. 7
( 16)


be considered 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 ocean surface and which con-
denses in the clouds within the storm, releasing energy. It might be expected
that warmer sea temperatures would mean more energy release, leading to
more frequent and intense storms. However, ocean temperature is not the
only parameter controlling the genesis of tropical storms; the nature of the
overall atmospheric ¬‚ow is also important. Further, although based on limited
data, observed variations in the intensity and frequency of tropical cyclones
show no clear trends in the last half of the twentieth century. AOGCMs can
take all the relevant factors into account but, because of the relatively large
size of their grid, they are unable to simulate reliably the detail of relatively
small disturbances such as tropical cyclones. From projections with these
models there is no consistent evidence of changes in the frequency of tropi-
cal cyclones or their areas of formation. However, during the last few years a
number of studies with regional models and more adequate resolution with
large-scale variables taken from AOGCMs (see next section on regional mod-
elling) project some consistent increases in peak wind intensities and mean
and peak precipitation intensities. An indication of the size of the increases is
provided from one study that projected an increase in 6% in peak surface wind
intensities and 20% increase in precipitation.29

Regarding storms at mid latitudes, the various factors that control their inci-
dence are complex. Two factors tend to an increased intensity of storms. The
¬rst, as with tropical storms, is that higher temperatures, especially 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 stormi-
ness might be expected, a result indicated by some model projections. However,
such a picture may be too simple; other models suggest changes in storm tracks
that bring very different changes in some regions and there is little overall con-
sistency between model projections.
For other extremes such as very small-scale phenomena (e.g. tornadoes,
thunderstorms, hail and lightning) that cannot be simulated in global mod-
els, although they may have important impacts, there is currently 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 future inci-
dence of extreme events. Although many of the trends are clear, many more
studies are required that provide quantitative assessments on regional scales of
likely changes in the frequency or intensity of extreme events or in climate

Regional climate models
Most of the likely changes that we have presented have been on the scale of con-
tinents. Can more speci¬c information be provided about change for smaller
regions? In Chapter 5 we referred to the limitation of global circulation mod-
els (GCMs) in the simulation of changes on the regional scale arising from
the coarse size of their horizontal grid “ typically 300 km or more.30 Also in
Chapter 5 we introduced the regional climate model (RCM) which typically pos-
sesses a resolution of 50 km and can be ˜nested™ in a global circulation model.
Examples are shown in Figures 6.13 and 7.9 of the improvement achieved by
RCMs in the simulation of extremes and in providing regional detail that in
many cases (especially for precipitation) 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.
162 C L I M AT E C H A N G E I N T H E T W E N T Y- F I R S T C E N T U RY A N D B E YO N D

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

Con¬dence in observed
changes (latter half of the Con¬dence in projected changes
twentieth century) Changes in phenomenon (during the twenty-¬rst century)

Very likelya Very likely
Higher maximum temperatures
and more hot days over nearly
all land areas
Very likely
Very likely Higher minimum temperatures,
fewer cold days and frost days
over nearly all land areas
Very likely Reduced diurnal temperature Very likely
range over most land areas
Increase of heat indexb over
Likely, over most areas Very likely, over most areas
land areas
More intense precipitation Very likely, over most areas
Likely, over many northern
hemisphere mid- to high-
latitude land areas
Likely, in most sub-tropical areas &
Increased summer continental
Likely, increases in total area
many mid-latitude continental areas
drying and associated risk of
affected over many land
(very likely, over Mediterranean,
south Australia, New Zealand)
Tropical cyclones Likely, increase in peak wind and
Likely, trends towards greater
precipitation intensities
storm intensity, no trend in
Extra-tropical cyclones Likely, increase in intensity over
Likely, net increase in
many areas (e.g. North Atlantic,
intensity and poleward shift
Central Europe and southern New
in track over many northern
hemisphere land areas

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

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 100
Most of the projections of future climate that
have been published cover the twenty-¬rst

Probability (%)
century. For instance, the curves plotted in
Figures 6.1 to 6.6 extend to the year 2100. They 1
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 revo-
lution until 2000 the burning of fossil fuels 0.01
released approximately 300 Gt of carbon in <0.1 >10 >20 >30 >50
Thresholds (mm day )
the form of carbon dioxide into the atmos-
phere. Under the SRES A1B scenario it is pro- Figure 6.13 Example of simulations showing the
probability of winter days over the Alps with different
jected that a further 1500 Gt will be released
daily rainfall thresholds, as observed, simulated by a
by the year 2100. As Chapter 11 will show, the
300-km resolution GCM and by a 50-km resolution
reserves of fossil fuels in total are suf¬cient RCM. Red bars observed, green bars, simulated by
to enable their rate of use to continue to grow GCM; blue bars, simulated by the RCM. The RCM
well beyond the year 2100. If that were to hap- shows much better agreement with observations
pen the global average temperature would especially for higher thresholds.
continue to rise and could, in the twenty-sec-
ond 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 would almost certainly 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 48“9) and the +30% uncertainty in 2100
in the atmospheric concentrations of carbon dioxide shown in Figure 6.2 was
introduced to allow for it. Similarly the upper part of the uncertainty bars on
the right-hand side of Figure 6.4a for the various SRES marker scenarios makes
allowance for this uncertainty “ for the A2 scenario in 2100 it amounts to about
one degree Celsius. Some of the further implications of this feedback will be
considered in Chapter 10 on page 311.
Especially when considering the longer term, there is also the possibility of
surprises “ changes in the climate system that are unexpected. The discovery
of the ˜ozone hole™ is an example of a change in the atmosphere due to human
activities, which was a scienti¬c ˜surprise™. By their very nature such ˜surprises™
cannot, of course, be foreseen. However, there are various parts of the system
which are, as yet, not well understood, where such possibilities might be looked
for;31 for instance, in the deep ocean circulation (see box in Chapter 5, page 120)
164 C L I M AT E C H A N G E I N T H E T W E N T Y- F I R S T C E N T U RY A N D B E YO N D

or in the stability of the major ice sheets (see paragraph on climate response,
Chapter 5, page 132). 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 120
in Chapter 5, where Figure 5.18 illustrates the deep ocean currents that trans-
port heat and fresh water 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
both temperature and precipitation will increase substantially especially at
high latitudes (Figures 6.6 and 6.7), leading to warmer surface water and addi-
tional fresh water input to the oceans. Some increased melting of the Greenland
ice cap would add further fresh water. The cold dense salty water in the North
Atlantic source region for the THC will become less cold and less salty and there-
fore less dense. As a result the THC will weaken and less heat will ¬‚ow north-
ward from tropical regions to the north Atlantic. All coupled ocean“atmosphere
GCMs show this occurring during the twenty-¬rst century though to a varying
extent from a small amount to over 50% change; a typical example is shown in
Figure 6.14, which indicates a weakening of about 20% by 2100. Although there
is disagreement between 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 6.6) “ but
none shows actual cooling occurring in this region during the twenty-¬rst
The question is often raised as to whether an abrupt transition could occur in
the THC or whether it might actually be cut off as has occurred in the past (see
Chapter 4, page 89). From model projections so far it is considered very unlikely
that an abrupt transition will occur during the twenty-¬rst century. However,
the stability of the THC further into the future is bound to be of concern espe-
cially if global warming continues largely unchecked and if the rate of meltdown
of the Greenland ice cap accelerates. Intense research is being pursued “ both
observations and modelling “ to elucidate further likely changes in the THC and
their possible impact.


Circulation strength (Sv)




High emissions
Medium high
Medium low
“14 Low emissions

1900 1950 2000 2050 2100 2150 2200

Figure 6.14 Change in the strength of the thermohaline circulation (THC) in the north
Atlantic as simulated by the Hadley Centre climate model for four SRES scenarios (A1FI,
A2, B2 and B1). The unit of circulation is the Sverdrup, 10 6 m3 s“1.

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 that triggered the ice ages and the major climate
changes of the past. These variations are, of course, still going on: what in¬‚u-
ence are they having now?
Over the past 10 000 years, because of these orbital changes, the solar radia-
tion incident at 60° N in July has decreased by about 35 W m“2, which is quite
a large amount. But over 100 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 50 000 years the solar radiation incident in summer on the polar
regions will be unusually constant so that the present interglacial is expected
166 C L I M AT E C H A N G E I N T H E T W E N T Y- F I R S T C E N T U RY A N D B E YO N D

Does the Sun™s output change?
Some scientists have suggested that all climate variations, even short-term ones, might be the result of
changes in the Sun™s energy output. Such suggestions are bound to be somewhat speculative because the
only direct measurements of solar output that are available are those since 1978, from satellites outside the
disturbing effects of the Earth™s atmosphere. These measurements indicate a very constant solar output,
changing by about 0.1% between a maximum and a minimum in the cycle of solar magnetic activity indi-
cated by the number of sunspots.
It is known from astronomical records and from measurements of radioactive carbon in the atmosphere
that solar sunspot activity has, from time to time over the past few thousand years, shown large variations.
Of particular interest is the period known as the Maunder Minimum in the seventeenth century when
very few sunspots were recorded.33 At the time of the IPCC TAR in 2001, studies of recent measurements
of solar output correlated with other indicators of solar activity, when extrapolated to this earlier period,
suggested that the Sun was a little less bright in the seventeenth century, perhaps by about 0.4% in the
average solar energy incident on the Earth™s surface and that this reduction in solar energy may have been
a cause of the cooler period at that time known as the ˜Little Ice Age™.34 More recently some of the assump-
tions in this work have been questioned and estimates made that over the past two centuries variations in
the solar energy incident on the Earth™s surface are unlikely to be greater than about 0.1% (Figure 6.15).35
This is about the same as the change in the energy regime at the Earth™s surface due to two or three years™
increase in greenhouse gases at the current rate.

Figure 6.15 Reconstructions of
Flux transport simulations
the total solar irradiance from
(Wang et al 2005)
1600 to the present showing
Range of cycle + background
estimates (blue) of the range of
(Lean 2000)
the irradiance variations arising
from the 11-year solar activity
Total solar irradiance

cycle and the period in the
seventeenth century when no
W m“2

sunspots were recorded. The lower
envelope is the reconstruction by
J. Lean, in which the long-term
trend was inferred from brightness
changes in sun-like stars. The
1364 recent reconstruction by Y. Wang
et al. (purple) is based on solar
considerations alone.
1600 1700 1800 1900 2000

to last for an exceptionally long period.32 Suggestions therefore that the cur-
rent increase of greenhouse gases might delay the onset of the next ice age are
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 I mentioned in Chapter 3
(see Figure 3.11) and as is described in the box, 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 indirect mechanisms whereby effects on
the Sun might in¬‚uence climate on Earth. Changes in solar ultraviolet radia-
tion will in¬‚uence atmospheric ozone and hence could have some in¬‚uence
on climate. It has been suggested that the galactic cosmic ray ¬‚ux, modi¬ed
by the varying Sun™s magnetic ¬eld, could in¬‚uence cloudiness and hence cli-
mate. Although studies have been pursued on such connections, their in¬‚uence
remains speculative. So far as the last few decades are concerned, there is ¬rm
evidence from both observational and theoretical studies that none of these
mechanisms could have contributed signi¬cantly to the rapid global tempera-
ture rise that has been observed.36
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 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 that
last for longer in the stratosphere, is much smaller.


• Increase in greenhouse gases is by far the largest of the factors that can lead
to climate change during the twenty-¬rst century.
• Likely climate changes for a range of scenarios of greenhouse gas emissions
have been described in terms of global average temperature and in terms
168 C L I M AT E C H A N G E I N T H E T W E N T Y- F I R S T C E N T U RY A N D B E YO N D

of regional change of temperature and precipitation and the occurrence of
• The rate of change is likely to be larger than any the Earth has seen at any
time during the past 10 000 years.
• The changes that are likely to have the greatest impact will be changes
in the frequencies, intensities and locations of climate extremes, especially
droughts and ¬‚oods.
• Suf¬cient fossil fuel reserves are available to provide for continuing growth
in fossil fuel emissions of carbon dioxide well into the twenty-second cen-
tury. If this occurred the climate change could be very large indeed and lead
to 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 ter-
minate the rate of change.

1 Suggest, for Figure 6.8, an appropriate temperature scale for a place
you know. De¬ne what might be meant by a very hot day and estimate
the percentage increase in the probability of such days if the average
temperature increases by 1, 2 and 4 °C.
2 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?
3 Look at the assumptions underlying the full range of SRES emission
scenarios in the IPCC 2007 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?
4 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?
5 How important do you consider it is to emphasise the possibility of ˜surprises™
when presenting projections of likely future climate change?
6 Estimate the effect, on the projected carbon dioxide concentrations for
2100 shown in Figure 6.2, the projected radiative forcing for 2100 shown in
Table 6.1 and the projected temperatures for 2100 shown in Figure 6.4a, of
N OT E S F O R C H A P T E R 6

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).
7 From newspapers or websites look up articles purporting to say that there
is no human-induced climate change or that what there is does not matter.
Assess them in the light of this and other chapters in this book. Do you think
any of their arguments are credible?

Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M.,
Miller, H.L. (eds.) 2007. Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change. Cambridge: Cambridge University
Technical Summary (summarises basic information about future climate projections)
Chapter 10 Global Climate Projections
Chapter 11 Regional Climate Projections
Nakicenovic, N. et al. (eds.) 2000. IPCC Special Report on Emissions Scenarios
Cambridge: Cambrige University Press.
WMO/UNEP, 2007. Climate Change 2007, IPCC Synthesis Report, www.ipcc.ch
McGuf¬e, K., Henderson-Sellers, A. 2005. A Climate Modeling Primer, third edition.
New York: Wiley.
Palmer, T. Hagedorn, R. (eds.) 2006. Predictability of Weather and Climate. Cambridge:
Cambridge University Press.
Schnellnhuber, H.J. et al. (eds.) 2006. Avoiding Dangerous Climate Change. Cambridge:
Cambridge University Press.

3 The +30% amounting to an addition of between 200
1 Nakicenovic, N. et al. (eds.) 2000. Special Report on
and 300 ppm to the carbon dioxide concentration in
Emission Scenarios (SRES): A Special Report of the IPCC.
2100 (see box on carbon feedbacks on page 48).
Cambridge: Cambridge University Press.
4 This summary is based on the Summary of SRES in
2 For details of IS 92a see Leggett, J., Pepper, W. J.,
the Summary for policymakers. In Houghton et al.
Swart, R. J. 1992. Emission scenarios for the IPCC: an
(eds.), Climate Change 2001: The Scienti¬c Basis, p. 18.
update. In Houghton, J. T., Callender, B. A., Varney,
Also Summary for policymakers, in Solomon et al.
S. K. (eds.) Climate Change 1992: The Supplementary
(eds.) Climate Change 2007: The Physical Science Basis.
Report to the IPCC Assessments. Cambridge: Cambridge
5 The World Energy Council Report. 1995 Global Energy
University Press, pp. 69“95. Small modi¬cations have
Perspectives to 2050 and Beyond. London: World Energy
been made to the IS 92a scenario to take into account
Council projects at 2050 global sulphur emissions
developments in the Montreal Protocol.
170 C L I M AT E C H A N G E I N T H E T W E N T Y- F I R S T C E N T U RY A N D B E YO N D

that are little more than half the 1990 levels. Also 1992. Climate modelling, climate prediction and
see United Nations Environmental Programme. 2007. model validation. In Houghton et al. (eds.) Climate
Global Environmental Outlook GEO4, Nairobi, Kenya: Change 1992: The Supplementary Report, pp. 171“5.
UNEP, Chapter 9, pp. 435“445. Also see Forster, P., Ramaswamy, V. et al., Chapter
6 See Figure 3.11. 2, Section 2.9, in Solomon et al. (eds.) Climate Change
7 Metz, B., David, O., Bosch, P., Dave, R., Meyer, L. 2007: The Physical Science Basis.
(eds.) 2007. Climate Change 2007: Mitigation 15 Alternatively gases other than carbon dioxide can
of Climate Change. Contribution of Working be converted to equivalent amounts of carbon
Group III to the Fourth Assessment Report of the dioxide by using their Global Warming Potentials
Intergovernmental Panel on Climate Change. (see Chapter 3 and Table 10.2).
Cambridge: Cambridge University Press, 16 The relationship between radiative forcing R and
Figure 3.12, Chapter 3. CO2 concentration C is R = 5.3 ln(C/C0) where C0 = the
8 Detailed values listed in Ramaswamy, V. et al. 2001. pre-industrial concentration of 280 ppm.
Chapter 6, in Houghton et al. (eds.), Climate Change 17 See for instance, Metz et al. (eds.) Climate Change
2001: The Science Basis; also see Meehl, G.A., Stocker, 2007: Mitigation, Chapter 3, Fig 3.12 . For more
T.F. et al. 2007. Chapter 10, in Solomon et al. (eds.) information about aerosol assumptions for the
Climate Change 2007: The Physical Science Basis. twenty-¬ rst century and how aerosol forcing
9 Note that because the response of global average is treated in models see Johns, T.C. et al. 2003.
temperature to the increase of carbon dioxide is Climate Dynamics, 20, 583“612.
logarithmic in the carbon dioxide concentration, 18 Related through the Clausius“Clapeyron equation,
e“1 de/dT = L/RT2, where e is the saturation vapour
the increase of global average temperature for
doubling of carbon dioxide concentration is the pressure at temperature T, L the latent heat of
same whatever the concentration that forms the evaporation and R the gas constant.
base for the doubling, e.g. doubling from 280 19 Allen, M. R., Ingram, W. J. 2002. Nature, 419,
ppm or from 360 ppm produces the same rise in 224“32.
global temperature. For a discussion of ˜climate 20 Christensen, J.H., Hewitson, B. et al. 2007. Regional
sensitivity™ see Cubasch, U., Meehl, G. A. et al. climate projections. Chapter 11, Executive
2001. Chapter 9, in Houghton et al. (eds.) Climate Summary, in Solomon et al. (eds.) Climate Change
Change 2001: The Scienti¬c Basis, also in Meehl, et al., 2007: The Physical Science Basis.
Chapter 10, in Solomon et al. (eds.) Climate Change 21 For more on this see Palmer, T. N. 1993. Weather, 48,
2007: The Physical Science Basis. 314“25; and Palmer, T. N. 1999. Journal of Climate, 12,
10 Summary for Policy Makers, ibid., p. 9. 575“91.
11 Ibid. 22 See Figure 10.16 in Meehl, et al., Chapter 10, in
12 James Hansen, Bjerknes Lecture at American Solomon et al. (eds.) Climate Change 2007: The Physical
Geophysical Union, 17 December 2008 at www. Science Basis.
columbia.edu/˜jeh1/2008/AGUBjerknes_20081217.pdf. 23 For a review of the science of extreme events see
13 See Harvey, D. D. 1997. An introduction to Mitchell, J.F.B., et al, 2006, Philosophical Transactions
simple climate models used in the IPCC Second of the Royal Society A, 364, 2117“33; also see Meehl,
Assessment Report. In IPCC Technical Paper 2. et al., Chapter 10 in Solomon et al. (eds.) Climate
Geneva: IPCC. Change 2007: The Physical Science Basis.
14 The assumption that greenhouse gases may be 24 For a more detailed discussion of the effect of global
treated as equivalent to each other is a good one for warming on the hydrological cycle, see Allen, M. R.,
many purposes. However, because of the differences Ingram, W. J. 2002.
in their radiative properties, accurate modelling 25 Tebaldi, C. et al. 2002. Climatic Change, 79, 185“211.
of their effect should treat them separately. More 26 Milly, P. C. D. et al. 2002. Increasing risk of great
details of this problem are given in Gates, W. L. et al. ¬‚oods in a changing climate. Nature, 415, 514“17;
N OT E S F O R C H A P T E R 6

see also Meehl et al., in Solomon et al. (eds.) Climate 31 See also Table 7.4.
Change 2007: The Physical Science Basis. 32 Berger, A., Loutre, M. F. 2002. Science, 297, 1287“8.
27 Burke, E. J., Brown, S. J. Christidis, N. 2006. 33 Studies of other stars are providing further
Modeling the recent evolution of global drought and information; see Nesme-Ribes, E. et al. 1996. Scienti¬c
projections for the 21st century with the Hadley American, August, 31“6.
Centre climate model. Journal of Hydrometrology, 7, 34 Lean, J. 2000 Evolution of the Sun™s spectral
1113“25. irradiance since the Maunder Minimum. Geophysics
28 De¬ned with relation to the Palmer Drought Research Letters, 27, 2425“8.
Severity Index. 35 Wang Y. et al 2005. Modeling the Sun™s magnetic
29 Knutson, T.R., Tuleya, R.E. 2004. Journal of Climate, ¬eld and irradiance since 1713, Astrophysical Journal,
17, 3477“95. See also Meehl, et al ., in Solomon 625, 522“38.
et al . (eds.) Climate Change 2007: The Physical Science 36 Lockwood M., Frohlich, C. 2007. Recent oppositely
Basis. directed trends in solar climate forcings and the
30 For de¬nition of continental and regional scales see global mean surface air temperature. Proceedings of
Note 28 in Chapter 5. the Royal Society A doi:10.1098/rspa.2007.1880.
7 The impacts of climate change

Droughts in the Masai region of Africa.

T HE LAST two chapters have detailed the climate change in terms of temperature and rainfall
that we can expect during the twenty-¬rst century because of human activities. 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?;
how will human health be affected? and can the cost of the likely damage be estimated? 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 eco-
systems, 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, Scandinavia
or northern Canada increased temperature will tend to lengthen the grow-
ing season with the possibility in these regions of growing a greater variety of
crops. Also, in winter there will be lower mortality and heating requirements.
Further, in some places, increased carbon dioxide will aid the growth of some
types of plants leading to increased crop yields.
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 pos-
sibly costly adaptation to a new climate will be required by the affected

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 elements of climate change, including mean climate characteristics, cli-
mate 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 potential damage, to take advantage of opportunities or to cope with the
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, adapt-
ability and vulnerability of a system.
174 T H E I M PAC T S O F C L I M AT E C H A N G E

community. An alternative might be for the affected community to migrate
to a region where less adaptation would be needed “ a solution that 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 glo-
bal warming.
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 environmental prob-
lem. For instance, the loss of soil and its impoverishment (through poor agricul-
tural 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 activities will be put in the context of other fac-
tors that might alleviate or exacerbate their impact.
The assessment of climate change impacts, adaptations and vulnerability
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 disci-
plines; the process is called Integrated Assessment (see box in Chapter 9 on
page 280).
Table 7.1 summarises some expected impacts for different increases in global
average temperature that might occur during the twenty-¬rst century. The fol-
lowing paragraphs consider detail of the various impacts in turn and then bring
them together in a consideration of the overall impact.

Table 7.1 Examples of global impacts projected for changes in climate associated with
different increases in global average surface temperature in the twenty-¬rst century.
Add 0.6 °C to obtain temperature increases from pre-industrial times. Also shown are
the projections of temperature increases associated with SRES scenarios as in Figure 6.4.
Adaptation to climate change is not included in these estimates.
SRES: AR4 WG I multiple sources
B1 2080s
B2 2090s


Increased water availability in moist tropics and high latitudes
Decreasing water availability and increasing drought in mid-latitudes and semi-arid low latitudes
Additional people
with increased
0.4 to 1.7 billion 1.0 to 2.0 billion 1.1 to 3.2 billion
water stress

Increasing amphibian About 20 to 30% species at increasingly Major extinctions around the globe
extinction high risk of extinction

ECOSYSTEMS Increased coral bleaching Most coral bleached Wildspread coral mortality

Terrestrial biosphere tends toward a net carbon source, as:
Increasing species range shifts and wildfire risk
“15% “40% of ecosystems affected

Low latitudes
Decreases for some cereals All cereals decrease
FOOD productivity
Increases for some cereals Decreases in some regions
Mid to high latitudes

Increased damage from floods and storms

About 30% loss
COAST of coastal wetlands
Additional people at risk of
0 to 3 million 2 to 15 million
coastal flooding each year

Increasing burden from malnutrition, diarrhoeal, cardio-respiratory and infectious diseases

Increased morbidity and mortality from heatwaves, floods and droughts
Changed distribution of some disease vectors Substantial burden on health services

Local retreat of ice in Long term commitment to several Leading to reconfiguration
Greenland and West metres of sea-level rise due to ice of coastlines world wide and
SINGULAR Antarctic sheet loss inundation of low-lying areas
Ecosystem changes due to weakening of the meridional overturning circulation

0 1 2 3 4 5°
Global mean annual temperature change relative to 1980“99 (°C)
176 T H E I M PAC T S O F C L I M AT E C H A N G E

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 4.6). 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.
The main cause of the large sea level changes was the melting or growth of
the large ice-sheets that cover the polar regions. The low sea level before 18 000
years ago was due to 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. Also the 5 or 6 m higher sea level during the last warm interglacial
period resulted from a reduction in the Antarctic and 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 about 20 cm.4 The largest contribution to this rise 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 after the end of the last ice age. Contributions
from the ice caps of Greenland and Antarctica are relatively small. A further
small contribution to sea level change arises from changes in terrestrial storage
of water, for instance from the growth of reservoirs or irrigation.
Since around 1990 much improved observations of changes of sea level with
global coverage have become possible through satellite-borne altimeters that
can measure with great accuracy the height of the sea surface at any location.
In Figure 7.1 are shown the largest contributions to sea level rise for the peri-
ods 1961“2003 and 1993“2003 as estimated from climate models and indicates
that these contributions when summed show good agreement with observa-
tions, providing some con¬dence in the modelling methods. The ¬gure also
illustrates the substantial increase in the rate of sea level rise, especially due to
thermal expansion, experienced during the most recent decade, 1993“2003.
The same methods and models used to estimate twentieth-century sea level
trends have been applied to provide estimates of the sea level rise during the
twenty-¬rst century. An example for SRES scenario A1B is shown in Figure
7.2 with an uncertainty range (5% to 95% probability) for the decade 2090“99
H OW M U C H W I L L S E A L E V E L R I S E ?

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 because 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 tem-
peratures 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 dec-
ades 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 5.5

Figure 7.1 Estimates for
1961“2003 (blue) and Thermal expansion
1993“2003 (pink) of
contributions to global mean Glaciers and ice caps
sea level change (upper four
entries), the sum of these Greenland
contributions and the observed
rate of rise (middle two) and the Antarctica
difference between the observed
rate and the estimates (lower).
The bars represent a range of
uncertainty of 90% probability.
Errors of the separate terms have
Difference (Obs “Sum)
been combined in quadrature to
obtain the error on their sum.
“1.0 “0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Rate of sea level rise (mm yr “1)

relative to 1980“99 of 0.21“0.48 m. Apart from a smaller uncertainty range this
shows little change from the 0.12“0.7 m range for the A1B scenario given in the
IPCC 2001 Report. The overall range covering the six SRES marker scenarios
is from 0.18 to 0.59 m. The largest contribution to sea level rise (0.13“0.32 m
for A1B) is expected to continue to be from thermal expansion of ocean water

Sea-ice is frozen sea water ¬‚oating on the surface of the ocean. Some sea-ice is semi-permanent, persisting
from year to year, and some is seasonal, melting and refreezing from season to season. The sea-ice cover
reaches its minimum extent at the end of each summer and the remaining ice is called the perennial ice
cover. The 2007 Arctic summer sea-ice (white) reached the lowest extent of perennial ice cover on record
“ nearly 25% less than the previous lowest in 2005 (orange). The average minimum sea-ice from 1979 to
2007 is shown in green. The area of perennial ice has been steadily decreasing since the satellite record
began in 1979, at a rate of about 10% per decade. Such a dramatic loss has implications for ecology, climate
and industry. The 2008 sea-ice extent was even less than that of 2007. With this increasingly rapid rate of
change, it is possible that Arctic summer sea-ice could reduce to zero by 2020.

calculated in detail from the ocean component of climate models. The second
largest (0.08“0.16 m for A1B) is expected from the melting of glaciers and ice
caps outside Greenland and Antarctica. It is derived from estimates of their
mass balance “ the difference between the amount of snowfall on them (mainly
in winter) and melting (mainly in summer); both winter snowfall and average
summer temperature are critical and have to be carefully estimated using cli-
mate models.
H OW M U C H W I L L S E A L E V E L R I S E ?

Figure 7.2 Global mean sea Estimates Instrumental record Projections
level in the past and as projected of the past of the future
for the future. From 1870 is a
reconstruction of the global
mean from tide gauges; the

Sea level change (mm)
green line is global mean sea
level as observed from satellite 200
altimetry. Beyond 2004 is the
range of model projections (the 100

5% to 95% uncertainty range)
for the SRES A1B scenario for
the twenty-¬rst century relative
to the 1980“99 mean from the
sum of estimates of the different “200
contributions (major ones
identi¬ed in Figure 7.1). 1800 1850 1900 1950 2000 2050 2100

The third largest contribution is expected from changes in the Antarctic and
Greenland ice-sheets. It is perhaps surprising that the net contribution from them
over the twentieth century is small and is still not large (Figure 7.1). For both ice-
sheets there are two competing effects. In a warmer world, there is more water
vapour in the atmosphere which leads to more snowfall. But as surface tempera-
ture increases, especially at high latitudes, 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. During the last
few decades, both ice-sheets have been close to balance (Figure 7.1). For the twen-
ty-¬rst century, the IPCC AR4 2007 projects that Antarctica will continue close
to balance but that for Greenland, ablation will be greater than accumulation
leading to a net loss amounting to less than 0.1 m by the end of the century.
During the last year, many pictures have been published of rapid ice melt-
ing and discharge from the Greenland ice cap and concern expressed that the
rate of melting could accelerate substantially as the warming progresses. This
possibility was recognised in IPCC AR4 where it was pointed out that if ice ¬‚ow
from Greenland increased linearly with temperature rise the upper range of sea
level rise in 2100 (Figure 7.2) would increase by 0.1 to 0.2 m. However, the rate
of increase may accelerate more rapidly. Four indications of this, the ¬rst two
suggested by P. Christoffersen and M. J. Hambrey6 and the last two by J. Hansen
and his co-authors,7 are:

(1) Observations from satellite radar altimeters show that the total ice-mass
loss from the Greenland ice cap rose from 90 km3 in 1996 to 140 km3 in
2000 and 220 km3 in 2005. Similar losses have been observed from the West
180 T H E I M PAC T S O F C L I M AT E C H A N G E

Antarctic ice-sheet. A loss of 400 km3 per year transfers into a global sea
level rise of about 1 mm per year or 0.1 m per century.
(2) Observations of acceleration in the movement of coastal outlet glaciers to
now more than 10 km per year. Similar losses and movement are occurring
with the West Antarctic ice-sheet.
(3) Observations of increased melt water, from the operation of the ice-albedo
feedback (see Chapter 5, page 114), penetrating to the bed of the ice-sheet
and through its lubrication enhancing ice motion and instabilities near the
ice-sheet base.
(4) Palaeo evidence of periods of rapid melting with associated global sea level
rise of up to several metres per century occurring for instance during the
recovery from the last ice age about 14 000 years ago.

The possibility of acceleration due to these non-linear processes was acknowl-
edged by IPCC, AR4 although they did not feel able to provide any quanti¬cation
of their likely size. A careful assessment published in Science8 as this book is
going to press concludes with a best estimate, including accelerated conditions
of ice melt, of 0.8 m total sea level rise by 2100. It also concludes that a rise of
up to 2 m by 2100 ˜could occur under physically possible glaciological condi-
tions but only if all variables are quickly accelerated to extremely high limits™.
Improved quanti¬cation should be possible as new satellite data (e.g. from the
GRACE gravity mission) is obtained and interpreted.9
If we look into the future beyond the twenty-¬rst century, as temperatures
around Greenland rise more than 3 °C above their pre-industrial level, model
studies show that meltdown of the ice cap will begin; its complete melting will
cause a sea level rise of about 7 m. The time taken for meltdown to occur will
depend on the amount of temperature rise; estimates for the time to 50% melt-
down vary from a few centuries to more than a millennium.
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 5-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 sur-
rounding ice shelves are weakened. From studies so far of ice dynamics and
¬‚ow there is no agreement that rapid disintegration is likely although, as with
Greenland, it is recognised that the possibility exists.
The projections in Figure 7.2 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 result-
ing from global warming will therefore lag behind temperature change at the

surface. 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 7.2 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.10 The effects of thermal expansion in the
oceans varies considerably with location. Further, movements of the land occur-
ring for natural reasons due, for instance, to tectonic movements or because of
human activities (for instance, the removal of groundwater) can have compara-
ble 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.

Impacts in coastal areas
A rise in average sea level of 10 to 20 cm by 2030 and about up to 1 metre by
the end of the next century 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.11 Within these, the
lowest lying are some of the most fertile and densely populated. To people liv-
ing in these areas, even half a metre increase in sea level can add enormously
to their problems. Their vulnerability is increased by the likelihood of storm
surges either due to more intense tropical cyclones or mid latitude storms and
by other problems such as local land subsidence and the increased intrusion of
salt into groundwater.
Especially vulnerable are large river deltas; in the largest 40 of these in the
world over 300 million people live who are increasingly affected by the rate of
sea level rise that is occurring even in the absence of climate change (Figure 7.3).
Since 1980 over a quarter of a million lives have been lost due to tropical cyclones
or storm surges “ as I write this paragraph in May 2008, over 100 000 have been
lost in a cyclone and storm surge in the Irrawaddy Delta in Myanmar. By 2080,
even with only half a metre of sea level rise and no further ¬‚ood defence, over
100 million in these deltas will be liable to ¬‚ooding.12 As an example of a delta
area I shall ¬rst consider Bangladesh; I shall then consider the Netherlands as
an example of an area very close to sea level where sea defences are already in
place. Thirdly, I shall look at the plight of small low-lying islands in the Paci¬c
and other oceans.
Bangladesh is a densely populated country of about 150 million people located in
the complex delta region of the Ganges, Brahmaputra and Meghna Rivers.13 About
182 T H E I M PAC T S O F C L I M AT E C H A N G E


Shatt Ganges
Mississippi el Arab
Sebou Indus Brahmaputra Changjiang
Nile Mahanadi Zhujiang
Grijalva Godavari
Orinoco Volta Mekong
Sao Francisco


Figure 7.3 Relative vulnerability of coastal deltas as indicated by estimates of the population potentially
displaced by current sea level trends to 2050 (extreme, > 1 million; high, 1 million to 50 000; medium, 50 000
to 5000). Climate change would exacerbate these impacts.

Figure 7.4 Land affected in Bangladesh by various
amounts of sea level rise. The 1, 2, 3 and 5 m
contours are shown.

10% of the country™s habitable land (with about
6 million population) would be lost with half
Bangladesh a metre of sea level rise and about 20% (with
about 15 million population) would be lost with
a 1-m rise (Figure 7.4).14 Estimates of the sea level
rise are of about 1 m by 2050 (compounded of
70 cm due to subsidence because of land move-
ments 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)15 “ although there is sig-
ni¬cant uncertainty in these estimates.
It is quite impractical to consider full protec-
tion of the long and complicated coastline of
Bangladesh from sea level rise. Its most obvious
effect, therefore, is that substantial amounts
Floods help make the cultivable land in Bangladesh fertile and this helps the agriculture sector of the
country. But excessive ¬‚ooding is considered a calamity. The rise in sea water levels, the narrow north tip to
the Bay of Bengal, tropical storms that whip up wind speeds of up to 140 mph (225 km h ’1) send waves up
to 26 feet (8 m tall) crashing into the coast, the shallow sea bed , the fact that water coming down from the
Rivers Ganges and Brahmaputra can not escape when the water level rises and the monsoons, all contribute
to the severe ¬‚ooding of the Bangladesh coastline.

of good agricultural land will be lost. This is serious: half the country™s economy
comes from agriculture and 83% of the nation™s population depends on agricul-
ture 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 25 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 mil-
lion people. Well over 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 agricultural
land; that is, the intrusion of salt water into fresh groundwater resources. At the
present time, it is estimated that in some parts of Bangladesh salt water extends
seasonally inland over 150 km. With a 1-m rise in sea level, the area affected by
184 T H E I M PAC T S O F C L I M AT E C H A N G E

saline intrusion could increase substantially although, since it is also likely that
climate change will bring increased monsoon rainfall, some of the intrusion of
salt water could be alleviated.16
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 riv-
ers 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 12% of the country™s arable land with a population of over 7 million
people would be affected by a 1-m rise of sea level.17 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.18
Many other examples of vulnerable delta regions, especially in Southeast
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 km 2 (about the area
of the Netherlands)19 where over 30 million people currently live. A particu-
lar delta that has been extensively studied is that of the Mississippi in North
America. These studies underline 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 move-
ments of the Earth™s crust. Also, the building of canals and dykes has inhibited
the input of sediments from the ocean.20 Studies of this kind emphasise 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.

In September 2008 a report by the government-appointed Delta Commission concluded that the
Netherlands must spend billions of euros on dyke upgrades and coastal expansion to avoid the ravages
of rising sea levels due to global warming over the coming decades

We now turn to the Netherlands, a country more than half of which con-
sists of coastal lowlands, mainly below present sea level. It is one of the most
densely populated areas in the world; 8 million of the 14 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, cur-
rents, 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.21 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 salt water into freshwater aquifers. It is estimated22 that an expenditure of
186 T H E I M PAC T S O F C L I M AT E C H A N G E

about $US12 000 million 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.23
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 sub-
stantially “ some would have to be abandoned “ and remove up to 50% 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.23
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
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 wetlands and man-
grove swamps currently occupy an area of about 1 million square kilometres
(the ¬gure is not known very precisely), equal approximately to twice the area
of France. They contain much biodiversity 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 lev-
els 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 extend 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 wet-
lands 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 extrac-
tion), coastal wetlands are currently being lost at a rate of 0.5“1.5% per year.
Sea level rise because of climate change would further exacerbate this loss.25
To summarise the impact of the half metre or more of sea level rise due to
global warming which could occur during the twenty-¬rst century: global
warming is not the only reason for sea level rise but it is likely to exacerbate
the impacts of other environmental problems. Careful 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 particu-
larly vulnerable, sea level rise will lead to substantial loss of agricultural land
and salt intrusion into freshwater resources. In Bangladesh, for instance, over
10 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 40 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.26
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 protection 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.
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.

Increasing human use of fresh water resources
The global water cycle is a fundamental component of the climate system. Water
is cycled between the oceans, the atmosphere and the land surface (Figure 7.5).
Through evaporation and condensation it provides the main means whereby
energy is transferred to the atmosphere and within it. Water is essential to all
forms of life; the main reason for the wide range of life forms, both plant and
animal, on the Earth is the extremely wide range of variation in the availability
188 T H E I M PAC T S O F C L I M AT E C H A N G E

of water. In wet tropical forests, the jungle teems with life of enormous variety.
In drier regions sparse vegetation exists, of a kind that can survive for long
periods with the minimum of water; animals there are also well adapted to dry
Water is also a key substance for humankind; we need to drink it, we need it
for the production of food, for health and hygiene, for industry and transport.
Humans have learnt that the ways of providing for livelihood can be adapted
to a wide variety of circumstances regarding water supply except, perhaps, for
the completely dry desert. Water availability for domestic, industrial and agri-
cultural use averaged per capita in different countries varies from less than
100 m3 (22 000 imperial gallons) per year to over 100 000 m3 (22 million imperial
gallons)27 “ although quoting average numbers of that kind hides the enormous
disparity between those in very poor areas who may walk many hours each day
to fetch a few gallons and many in the developed world who have access to virtu-
ally unlimited supplies at the turn of a tap.
Increases in freshwater use are driven by changes in population, lifestyle,
economy, technology and most particularly by demand for food which drives
irrigated agriculture. During the last 50 years water use worldwide has grown
over threefold (Figure 7.6); it now amounts to about 10% of the estimated global
total of the river and groundwater ¬‚ow from land to sea (Figure 7.5). Two-thirds
of human water use is currently for agriculture, much of it for irrigation; about
a quarter is used by industry; only 10% or so is used domestically. The demand
is so great in some river basins, for instance the Rio Grande and the Colorado in
North America, that almost no water from them reaches the sea. Increasingly,
water stored over hundreds or thousands of years in underground aquifers is
being tapped for current use and there are now many places in the world where
groundwater is being used much faster than it is being replenished; every year
the water has to be extracted from deeper levels. For instance, over more than
half the land area of the United States over a quarter of the groundwater with-
drawn is not replenished and around Beijing in China the water table is falling
by 2 m a year as groundwater is pumped out. These are just examples of greatly
increased vulnerability regarding water supplies that arise from rapid growth
of demand.
The extent to which a country is water stressed is related to the proportion of
the available freshwater supply that is withdrawn for use. In global scale assess-
ments, basins with water stress are de¬ned either as having per capita water
availability below 1000 m3 per year (based on long-term average run-off) or as
having a ratio of withdrawals to long-term average annual run-off above 0.4.28
Under this de¬nition, some 1.5 to 2 billion people, one-quarter to one-third of
the world™s population, live in water-stressed countries “ in parts of Africa, the

Vapour Transport 40


Evaporation Transpiration

425 Precipitation
Evaporation 385

Return Flow


Figure 7.5 The global water cycle (in thousands of cubic kilometres per year), showing the key processes
of evaporation, precipitation, transport as vapour by atmospheric movements and transport from the land
to the oceans by run-off or groundwater ¬‚ow.

Mediterranean region, the Near East, South Asia, northern China, Australia,
USA, Mexico, northeast Brazil and the western coast of South America. This
is illustrated in Figure 7.7 where examples are given of some of the current
vulnerabilities of fresh water resources and their management. The number
living in water-stressed countries is projected to rise steeply in coming decades
even without taking into account any effect on water supplies due to climate
A further vulnerability arises because many of the world™s major sources of
water are shared. About half the land area of the world is within water basins
that fall between two or more countries. There are 44 countries for which at
least 80% of their land area falls within such international basins. The Danube,
for instance, passes through 12 countries that use its water, the Nile water
through nine, the Ganges“Brahmaputra through ¬ve. Other countries where
190 T H E I M PAC T S O F C L I M AT E C H A N G E

Figure 7.6 Global water
withdrawal for different
purposes, 1900“95, and
projected to the year 2025
in cubic kilometres per year.
Municipal needs


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