. 8
( 16)


Losses from reservoirs are
Withdrawal (km3 yr “1)

also included. As some water
withdrawn is reused, the
total water consumption
amounts to about 60% of
the total water withdrawal.


1900 1920 1940 1960 1980 2000 2020

water is scarce are critically dependent on sharing the resources of rivers such
as the Euphrates and the Jordan. The achievements of agreements to share
water often bring with them demands for more effective use of the water and
better management. Failure to agree brings increased possibility of tension and
con¬‚ ict. The former United Nations Secretary General, Boutros Boutros-Ghali,
has said that ˜The next war in the Middle East will be fought over water, not

The impact of climate change on fresh
water resources
The availability of fresh water will be substantially changed in a world affected
by global warming. Although uncertainties remain for projections of precipita-
tion particularly on the regional or even river-basin scale, it is possible to iden-
tify many areas where substantial increases or decreases are likely (Figure 6.7).
For instance, precipitation is expected to increase in high latitudes and in parts
of the tropics and decrease in many mid latitude and sub-tropical regions espe-
cially in summer. Further, increase of temperature will mean that a higher
proportion of the water falling on the Earth™s surface will evaporate. In regions
with increased precipitation, some or all of the loss due to evaporation may
be made up. However, in regions with unchanged or less precipitation, there
will be substantially less water available at the surface. The combined effect of

Damage to riparian
ecosystems due to
flood protection Huanghe river
along river Elbe has temporarily
Multi-year run dry due to
droughts precipitation
in USA and decrease and
southern irrigation
Canada Health problems
Rural water due to arsenic
supply affected and fluoride in
Land subsidence
by extended groundwater in
and landslides in
dry season in India
Mexico City

Flood disasters in
Bangladesh (more
Area of Lake
Water supply affected by
than 70% of the
Chad declining
shrinking glaciers in Andes
country inundated
in 1998) Damage to aquatic
ecosystems due to
Water supply reduced by erosion
decreased streamflow
and sedimentation in reservoirs in
and increased salinity
northeast Brazil
in Murray“Darling basin

Water stress indicator: withdrawal to availability ratio
Water withdrawal: water used for irrigation,
mid stress
no stress high stress very high stress
low stress
livestock, domestic and industrial purposes (2000)
Water availability: average annual water
0 0.1 0.2 0.4 0.8
availability based on the 30-year period 1961 “ 90
No/low stress and per capita water
availability <1700m 3/ yr “1

Figure 7.7 Examples of current vulnerabilities of fresh water resources and their management; in the
background, a water stress map based on Alcamo et al. (2003a). See text for relation to climate change.

less rainfall and more evaporation means less soil moisture available for crop
growth and also less run-off “ in regions with marginal rainfall this loss of
soil moisture can be critical. Although, increased carbon dioxide also tends to
reduce plant transpiration and less water use by plants, a factor that requires
further investigation.30
The run-off in rivers and streams is what is left from the precipitation that
falls on the land after some has been taken by evaporation and by transpiration
from plants; it is the major part of what is available for human use. The amount
of run-off is highly sensitive to changes in climate; even small changes in the
amount of precipitation or in the temperature (affecting the amount of evapo-
ration) can have a big in¬‚uence on it. To illustrate this, Figure 7.8 shows esti-
mates of the mean change in annual run-off between 1980“2000 and 2081“2100
under the SRES A1B scenario. There are changes of up to plus 50% or minus
192 T H E I M PAC T S O F C L I M AT E C H A N G E

Desiccation of Lake Chad: (a) 1973 (b) 2007. The lake is very shallow and is particularly sensitive to small
changes in average depth, and seasonal variation. An increased demand on the lake™s water from the local
population has probably accelerated its shrinkage over the past 40 years; also, over-grazing in the area
surrounding the lake causes deserti¬cation and a decline in vegetation.

50% in many places. Water availability in many locations and watersheds will
change a great deal as the century progresses. Note that Figure 7.8 describes
average annual conditions. Superimposed on the changes of Figure 7.7 will be
the variability of climate, in particular the likely increase in the incidence and
intensity of climate extremes. The boxes in Figure 7.8 illustrate some particular
impacts of expected changes. Eight of the changes of particular concern are
the following.31

(1) Mentioned earlier in the chapter was that as the rate of warming increases,
up to one-half of the mass of mountain glaciers and small ice caps outside the
polar regions may melt away over the next hundred years. In fact if current
warming rates are maintained it is projected that Himalayan glaciers could
decay by 80% of their area by the 2030s. Snow melt is an important source of
run-off and watersheds will be severely affected by glacier and snow cover
decline. As temperatures rise, winter run-off will initially increase while
spring high water, summer and autumn ¬‚ows will be reduced. Particularly
seriously affected will be river basins dependent on the Hindu-Kush“Himalaya
glaciated region in Asia (e.g. the Indus, Ganges“Brahmaputra and Yangtze riv-
ers) where more than one-sixth of the world™s population currently lives, and
those dependent on glaciers in the South American Andes. In many areas,
there could be large changes in the seasonal distribution of river ¬‚ow and
water supply for hydroelectric generation and agriculture. For instance, in
Europe a decrease in hydropower potential of about 10% has been projected
by the 2070s.

5. Electricity
4. Flooded area
for annual peak
potential at
discharge in
increases by at
least 25% with
a global
by more
than 25% by
increase of 2 °C
the 2070s
2. Streamflow decreases
such that present water
demand could not be
satisfied after 2020, and
loss of salmon habitat 6. Increase of
pathogen load due
to more heavy 1. Thickness of small
precipitation events island freshwater lens
in areas without declines from 25 to
good water supply
3. Groundwater recharge 10 m due to 0.1 m sea
and sanitation
decreases by more than level rise by 2040“ 80
70% by the 2050s

“50 “30 “20 “10 “5 0 5 10 20 30 50

Figure 7.8 Illustrative map of future climate change impacts on fresh water which are a threat to the
sustainable development of the affected regions. The background map is of the ensemble mean change in
annual run-off in per cent between (1981“2000) and (2081“ 2100) for the SRES A1B emissions scenario.

(2) Many semi-arid areas (e.g. Mediterranean basin, western USA, southern
Africa, northeastern Brazil and parts of Australia) will suffer serious
decreases in water resources due to climate change. These problems will be
particularly acute in semi-arid or arid low-income countries, where precipi-
tation and stream¬‚ow are concentrated over a few months and where the
variability of precipitation is likely to increase as climate changes.
(3) Due to increases in population in addition to climate change, the number
of people living in severely stressed river basins is projected to increase
from about 1.5 billion in 1995 to 3 to 5 billion in 2050 for the SRES B2
(4) As was explained in Chapter 6, the more intense hydrological cycle associ-
ated with global warming will lead to increased frequency and intensity
of both ¬‚oods and droughts. As was mentioned in Chapter 1, ¬‚oods and
droughts affect more people across the globe than all other natural dis-
asters and their impact has been increasingly severe in recent decades.
Increases by 2050 in many parts of the world in the frequency and severity
194 T H E I M PAC T S O F C L I M AT E C H A N G E

of both ¬‚oods and droughts of about a factor of 5 that were mentioned
in Chapter 6 will have very large implications for water availability and
(5) Groundwater recharge will decrease considerably in some already water-
stressed regions where vulnerability may be exacerbated by increase in
population and water demand.
(6) Sea level rise together with greater use of groundwater will extend areas
of salination of groundwater and estuaries, resulting in a decrease in fresh
water availability for humans and ecosystems in coastal areas.
(7) Higher water temperatures, increased precipitation intensity and longer
periods of low ¬‚ows exacerbate many forms of water pollution, with
impacts on ecosystems, human health and water system reliability and
operating costs.
(8) A further reason, not unconnected with global warming, for the vulner-
ability of water supplies is the link between rainfall and changes in land
use. Extensive deforestation can lead to large changes in rainfall (see box
on page 208). A similar tendency to reduced rainfall can be expected if
there is a reduction in vegetation over large areas of semi-arid regions.
Such changes can have a devastating and widespread effect and assist in
the process of deserti¬cation. This is a potential threat to the drylands
covering about one-quarter of the land area of the world (see box on
page 197).

The monsoon regions of Southeast Asia are an example of an area that may
be particularly vulnerable to both ¬‚oods and droughts. Figure 7.9 shows the
predicted change in summer precipitation over the Indian sub-continent as
simulated by a regional climate model (RCM) for 2050 under a scenario similar
to SRES A1B. Note the improvement in detail of the precipitation pattern that
results from the use of the increased resolution of the regional model com-
pared with the global model (GCM), for instance over the Western Ghats (the
mountains that rise steeply from India™s west coast) there are large increases
not present in the global model simulation. The most serious reductions in
water availability simulated by the regional model are in the arid regions
of northwest India and Pakistan where average precipitation is reduced to
less than 1 mm day“1 “ that coupled with higher temperatures leads to a 60%
decline in soil moisture. Substantial increases in average precipitation are
projected for areas in eastern India and in ¬‚ood-prone Bangladesh where
the projected increase is about 20%. What is urgently required for this part
of the world and elsewhere is much better information linking changes in

Figure 7.9 Predicted
changes in monsoon
rainfall (mm day’1) over
India between the present
day and the middle of
the twenty-¬rst century
from a 300-km resolution
GCM and from a 50-km
resolution RCM. The RCM
pattern is very different
in some respects from the
coarser resolution pattern
of the GCM.

“3 “1 “0.2 0 0.2 1 3
Precipitation (mm day )

average parameters with likely changes in frequency, intensity and location
of extreme events.
It will be clear from the likely impacts as listed above that strong actions will
have to be taken to lessen the impact of changes in water availability or sup-
ply and to adapt to the changes. Some of the actions that can be taken are the
following. 32
Firstly, increasing the ef¬ ciency of water use. For instance, irrigation applied
to about one-sixth of the world™s farmland producing about one-third of the
world™s crops and accounting for about two-thirds of world water use, can be
made much more ef¬ cient. Most irrigation is through open ditches, which
is very wasteful of water; over 60% is lost through evaporation and seepage.
Microirrigation techniques, in which perforated pipes deliver water directly to
the plants, provide large opportunities for water conservation, making it possi-
ble to expand irrigated ¬elds without building new dams. Many other ef¬ciency
measures are also available, for instance, to recycle water where possible, to
promote indigenous practices for sustainable water use (e.g. local rainwater stor-
age), to conserve water (and also soil) by avoiding deforestation or increasing for-
ested areas and to use economic incentives to encourage water conservation.
Secondly, looking for new water supplies. For instance, by increasing water
storage in reservoirs or dams, by desalination of sea water, by transferring water
196 T H E I M PAC T S O F C L I M AT E C H A N G E

from areas of greater abundance or by prospecting and extracting groundwater
in appropriate areas.
Thirdly, by introducing more informed management. For instance, regions
such as Southeast Asia that are dependent on unregulated river systems are
more vulnerable to change than regions such as western Russia and the west-
ern United States that have large, regulated water resource systems. Ma ny
interested experts and bodies are promoting Integrated water management that
involves all sectors “ agricultural, domestic and industrial “ relates to existing
infrastructure and plans for new infrastructure and also, most importantly,
includes preparation for disasters such as major ¬‚oods and droughts.
Most of these actions will cost money, although many of them may be much
more cost-effective ways of coping with future change in water resources than
attempting to develop major new facilities.33

Impact on agriculture and food supply
Every farmer understands the need to grow crops or rear animals that are
suited to the local climate. The distribution of temperature and rainfall dur-
ing the year are key factors in making decisions regarding what crops to grow.
These will change in the world in¬‚uenced by global warming. The patterns of
what crops are grown where will therefore also change. But these changes will
be complex; economic and other factors will take their place alongside climate
change in the decision-making process.
There is enormous capacity for adaptation in the growth of crops for food
“ as is illustrated by what was called the Green Revolution of the 1960s, when
the development of new strains of many species of crops resulted in large
increases in productivity. Between the mid 1960s and the mid 1980s global
food production rose by an average annual rate of 2.4% “ faster than global
population “ more than doubling over that 30-year period. Grain production
grew even faster, at an annual rate of 2.9%.36 Since the mid 1980s growth
in production has continued at about 2% per year. There are concerns that
factors such as the degradation of many of the world™s soils largely through
erosion and the slowed rate of expansion of irrigation because less fresh
water is available will tend to reduce the potential for increased agricultural
production in the future. However, with declining rates of population growth
(SRES scenarios A1, B1, B2) and continued economic development, there
remains optimism that, in the absence of major climate change, growth in
world food supply is likely to continue at least to match growth in demand
and that the numbers of undernourished in the world will substantially

Drylands (de¬ned as those areas where precipitation is low and where rainfall typically consists of small,
erratic, short, high-intensity storms) cover about 40% of the total land area of the world and support
over one-¬fth of the world™s population. Figure 7.10 shows how these arid areas are distributed over the
Deserti¬cation in these drylands is the degradation of land brought about by climate variations or human
activities that have led to decreased vegetation, reduction of available water, reduction of crop yields
and erosion of soil. The United Nations Convention to Combat Deserti¬cation (UNCCD) set up in 1996
estimates that over 70% of these drylands, covering over 25% of the world™s land area, are degraded34
and therefore affected by deserti¬cation. The degradation can be exacerbated by excessive land use or
increased human needs (generally because of increased population), or political or economic pressures (for
instance, the need to grow cash crops to raise foreign currency). It is often triggered or intensi¬ed by a
naturally occurring drought.
The progress of deserti¬cation in some of the drylands will be increased by the more frequent or more
intense droughts that are likely to result from climate change during the twenty-¬rst century.
Recent research has demonstrated the complex nature of the effects of climate change on dryland eco-
systems, on the interactions between the species they contain and with the local human communities who
live in the dryland areas. Much more understanding is required to assess what is likely to occur and how
adverse effects can be minimised.35

Figure 7.10 The world™s drylands, by continent. The 11%
total area of drylands is about 60 million square
kilometres (about 40% of the total land area), of 5%
which 10 million are hyper-arid deserts. 32%




Asia North America

Africa Europe

South America Australia
198 T H E I M PAC T S O F C L I M AT E C H A N G E

What will be the effect of climate change on agriculture and food supply?
With the detailed knowledge of the conditions required by different species and
the expertise in breeding techniques and genetic manipulation available today,
there should be little dif¬culty in matching crops to new climatic conditions
over large parts of the world. At least, that is the case for crops that mature
over a year or two. Forests reach maturity over much longer periods, from dec-
ades up to a century or even more. The projected rate of climate change is such
that, during this time, trees may ¬nd themselves in a climate to which they are
far from suited. The temperature regime or the rainfall may be substantially
changed, resulting in stunted growth or a greater susceptibility to disease, pests
and ¬res. The impact of climate change on forests is considered in more detail
in the next section.
An example of adaptation to changing climate is the way in which farmers in
Peru adjust the crops they grow depending on the climate forecast for the year.38
Peru is a country whose climate is strongly in¬‚uenced by the cycle of El Ni±o
events described in Chapters 1 and 5. Two of the primary crops grown in Peru,
rice and cotton, are very sensitive to the amount and the timing of rainfall.
Rice requires large amounts of water; cotton has deeper roots and is capable of
yielding greater production during years of low rainfall. In 1983, following the
1982“3 El Ni±o event, agricultural production dropped by 14%. By 1987 forecasts
of the onset of El Ni±o events had become suf¬ciently good for Peruvian farm-
ers to take them into account in their planning. In 1987, following the 1986“7 El
Ni±o, production actually increased by 3%, thanks to a useful forecast.
Four factors are particularly important in considering the effect of cli-
mate change on agriculture and food production. The availability of water
is the most important of the factors. The vulnerability of water supplies to
climate change carries over into a vulnerability in the growing of crops and
the production of food. Thus the arid or semi-arid areas, mostly in developing
countries, are most at risk. A second factor, which tends to lead to increased
production as a result of climate change, is the boost to growth that is given,
particularly to some crops, by increased atmospheric carbon dioxide (see box
below). A third factor is the effect of temperature changes; as temperatures
rise, yields of some crops are substantially reduced.39 A fourth factor is the
in¬‚uence of climate extremes, heatwaves, ¬‚oods and droughts that seriously
interfere with food production.
Detailed studies have been carried out of the sensitivity to climate change
during the twenty-¬rst century of the major crops which make up a large pro-
portion of the world™s food supply (see box below). They have used the results
of climate models to estimate changes in temperature and precipitation. Many
of them include the effect of carbon dioxide fertilisation and some also model

The carbon dioxide ˜fertilisation™ effect
An important positive effect of increased carbon dioxide concentrations in the atmosphere is the boost
to growth in plants given by the additional carbon dioxide. Higher carbon dioxide concentrations stimu-
late photosynthesis, enabling plants to ¬x carbon at a higher rate. This is why in glasshouses additional
carbon dioxide may be introduced arti¬cially to increase productivity. The effect is particularly applica-
ble to what are called C3 plants (such as wheat, rice and soya bean), but less so to C4 plants (for exam-
ple, maize, sorghum, sugar cane, millet and many pasture and forage grasses). Under ideal conditions it
can be a large effect; for C3 crops under doubled carbon dioxide, an average of +30%,40 although grain
and forage quality tends to decline with carbon dioxide enrichment and higher temperatures. However,
under real conditions on the large scale where water and nutrient availability are also important factors
in¬‚uencing plant growth, experiments show increases under unstressed conditions in the range 10“25%
for C3 crops and 0“10% for C4 crops. Enhanced growth has been observed for young tree stands but
no signi¬cant response has been measured for mature forest stands. Ozone exposure limits carbon
dioxide response in both crops and forests.41 More research is required especially for many tropical crop
species and for crops grown under suboptimal conditions (low nutrients, weeds, pests and diseases).
More information is also needed about possible effects on the nutritional value of the crops with carbon
dioxide fertilisation.42

the effects of climate variability as well as changes in the means. Some also
include the possible effects of economic factors and of modest levels of adap-
tation. These studies in general indicate that the bene¬t of increased carbon
dioxide concentration on crop growth and yield does not always overcome the
effects of excessive heat and drought. For cereal crops in mid latitudes, potential
yields are projected to increase for small increases in temperature (2“3 °C) but
decrease for larger temperature rises (Figure 7.11).43 In most tropical and sub-
tropical regions, potential yields are projected to decrease for most increases in
temperature; this is because such crops are near their maximum temperature
tolerance. Where there is a large decrease in rainfall, tropical crop yields would
be even more adversely affected.
Taking the supply of food for the world as a whole, studies tend to show that,
with appropriate adaptation, the effect of climate change on total global food
supply is not likely to be large. However, none of them has adequately taken
into account the likely effect on food production of climate extremes (espe-
cially of the incidence of drought), of increasingly limited water availability or
of other factors such as the integrity of the world™s soils, which are currently
being degraded at an alarming rate.44 A serious issue exposed by the studies is
that climate change is likely to affect countries very differently. Production in
developed countries with relatively stable populations may increase, whereas
200 T H E I M PAC T S O F C L I M AT E C H A N G E

Modelling the impact of climate change on world food supply
An example illustrating the key elements of a detailed study of the impact of climate change on world food
supply is shown in Figure 7.12.45
A climate change scenario is ¬rst set up with a climate model of the kind described in Chapter 5.
Models of different crops that include the effects of temperature, precipitation and carbon dioxide
are applied to 124 different locations in 18 countries to produce projected crop yields that can be
compared with projected yields in the absence of climate change. Included also are farm-level adapta-
tions, e.g. planting date shifts, more climatically adapted varieties, irrigation and fertiliser application.
These estimates of yield are then aggregated to provide yield-change estimates by crop and country
or region.
These yield changes are then employed as inputs to a world food trade model that includes assump-
tions about global parameters such as population growth and economic change and links together
national and regional economic models for the agricultural sector through trade, world market prices
and ¬nancial ¬‚ows. The world food trade model can explore the effects of adjustments such as increased
agricultural investment, reallocation of agricultural resources according to economic returns (including
crop switching) and reclamation of additional arable land as a response to higher cereal prices. The
outputs from the total process provide information projected up to the 2080s on food production, food
prices and the number of people at risk of hunger (de¬ned as the population with an income insuf¬cient
either to produce or to procure their food requirements).
The main results with models of this kind for the 2080s regarding the impact of climate change,
for SRES scenarios A1, B1 or B2, are that yields at mid to high latitudes are expected to increase, and
at low latitudes (especially the arid and sub-humid tropics) to decrease. This pattern becomes more
pronounced as time progresses. The African continent is particularly likely to experience marked
reductions in yield, decreases in production and more people at risk of hunger as a result of climate
The authors emphasise that, although the models and the methods they have employed are com-
paratively complex, there are many factors that have not been taken into account. For instance, they
have not adequately considered the impact of changes in climate extremes, the availability of water
supplies for irrigation or the effects of future technological change on agricultural productivity. Further
(see Chapter 6 ), scientists have as yet limited con¬dence in the regional detail of climate change. The
results, therefore, although giving a general indication of the changes that could occur, should not be
treated as a detailed prediction. They highlight the importance of studies of this kind as a guide to
future action.

Figure 7.12 Key elements of a
Climate models study of crop yield and food
trade under a changed climate.

Observed Sensitivity
climate tests

Crop models:
CO2 effects
wheat, rice, maize, soya bean adaptations

Crop yield by site
and scenario:
evapotranspiration, irrigation,
season length

Aggregation of site results
Agroecological zone analysis

Yield functions by region
Yield = function of temperature
precipitation and CO2

Yield change estimates
Commodity group and
projections Population
Economic World food trade
growth rates
Economic consequences
Shifts in trade
Incidence of food poverty
202 T H E I M PAC T S O F C L I M AT E C H A N G E

(a) Wheat, mid- to high latitude (b) Wheat, low latitude
60 60

40 40
Yield change (%)

20 20

0 0

“ 20 “ 20

“ 40 “ 40

“ 60 “ 60
0 1 2 3 4 5 6 0 1 2 3 4 5 6
Mean local temperature change (°C) Mean local temperature change (°C)

Figure 7.11 Sensitivity of yield to climate change for wheat for mid to high latitude (a) and low latitude
(b). Cases without adaptation (red) and with adaptation (green). Derived from 69 published studies at
multiple simulation sites.

that in many developing countries (where large increases in population are
occurring) is likely to decline as a result of climate change. The disparity between
developed and developing nations will tend to become much larger, as will the
number of those at risk of hunger. The surplus of food in developed countries
is likely to increase, while developing countries will face increasing depriva-
tion as their declining food availability becomes much less able to provide for
the needs of their increasing populations. Such a situation will raise enormous
problems, one of which will be that of employment. Agriculture is the main
source of employment in developing countries; people need employment to be
able to buy food. With changing climate, as some agricultural regions shift, peo-
ple will tend to attempt to migrate to places where they might be employed in
agriculture. With the pressures of rising populations, such movement is likely
to be increasingly dif¬cult and we can expect large numbers of environmental
In looking to future needs, two activities that can be pursued now are par-
ticularly important. Firstly, there is a large need for technical advances in
agriculture in developing countries requiring investment and widespread
local training. In particular, there needs to be continued development of pro-
grammes for crop breeding and management, especially in conditions of heat
and drought. These can be immediately useful in the improvement of produc-
tivity in marginal environments today. Secondly, as was seen earlier when
considering fresh water supplies, improvements need to be made in the avail-
ability and management of water for irrigation, especially in arid or semi-arid
areas of the world.

The impact on ecosystems
A little over 10% of the world™s land area is under cultivation “ that was the area
addressed in the last section. The rest is to a greater or lesser extent unman-
aged by humans. In Figure 7.13 are illustrated the world™s major ecosystems (or
biomes) with their global areal extent showing how they have been transformed
by land use.
Ecosystems are of great importance to human communities. They pro-
vide supplies for human communities in the provision of food, water, fuel,
wood and biodiversity. They also provide important regulation especially for
components of the hydrological cycle. Further they possess a wide range of
important cultural value. All these together are commonly called ecosystem
The variety of plants and animals that constitute a local ecosystem is sensitive
to the climate, the type of soil and the availability of water. Ecologists divide
the world into regions characterised by their distinctive vegetation. This is well
illustrated by information about the distribution of vegetation over the world
during past climates (e.g. for the part of North America shown in Figure 7.14),
which indicates the ecosystems most likely to ¬‚ourish under different climatic
regimes. Changes in climate alter the suitability of a region for different species
(Figure 7.15), and change their competitiveness within an ecosystem, so that
even relatively small changes in climate will lead, over time, to large changes
in the composition of an ecosystem.
However, changes of the kind illustrated in Figure 7.14 took place over thou-
sands of years. With global warming similar changes in climate occur over
a few decades. Most ecosystems cannot respond or migrate that fast. Fossil
records indicate that the maximum rate at which most plant species have
migrated in the past is less than 1 km per year. Known constraints imposed
by the dispersal process (e.g. the mean period between germination and the
production of seeds and the mean distance that an individual seed can travel)
suggest that, without human intervention, many species would not be able to
keep up with the rate of movement of their preferred climate niche projected
for the twenty-¬ rst century, even if there were no barriers to their movement
imposed by land use.46 Natural ecosystems will therefore become increasingly
unmatched to their environment. How much this matters will vary from spe-
cies to species: some are more vulnerable to changes in average climate or
climate extremes than others. But all will become more prone to disease and
attack by pests. Any positive effect from added ˜fertilisation™ due to increased
carbon dioxide is likely to be more than outweighed by negative effects from
other factors.
204 T H E I M PAC T S O F C L I M AT E C H A N G E

Atmosphere ˜2000 800

Carbon stocks (PgC)
Atmosphere P“IND 600
Atmosphere LGM


Surface area (Mkm2)



D G&S(tr) G(te) ME F(tr) F(te) F(b) T FW C O

Figure 7.13 The world™s major ecosystems with their global areal extent (lower panel), transformed by land
use in yellow, untransformed in purple and total carbon stores (upper panel) in plant biomass (green), soil
(brown) and yedoma/permafrost (light blue). D, deserts; G & S(tr), tropical grasslands and savannas; G(te),
temperate grasslands; ME, Mediterranean ecosystems; F(tr), tropical forests; F(te), temperate forests; F(b),
boreal forests; T, tundra; FW, freshwater lakes and wetlands; C, croplands; O, oceans. Approximate carbon
content of the atmosphere is indicated for last glacial maximum (LGM), pre-industrial (P-IND) and current
(about 2000).

Forests cover about 30% of the world™s land area and are among the most
productive of terrestrial ecosystems. They represent a large store of carbon. Of
terrestrial carbon, 80% of above-ground and 40% of below-ground is in forests,
together storing about twice as much as is in the atmosphere (Figure 3.1). They
are particularly important in the context of climate change. Current levels of
deforestation are responsible for around 20% of the additional carbon dioxide
emitted to the atmosphere each year due to human activities. What effect will
climate change have on the world™s forests and how in turn might that effect
the climate?
Trees are long-lived and take a long time to reproduce, so cannot respond
quickly to climate change. Further, many trees are surprisingly sensitive to
the average climate in which they develop. The environmental conditions (e.g.
temperature and precipitation) under which a species can exist and repro-
duce are known as its niche. Climate niches for some typical tree species are
illustrated in Figure 7.16; under some conditions a change as small as 1 °C
in annual average temperature can make a substantial difference to a tree™s
productivity. For the likely changes in climate in the twenty-¬ rst century, a
substantial proportion of existing trees will be subject to unsuitable climate
conditions. This will be particularly the case in the boreal forests of the north-
ern hemisphere where, as trees become less healthy, they will be more prone


Boreal forest to spruce/spruce pine

Mixed conifer“ northern
hardwood forest

Boreal forest

Cool “ temperate deciduous forest

Warm“ temperate southeastern
evergreen forest

Sand dune scrub



(b) (c)

Figure 7.14 Vegetation maps of the southeastern United States during past climate regimes: (a) for
18 000 years ago at the maximum extent of the last ice age, (b) for 10 000 years ago, (c) for 5000
years ago when conditions were similar to present. A vegetation map for 200 years ago is similar to
that in (c).
206 T H E I M PAC T S O F C L I M AT E C H A N G E

Conifer forest showing trees damaged by ˜acid rain™. An important factor that will in¬‚uence the future
concentrations of sulphate particles is ˜acid rain™ pollution, caused mainly by the sulphur dioxide emissions.
This leads to the degradation of forests and ¬sh stocks in lakes, especially in regions downwind of major
industrial areas.

to pests, dieback and forest ¬ res. One estimate projects that, under a doubled
carbon dioxide scenario, up to 65% of the current boreal forested area could
be affected.47
A decline in the health of many forests in recent years has received consider-
able attention, especially in Europe and North America where much of it has
been attributed to acid rain and other pollution originating from heavy indus-
try, power stations and motor cars. Not all damage to trees, however, is thought
to have this origin. Studies in several regions of Canada, for instance, indicate
that the dieback of trees there is related to changes in climatic conditions, espe-
cially to successions of warmer winters and drier summers.48 In some cases it
may be the double effect of pollution and climate stress causing the problem;

Figure 7.15 The pattern of 30

world biome types related to Tropical
Woodland seasonal
mean annual temperature and Tropical
Thorn forest
precipitation. Other factors, rainforest
scrub Tropical
especially the seasonal variations Desert
of these quantities, affect the 20
detailed distribution patterns temperature
desert Grassland
(after Gates). rainforest

Mean annual temperature (°C)


forest Cold

“10 Arctic “

0 50 100 150 200 250 300 350 400 450
Mean annual precipitation (cm)

trees already weakened by the effects of pollution fail to cope with climate
stress when it comes. Stresses on the world™s forests due to climate change (see
box on page 208) will be concurrent with other problems associated with for-
ests, in particular those of continuing tropical deforestation and of increasing
demand for wood and wood products resulting from rapidly increasing popula-
tions especially in developing countries.
If a stable climate is eventually re-established, given adequate time (which
could be centuries), different trees will be able to ¬nd again at some location
their particular climatic niche. It is during the period of rapid change that
most trees will ¬nd themselves unsatisfactorily located from the climate point
of view. If, because of the rate of climate change, substantial stress and die-
back occur in boreal and tropical forests (see box below) a release of carbon
will occur. This positive feedback was mentioned in Chapter 3 (see the box on
page 48“9). Just how large this will be is uncertain but estimates as high as 240
Gt over the twenty-¬rst century for the above-ground component alone have
been quoted.50
208 T H E I M PAC T S O F C L I M AT E C H A N G E

Forest“climate interactions and feedbacks
Various interactions and feedbacks are in play between forests and climate. Extensive changes in the area
of forests due to deforestation can seriously affect the climate in the region of change. Changes in carbon
dioxide, temperature or rainfall associated with climate change can have a major impact on the health or
structure of forests that can in turn feed back on the climate. We consider some of these effects in turn.
Changes in land use such as those brought about by deforestation can affect the amount of rainfall,
for three main reasons. Over a forest there is a lot more evaporation of water (through the leaves of the
trees) than there is over grassland or bare soil, hence the air will contain more water vapour. Also, a forest
re¬‚ects 12“15% of the sunlight that falls on it, whereas grassland will re¬‚ect about 20% and desert sand
up to 40%. A third reason arises from the roughness of the surface so stimulating convection and other
dynamic activity where vegetation is present.
It was in fact an American meteorologist, Professor Jules Charney, who suggested in 1975 that, in the
context of the drought in the Sahel, there could be an important link between changes of vegetation and
rainfall. Early experiments with numerical models that included these physical processes demonstrated the
effect and indicated large reductions of rainfall when large areas of forest were replaced by grassland. In
the most extreme cases, the rainfall reduction was so large that grassland would no longer be supported
and the land would become semi-arid.
However, even in the absence of changes of vegetation because of human action, interactions occur
between the climate and the forest that can effect large changes. Three important feedbacks that lead to
reduced precipitation are:

• increased carbon dioxide causes stomatal closure within the leaves of the trees so reducing
• increased temperature tends to cause forest dieback, again leading to reduced evaporation;
• increased temperature causes increased respiration of carbon dioxide from the soil so leading to further
global warming “ the climate/carbon-cycle feedback mentioned in Chapter 3.

For Amazonia, when these three feedbacks are added to the effect on the forest of the local climate
change that is likely to occur because of global circulation changes, under a scenario of continuing
increase in carbon dioxide emissions, simulations suggest major losses of forest cover in the Amazon
basin during the twenty-¬rst century. Large areas would be replaced by shrubs and grasses and part of
Amazonia could become semi-desert.49 Such results are still subject to considerable uncertainties (for
instance, those associated with the model simulations of El Ni±o events under climate change conditions
and the connections between these events and the climate over Amazonia), but they illustrate the type
of impacts that might occur and emphasise the importance of understanding the interactions between
climate and vegetation.

Arolla pine


Temperature (°C)


(t ha“1)

500 750 1000 1250 1500 1750 2000
Precipitation (mm yr “1)

Norway spruce Common beech

12 12

10 10
Temperature (°C)

Temperature (°C)

8 8

6 6

4 4

2 2

0 0

“2 “2
500 750 1000 1250 1500 1750 2000 500 750 1000 1250 1500 1750 2000
Precipitation (mm yr “1) Precipitation (mm yr “1)

Figure 7.16 Simulated environmental realised niches (the realised niche describes the
conditions under which the species is actually found) for three tree species, Arolla
Pine, Norway Spruce and Common Beech. Plots are of biomass generated per year
against annual means of temperature and precipitation. Arolla Pine is a species with a
particularly narrow niche. The narrower the niche, the greater the potential sensitivity to
climate change.

The above discussion has related to the impact of climate change on natural
forests where the likely impacts are largely negative. Studies of the impacts on
managed forests are more positive.51 They suggest that with appropriate adap-
tation and land and product management, even without forestry projects that
increase the capture and storage of carbon (see Chapter 10), a small amount of
210 T H E I M PAC T S O F C L I M AT E C H A N G E

Current Arctic conditions Projected Arctic conditions
180° 180°





80 80
°N °N

90°W 90°E 90°W 90°E

0° 0°

Temperate Boreal Grassland Polar desert/ Tundra Ice
forest forest semi-desert
Observed ice extent Northwest passage
September 2002
Projected ice extent Northern sea route
September 2080“2100

Figure 7.17 Ice cover and vegetation in the Arctic and neighbouring regions as observed today and as
modelled for 2080“2100 under the IS 92a scenario. Latest projections are for complete disappearance of
Arctic sea-ice in late summer, possibly by 2020.

climate change could increase global timber supply and enhance existing mar-
ket trends towards rising market share in developing countries.
Large changes are also expected in the polar regions (Figure 7.17) both in the
amount of sea-ice cover and in vegetation with large implications for managed
and unmanaged ecosystems both on land and marine areas. Changes in dry-
lands, in their extent or the nature of their vegetation are also of great concern
(see box on page 197).
A further concern about natural ecosystems relates to the diversity of species
that they contain and the loss of species and hence of biodiversity due to the
impact of climate change. Signi¬cant disruptions of ecosystems from distur-
bances such as ¬re, drought, pest infestation, invasion of species, storms and
coral bleaching events are expected to increase. The stresses caused by climate
change, added to other stresses on ecological systems (e.g. land conversion,

land degradation, deforestation, harvesting and pollution) threaten substantial
damage to or complete loss of some unique ecosystems, and the extinction of
some endangered species. Coral reefs and atolls, mangroves, boreal and tropi-
cal forests, polar and alpine ecosystems, prairie wetlands and remnant native
grasslands are examples of systems threatened by climate change. In some
cases the threatened ecosystems are those that could mitigate against some
climate change impacts (e.g. coastal systems that buffer the impact of storms).
Possible adaptation methods to reduce the loss of biodiversity include the estab-
lishment of refuges, parks and reserves with corridors to allow migration of
species, and the use of captive breeding and translocation of species.52
So far we have been considering ecosystems on land. What about those in
the oceans; how will they be affected by climate change? Although we know
much less about ocean ecosystems, there is considerable evidence that biologi-
cal activity in the oceans has varied during the cycle of ice ages. Chapter 3
noted (see box on page 43) the likelihood that it was these variations in marine
biological activity which provided the main control on atmospheric carbon
dioxide concentrations during the past million years (see Figure 4.6). Changes
in ocean water temperature and possible changes in some of the patterns
of ocean circulation will result in changes in the regions where upwelling
occurs and where ¬sh congregate. Some ¬sheries could collapse and others
expand. At the moment the ¬shing industry is not well adapted to address
major change.53
Some of the most important marine ecosystems are found within coral
reefs that occur in many locations throughout the tropical and sub-tropical
world. They are especially rich in biodiversity and are particularly threatened
by global warming. Within them the species diversity contains more phyla
than rainforests and they harbour more than 25% of all known marine ¬sh.54
They represent a signi¬cant source of food for many coastal communities.
Corals are particularly sensitive to sea surface temperature and even 1 °C of
persistent warming can cause bleaching (paling in colour) and extensive mor-
tality accompanies persistent temperature anomalies of 3 °C or more. Much
recent bleaching, for instance that in 1998, has been associated with El Ni±o
Added to the stresses caused by climate change will be those that arise from
increased ocean acidi¬cation that results from the increase of carbon dioxide
in ocean water (Figure 7.18). These increased stresses will be most serious for
a wide range of planktonic and shallow benthic marine organisms that use
aragonite to make their shells or skeletons, such as corals and marine snails
(pteropods). Research on how serious these stresses will be is being actively
212 T H E I M PAC T S O F C L I M AT E C H A N G E

Coral bleaching is a vivid sign of corals responding to stress which can be induced by increased or reduced
water temperatures, increased solar irradiance, changes in water chemistry, starvation caused by a decline
in zooplankton levels as a result of over-¬shing, and increased sedimentation.

Summarising the impacts of climate change on ecosystems with a warming
of global average temperature of 2 °C or more from its pre-industrial value, there
are ¬ve areas of particular concern.57

(1) The resilience of many ecosystems (their ability to adapt) is likely to be
exceeded by an unprecedented combination of change in climate, associ-
ated disturbances (e.g. ¬‚ooding, drought, wild¬re, insects, ocean acidi¬-
cation) and in other drivers such as land-use change, pollution and over
exploitation of resources.
(2) The terrestrial biosphere is currently a net carbon sink (see Table 3.1). As
was mentioned in Chapter 3, during the twenty-¬rst century, it is likely to
become a net carbon source thus amplifying climate change.




Oceanic pH
7.8 2100


-25 -20 -15 -10 -5 0
Time (million years before present)

Figure 7.18 Rising CO2 concentration in the atmosphere leads to move CO2 dissolved
in the ocean with rapid increase in ocean acidity (lower pH) to levels not encountered
for millions of years. Past (blue spots, data from Pearson and Palmer 2000) and
contemporary variability of marine pH (purple spots, with dates). Future predictions are
model derived values assuming atmospheric carbon dioxide concentration of 500 ppm
in 2050 and 700 ppm in 2100.

(3) Approximately 20“30% of plant and animal species so far assessed (in an
unbiased sample) are likely to be at increasingly high risk of extinction.
(4) Substantial changes in structure and functioning of terrestrial ecosystems
are very likely to occur with some positive impacts due to the carbon diox-
ide fertilisation effect but with extensive forest and woodland decline in
mid to high latitudes and the tropics associated particularly with changing
disturbance regimes (e.g. through wild¬re and insects).
(5) Substantial changes in structure and functioning of marine and other
aquatic ecosystems are very likely to occur. In particular the combination
of climate change and ocean acidi¬cation will have a severe impact on

The impact on human health
Human health is dependent on a good environment. Many of the factors that
lead to a deteriorated environment also lead to poor health. Pollution of the
atmosphere, polluted or inadequate water supplies and poor soil (leading to
poor crops and inadequate nutrition) all present dangers to human health and
well-being and assist the spread of disease. As has been seen in considering the
impacts of global warming, many of these factors will be exacerbated through
the climate change occurring in the warmer world. The greater likelihood of
extremes of climate, such as droughts and ¬‚oods, will also bring greater risks to
214 T H E I M PAC T S O F C L I M AT E C H A N G E

health from increased malnutrition and from a prevalence of conditions more
likely to lead to the spread of diseases from a variety of causes.
How about direct effects of the climate change itself on human health? Humans
can adapt themselves and their buildings so as to live satisfactorily in very
varying conditions and have great ability to adapt to a wide range of climates.
The main dif¬culty in assessing the impact of climate change on health is that
of unravelling the in¬‚uences of climate from the large number of other factors
(including other environmental factors) that affect health.
The main direct effect on humans will be that of heat stress in the extreme
high temperatures that will become more frequent and more widespread espe-
cially in urban populations (see box and Figure 6.6). In large cities where heat-
waves commonly occur death rates can be doubled or tripled during days of
unusually high temperatures.58 Although such episodes may be followed by
periods with fewer deaths showing that some of the deaths would in any case
have occurred about that time, most of the increased mortality seems to be
directly associated with the excessive temperatures with which old people in
particular ¬nd it hard to cope. On the positive side, mortality due to periods
of severe cold in winter will be reduced. The results of studies are equivocal
regarding whether the reduction in winter mortality will be greater or less than
the increase in summer mortality. These studies have largely been con¬ned
to populations in developed countries, precluding a more general comparison
between changes in summer and winter mortality.
A further likely impact of climate change on health is the increased spread-
ing of diseases in a warmer world. Many insect carriers of disease thrive better
in warmer and wetter conditions. For instance, epidemics of diseases such as
viral encephalitides carried by mosquitoes are known to be associated with the
unusually wet conditions that occur in the Australian, American and African
continents associated with different phases of the El Ni±o cycle.60 Some dis-
eases, currently largely con¬ned to tropical regions, with warmer conditions
could spread into mid latitudes. Malaria is an example of such a disease that
is spread by mosquitoes under conditions that are optimum in the tempera-
ture range of 15“32 °C with humidities of 50“60%. It currently represents a
huge global public health problem, causing annually around 300 million new
infections and over 1 million deaths. Under climate change scenarios, most
predictive model studies indicate a net increase in the geographic range (and
in the populations at risk) of potential transmission of malaria and dengue
infections, each of which currently impinge on 40“50% of the world™s popula-
tion. Other diseases that are likely to spread for the same reason are yellow
fever and some viral encephalitides. In all cases, however, actual disease
occurrence will be strongly in¬‚uenced by local environmental conditions,

Heatwaves in Europe and India, 2003
Record extreme temperatures were experienced in Europe during June, July and August 2003. At many
locations temperature rose to over 40 °C. In France, Italy, the Netherlands, Portugal and Spain, over 20, 000
(possibly as many as 35 000) additional deaths were attributed to the unrelenting heat. Spain, Portugal,
France and countries in Central and Eastern Europe suffered from intense forest ¬ res.59 Figure 7.19
illustrates the rarity of this event showing that it is well outside normal climate variability. Studies indicate
that most of the risk of this event arose from increase in greenhouse gases due to human activities. They
also indicate that it will represent a normal year by 2050 and a cool year by 2100.
Extreme heat was also experienced in 2003 in other parts of the world; for instance in Andhra Pradesh
in India over 1000 people died through extreme temperatures above 45 °C that occurred most unusually
for 27 consecutive days.

Figure 7.19 Characteristics (a)
of the summer 2003 4
heatwave in Europe.
(a) June, July, August (JJA)
temperature anomaly with 2

Temperature anomaly (K)
respect to 1961“90; (b) to 1
(d) JJA temperatures for
Switzerland; (b) observed
during 1864“2003; “1
(c) simulated with a
regional model for the
period 1961“90; (d) “3
simulated for 2071“2100
under the SRES A2
scenario. The vertical bars
in (b) to (d) represent (b) Observations

mean summer surface 1864“2002


temperature for each
year of the time period
considered; the ¬tted
Gaussian distribution is Climate simulation

indicated in black.

(d) Climate simulation


14 16 18 20 22 24 26 28
Temperature (°C)
216 T H E I M PAC T S O F C L I M AT E C H A N G E

Impacts on Africa
Africa is one of the most vulnerable continents to climate change and variability, a situation that is exac-
erbated by existing developmental challenges such as endemic poverty; complex governance and institu-
tional dimensions; limited access to capital, including markets, infrastructure and technology; ecosystem
degradation; and complex disasters and con¬‚icts “ all of which contribute to Africa™s weak adaptive capac-
ity to climate change.61 Some projected climate impacts for Africa are summarised as follows:62

• The impacts of climate change in Africa are likely to be greatest where they co-occur with a range of
other stresses (e.g., unequal access to resources, enhanced food insecurity, poor health management
systems. These stresses, enhanced by climate variability and change, further enhance the vulnerabilities
of many people in Africa.**
• An increase of 5% to 8% (60 to 90 million ha) of arid and semi-arid land in Africa is projected by the
2080s under a range of climate-change scenarios.**
• Declining agricultural yields are probably due to drought and land degradation, especially in marginal
areas. Changes in the length of growing period have been noted under various scenarios. In the A1F1
SRES scenario, which has an emphasis on globally integrated ecconomic growth, areas of major change


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