming is also made more
complex because global warming is not the only human-induced envi-
ronmental problem. The loss of soil and its impoverishment (through
poor agricultural practice), the over-extraction of groundwater and the
damage due to acid rain are examples of environmental degradations on
local or regional scales that are having a substantial impact now.3 If they
are not corrected they will tend to exacerbate the negative impacts likely
to arise from global warming. For these reasons, the various effects of
climate change so far as they concern human communities and their ac-
tivities will be put in the context of other factors that might alleviate or
exacerbate their impact.
The assessment of climate change impacts, adaptations and vulner-
ability draws on a wide range of physical, biological and social science
disciplines and consequently employs a large variety of methods and
tools. It is therefore necessary to integrate information and knowledge
from these diverse disciplines; the process is called Integrated Assess-
ment (see box in Chapter 9 on page 237).
The following paragraphs will look at various impacts in turn and
then bring them together in a consideration of the overall impact.


How much will sea level rise?
There is plenty of evidence for large changes in sea level during the
Earth™s history. For instance, during the warm period before the onset of
the last ice age, about 120 000 years ago, the global average temperature
was a little warmer than today (Figure 4.4). Average sea level then was
about 5 or 6 m higher than it is today. When ice cover was at its maximum
towards the end of the ice age, some 18 000 years ago, sea level was over
100 m lower than today, suf¬cient, for instance, for Britain to be joined
to the continent of Europe.
It is often thought that the main cause of these sea level changes was
the melting or growth of the large ice-sheets that cover the polar regions.
It is certainly true that the main reason for the drop in sea level 18 000
years ago was the amount of water locked up in the large extension of the
polar ice-sheets. In the northern hemisphere these extended in Europe as
far south as southern England and in North America to south of the Great
Lakes. It must also be true that the main reason for the 5 or 6 m higher
146 The impacts of climate change



sea level during the last warm interglacial period was a reduction in the
Antarctic or Greenland ice-sheets. But changes over shorter periods are
largely governed by other factors that combine to produce a signi¬cant
effect on the average sea level.
During the twentieth century observations show that the average sea
level rose by between 10 and 20 cm.4 The largest contribution to this
rise (about one-third) is from thermal expansion of ocean water; as the
oceans warm the water expands and the sea level rises (see box below).
Other signi¬cant contributions come from the melting of glaciers and as
a result of long-term adjustments that are still occurring because of the
removal of the major ice-sheets 20 000 years or so ago. The contributions
from the ice caps of Greenland and Antarctica are believed to be small.
A further contribution to sea level change of uncertain magnitude arises
from changes in terrestrial storage of water, for instance from the growth
of reservoirs or irrigation.



Thermal expansion of the oceans
A large component of sea level rise is due to thermal expansion of the
oceans. Calculation of the precise amount of expansion is complex be-
cause it depends critically on the water temperature. For cold water the
expansion for a given change of temperature is small. The maximum
density of sea water occurs at temperatures close to 0 —¦ C; for a small
temperature rise at a temperature close to 0 —¦ C, therefore, the expansion
is negligible. At 5 —¦ C (a typical temperature at high latitudes), a rise of
1 —¦ C causes an increase of water volume of about 1 part in 10 000 and
at 25 —¦ C (typical of tropical latitudes) the same temperature rise of 1 —¦ C
increases the volume by about 3 parts in 10 000. For instance, if the top
100 m of ocean (which is approximately the depth of what is called the
mixed layer) was at 25 —¦ C, a rise to 26 —¦ C would increase its depth by
about 3 cm.
A further complication is that not all the ocean changes temperature
at the same rate. The mixed layer fairly rapidly comes into equilibrium
with changes induced by changes in the atmosphere. The rest of the ocean
changes comparatively slowly (the whole of the top kilometre will, for
instance, take many decades to warm); some parts may not change at all.
Therefore, to calculate the sea level rise due to thermal expansion “ its
global average and its regional variations “ it is necessary to employ the
results of an ocean climate model, of the kind described in Chapter 5.



Various contributions to the likely sea level rise in the twenty-¬rst
century can be identi¬ed. Again the largest is from the thermal expansion
of ocean water. The other main contribution comes from the melting of
How much will sea level rise? 147



glaciers. If all glaciers outside Antarctica and Greenland were to melt, the
rise in sea level would be about 50 cm (between 40 and 60 cm). Substan-
tial glacier retreat has occurred in recent decades adding an estimated
2“4 cm to the sea level rise in the twentieth century. Modelling the effect
of climate change on the behaviour of glaciers is, however, complex.
The growth or decay of a glacier depends on the balance between the
amount of snowfall on it, especially in winter, and the amount of melting
in the summer. Both winter snowfall and average summer temperature
are important, and both must be taken into account in future projections
of the rate of glacier melting.
The average sea level rise during the twenty-¬rst century for each of
the Special Report on Emission Scenarios (SRES) has been calculated
by adding up the various contributions. Those due to thermal expan-
sion (typically about sixty per cent of the total) and land ice changes
(typically about twenty-¬ve per cent of the total) were calculated us-
ing a simple climate model calibrated separately for each of seven cou-
pled atmosphere“ocean general circulation models (AOGCMs) (as in the
calculations for changes in global average temperature in Figure 6.4).
The relatively small contributions from changes in permafrost, the effect
of sediment deposition and the long-term adjustment of the ice-sheets
to past climate change were then added. The results are shown in Fig-
ure 7.1, where it will be seen that the uncertainties in the estimates are
substantial. Apart from the uncertainties inherent in the emissions scen-
arios, there is the uncertainty in the actual temperature rise (and hence in
the contribution from thermal expansion) depending on the value cho-
sen for the climate sensitivity (Figure 6.4). Different models also give
substantially different estimates of the amount of sea level rise due to
the melt from glaciers and small ice caps. The total range of uncertainty
by 2100 is from about 10 cm to 90 cm.
The projections in Figure 7.1 apply to the next 100 years. During
that period, because of the slow mixing that occurs throughout much of
the oceans, only a small part of the ocean will have warmed signi¬cantly.
Sea level rise resulting from global warming will therefore lag behind
temperature change at the surface (Figure 7.2). During the following
centuries, as the rest of the oceans gradually warm, sea level will continue
to rise at about the same rate, even if the average temperature at the
surface were to be stabilised.
The estimates of average sea level rise in Figure 7.1 provide a general
guide as to what can be expected during the twenty-¬rst century. Sea level
rise, however, will not be uniform over the globe. The effects of thermal
expansion in the oceans will vary considerably with location. Further,
movements of the land occurring for natural reasons due, for instance,
to tectonic movements or because of human activities (for instance, the
148 The impacts of climate change



1.0
A1B
A1FI
A1T
A2
0.8 B1
B2
Sea level rise (m)




0.6




0.4




0.2




0.0
1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Year

Figure 7.1 Global average sea level rise 1990“2100 for the SRES scenarios.
Each of the six lines identi¬ed in the key is the average of the AOGCMs for one
of the six illustrative SRES scenarios. The region in dark shading shows the range
of the average of AOGCMs for all thirty-¬ve SRES scenarios. The region in light
shading shows the range of all AOGCMs for all thirty-¬ve SRES scenarios. The
region delineated by the outermost lines shows the range of all AOGCMs and
scenarios including uncertainties in land ice changes, permafrost changes and
sediment deposition. Note that this range does not allow for uncertainty relating
to ice-dynamical changes in the West Antarctic ice-sheet (see text). The bars at
the right show the range in 2100 of all AOGCMs for the six illustrative scenarios.


Figure 7.2 Estimate of sea
level rise under a scenario with
increasing greenhouse gases
until the year 2030, at which
time it is assumed that
greenhouse gases are
stabilised so that there is no
further radiative forcing of the
climate. An additional rise in
sea level occurs during the
remainder of the century as
the increase in temperature
penetrates to more of the
ocean. The rise will continue
at about the same rate for the
following centuries as the rest
of the ocean warms.
How much will sea level rise? 149



removal of groundwater) can have comparable effects to the rate of sea
level rise arising from global warming. At any given place, all these
factors have to be taken into account in determining the likely value of
future sea level rise.
It is interesting and perhaps surprising that the net contribution ex-
pected from changes in the Antarctic and Greenland ice-sheets is small.
For both ice-sheets there are two competing effects.5 In a warmer world,
there is more water vapour in the atmosphere which leads to more snow-
fall. But there is also more ablation (erosion by melting) of the ice around
the boundaries of the ice-sheets where melting of the ice and calving of
icebergs occur during the summer months. For Antarctica, the estimates
are that accumulation is greater than ablation, leading to a small net
growth. For Greenland, ablation is greater than accumulation. For the
two taken together, under current conditions, the net effect is about zero,
although there is considerable uncertainty in that estimate.
If we look further into the future, however, larger changes in the
ice-sheets may begin to occur. The Greenland ice-sheet is the more vul-
nerable; its complete melting will cause a sea level rise of about 7 m.
Model studies of the ice-sheet show that, with a temperature rise of
more than 3 —¦ C, ablation will signi¬cantly overtake accumulation and
meltdown of the ice cap will begin. Figure 7.3 illustrates the rate of sea


600
Global sea level change (cm)




+8°C
500

400

300 +5.5°C

200

+3°C
100

0
2000 2200 2400 2600 2800 3000
Year AD
Figure 7.3 Global average sea level changes due to the response of the
Greenland ice-sheet to three climate warming scenarios during the third
millennium. The labels on the curves refer to the mean annual temperature rise
over Greenland by 3000 AD as predicted by a two-dimensional climate and
ocean model forced by greenhouse gas concentration rises until 2130 and kept
constant after that. Note that projected temperature rises over Greenland are
generally greater than those for global averages (by a factor of 1.2 to 3.1 for the
AOGCMs used in generating Figure 6.4).
150 The impacts of climate change



level rise over the next millennium that might be expected with different
levels of temperature change at Greenland. A warming of 5.5 —¦ C, for
instance, if sustained for 1000 years, would be likely to result in about
3 m of sea level rise.
The portion of the Antarctic ice-sheet that is of most concern is that
in the west of Antarctica (around 90—¦ W longitude); its disintegration
would result in about a 6-m sea level rise. Because a large portion of
it is grounded well below sea level it has been suggested that rapid
ice discharge could occur if the surrounding ice shelves are weakened.
Although studies are far from conclusive, current ice dynamic models
do not indicate that rapid disintegration is likely and suggest that, over
the next millennium, the contribution of the west Antarctic ice-sheet to
sea level rise will be less than 3 m.


The impacts of sea level rise
A rise in average sea level of 10 cm by 2030 and about half a metre by the
end of the next century (typical values from Figure 7.1) may not seem a
great deal. Many people live suf¬ciently above the level of high water not
to be directly affected. However, half of humanity inhabits the coastal
zones around the world. Within these, the lowest lying are some of the
most fertile and densely populated. To people living in these areas, even
a fraction of a metre increase in sea level can add enormously to their
problems. Some of the areas that are especially vulnerable are, ¬rstly,
large river delta areas, for instance Bangladesh; secondly, areas very
close to sea level where sea defences are already in place, for instance
the Netherlands; and, thirdly, small low-lying islands in the Paci¬c and
other oceans. We shall look at these in turn.
Bangladesh is a densely populated country of about 120 million
people located in the complex delta region of the Ganges, Brahmaputra
and Meghna Rivers.6 About ten per cent of the country™s habitable land
(with about six million population) would be lost with half a metre of sea
level rise and about twenty per cent (with about 15 million population)
would be lost with a 1-m rise7 (Figure 7.4). Estimates of the sea level
rise are of about 1 m by 2050 (compounded of 70 cm due to subsidence
because of land movements and removal of groundwater and 30 cm from
the effects of global warming) and nearly 2 m by 2100 (1.2 m due to
subsidence and 70 cm from global warming)8 “ although there is a large
uncertainty in these estimates.
It is quite impractical to consider full protection of the long and
complicated coastline of Bangladesh from sea level rise. Its most obvious
effect, therefore, is that substantial amounts of good agricultural land
will be lost. This is serious: half the country™s economy comes from
The impacts of sea level rise 151



Figure 7.4 Land affected
in Bangladesh by various
amounts of sea level rise.




agriculture and eighty-¬ve per cent of the nation™s population depends
on agriculture for its livelihood. Many of these people are at the very
edge of subsistence.
But the loss of land is not the only effect of sea level rise. Bangladesh
is extremely prone to damage from storm surges. Every year, on average,
at least one major cyclone attacks Bangladesh. During the past twenty-
¬ve years there have been two very large disasters with extensive ¬‚ooding
and loss of life. The storm surge in November 1970 is probably the largest
of the world™s natural disasters in recent times; it is estimated to have
claimed the lives of over a quarter of a million people. Well over a 100 000
are thought to have lost their lives in a similar storm in April 1991. Even
small rises in sea level add to the vulnerability of the region to such
storms.
There is a further effect of sea level rise on the productivity of agri-
cultural land; that is, the intrusion of saltwater into fresh groundwater
152 The impacts of climate change



resources. At the present time, it is estimated that in some parts of
Bangladesh saltwater extends seasonally inland over 150 km. With a
1-m rise in sea level, the area affected by saline intrusion could increase
substantially9 although, since it is also likely that climate change will
bring increased monsoon rainfall, some of the intrusion of saltwater
could be alleviated.10
What possible responses can Bangladesh make to these likely future
problems? Over the timescale of change that is currently envisaged it
can be supposed that the ¬shing industry can relocate and respond with
¬‚exibility to changing ¬shing areas and changing conditions. It is less
easy to see what the population of the affected agricultural areas can do to
relocate or to adapt. No signi¬cant areas of agricultural land are available
elsewhere in Bangladesh to replace that lost to the sea, nor is there
anywhere else in Bangladesh where the population of the delta region
can easily be located. It is clear that very careful study and management
of all aspects of the problem is required. The sediment brought down by
the rivers into the delta region is of particular importance. The amount
of sediment and how it is used can have a large effect on the level of the
land affected by sea level rise. Careful management is therefore required
upstream as well as in the delta itself; groundwater and sea defences
must also be managed carefully if some alleviation of the effects of sea
level rise is to be achieved.
A similar situation exists in the Nile delta region of Egypt. The likely
rise in sea level this century is made up from local subsidence and global
warming in much the same way as for Bangladesh “ approximately 1 m
by 2050 and 2 m by 2100. About twelve per cent of the country™s arable
land with a population of over seven million people would be affected by
a 1-m rise of sea level.11 Some protection from the sea is afforded by the
extensive sand dunes but only up to half a metre or so of sea level rise.12
Many other examples of vulnerable delta regions, especially in south-
east Asia and Africa, can be given where the problems would be similar
to those in Bangladesh and in Egypt. For instance, several large and low-
lying alluvial plains are distributed along the eastern coastline of China.
A sea level rise of just half a metre would inundate an area of about
40 000 km2 (about the area of The Netherlands)13 where over thirty mil-
lion people currently live. A particular delta which has been extensively
studied is that of the Mississippi in North America. These studies under-
line the point that human activities and industry are already exacerbating
the potential problems of sea level rise due to global warming. Because
of river management little sediment is delivered by the river to the delta
to counter the subsidence occurring because of long-term movements of
the Earth™s crust. Also, the building of canals and dykes has inhibited
the input of sediments from the ocean.14 Studies of this kind emphasise
The impacts of sea level rise 153



the importance of careful management of all activities in¬‚uencing such
regions, and the necessity of making maximum use of natural processes
in ensuring their continued viability.
We now turn to The Netherlands, a country more than half of which
consists of coastal lowlands, mainly below present sea level. It is one
of the most densely populated areas in the world; eight of the fourteen
million inhabitants of the region live in large cities such as Rotterdam,
The Hague and Amsterdam. An elaborate system of about 400 km of
dykes and coastal dunes, built up over many years, protects it from the
sea. Recent methods of protection, rather than creating solid bulwarks,
make use of the effects of various forces (tides, currents, waves, wind
and gravity) on the sands and sediments so as to create a stable barrier
against the sea “ similar policies are advocated for the protection of
the Norfolk coast in eastern England.15 Protection against sea level rise
next century will require no new technology. Dykes and sand dunes will
need to be raised; additional pumping will also be necessary to combat
the incursion of saltwater into freshwater aquifers. It is estimated16 that
an expenditure of about twelve thousand million dollars (US) would be
required for protection against a sea level rise of 1 m.
The third type of area of especial vulnerability is the low-lying small
island.17 Half a million people live in archipelagos of small islands and
coral atolls, such as the Maldives in the Indian Ocean, consisting of
1190 individual islands, and the Marshall Islands in the Paci¬c, which
lie almost entirely within 3 m of sea level. Half a metre or more of sea
level rise would reduce their areas substantially “ some would have to
be abandoned “ and remove up to ¬fty per cent of their groundwater.
The cost of protection from the sea is far beyond the resources of these
islands™ populations. For coral atolls, rise in sea level at a rate of up to
about half a metre per century can be matched by coral growth, providing
that growth is not disturbed by human interference and providing also
that the growth is not inhibited by a rise in the maximum sea temperature
exceeding about 1“2 —¦ C.18
These are some examples of areas particularly vulnerable to sea level
rise. Many other areas in the world will be affected in similar, although
perhaps less dramatic, ways. Many of the world™s cities are close to sea
level and are being increasingly affected by subsidence because of the
withdrawal of groundwater. The rise of sea level due to global warming
will add to this problem. There is no technical dif¬culty for most cities in
taking care of these problems, but the cost of doing so must be included
when calculating the overall impact of global warming.
So far, in considering the impact of sea level rise, places of dense
population where there is a large effect on people have been considered.
There are also areas of importance where few people live. The world™s
154 The impacts of climate change



wetlands and mangrove swamps currently occupy an area of about a
million square kilometres (the ¬gure is not known very precisely), equal
approximately to twice the area of France. They contain much biodi-
versity and their biological productivity equals or exceeds that of any
other natural or agricultural system. Over two-thirds of the ¬sh caught
for human consumption, as well as many birds and animals, depend on
coastal marshes and swamps for part of their life cycles, so they are
vital to the total world ecology. Such areas can adjust to slow levels of
sea level rise, but there is no evidence that they could keep pace with
a rate of rise of greater than about 2 mm per year “ 20 cm per century.
What will tend to occur, therefore, is that the area of wetlands will ex-
tend inland, sometimes with a loss of good agricultural land. However,
because in many places such extension will be inhibited by the presence
of ¬‚ood embankments and other human constructions, erosion of the
seaward boundaries of the wetlands will lead more usually to a loss of
wetland area. Because of a variety of human activities (such as shoreline
protection, blocking of sediment sources, land reclamation, aquaculture
development and oil, gas and water extraction), coastal wetlands are cur-
rently being lost at a rate of 0.5“1.5% per year. Sea level rise because of
climate change would further exacerbate this loss.19
To summarise the impact of the half-metre or so of sea level rise
due to global warming which could occur during the twenty-¬rst cen-
tury: global warming is not the only reason for sea level rise but it is
likely to exacerbate the impacts of other environmental problems. Care-
ful management of human activities in the affected areas can do a lot to
alleviate the likely effects, but substantial adverse impacts will remain.
In delta regions, which are particularly vulnerable, sea level rise will lead
to substantial loss of agricultural land and salt intrusion into freshwater
resources. In Bangladesh, for instance, over ten million people are likely
to be affected by such loss. A further problem in Bangladesh and other
low-lying tropical areas will be the increased intensity and frequency
of disasters because of storm surges. Each year, the number of people
worldwide experiencing ¬‚ooding because of storm surges is estimated
now at about forty million. With a 40-cm sea level rise by the 2080s this
number is estimated to quadruple “ a number that might be reduced by
half if coastal protection is enhanced in proportion to gross domestic
product (GDP) growth.20 Low-lying small islands will also suffer loss of
land and freshwater supplies. Countries like The Netherlands and many
cities in coastal regions will have to spend substantial sums on protec-
tion against the sea. Signi¬cant amounts of land will also be lost near
the important wetland areas of the world. Attempts to put costs against
these impacts, in both money and human terms, will be considered later
in the chapter.
Increasing human use of fresh water resources 155



In this section we have considered the impacts of sea level rise for the
twenty-¬rst century. Because, as we have seen, the ocean takes centuries
to adjust to an increase in surface temperature, the longer-term impacts
of sea level rise also need to be emphasised. Even if the concentrations of
greenhouse gases in the atmosphere were stabilised so that anthropogenic
climate change is halted, the sea level will continue to rise for many
centuries as the whole ocean adjusts to the new climate.

Figure 7.5 The global
water cycle (in thousands
Increasing human use of fresh water resources
of cubic kilometres per
The global water cycle is a fundamental component of the climate sys- year), showing the key
processes of evaporation,
tem. Water is cycled between the oceans, the atmosphere and the land
precipitation, transport as
surface (Figure 7.5). Through evaporation and condensation it provides
vapour by atmospheric
the main means whereby energy is transferred to the atmosphere and
movements and transport
within it. Water is essential to all forms of life; the main reason for the from the land to the
wide range of life forms, both plant and animal, on the Earth is the oceans by runoff or
extremely wide range of variation in the availability of water. In wet groundwater ¬‚ow.




VAPOUR TRANSPORT 40




71

EVAPORATION TRANSPIRATION



PRECIPITATION
111
425 PRECIPITATION
EVAPORATION 385
SURFACE
RUNOFF
Lake



River
RETURN FLOW
40
Land
Ocean
PERCOLATION

GROUNDWATER
FLOW
156 The impacts of climate change
Withdrawal, km3 (year ’1)




Figure 7.6 Global water withdrawal for different purposes, 1900“1995, and
projected to the year 2025 in cubic kilometres per year. Losses from reservoirs
are also included. As some water withdrawn is reused, the total water
consumption amounts to about sixty per cent of the total water withdrawal.

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