Download Climate change and sustainable water resources: placing the threat

Hydrological Sciences—Journal—des Sciences Hydrologiques, 44(4) August 1999
54]
Special issue: Barriers to Sustainable Management of Water Quantity and Quality
Climate change and sustainable water resources:
placing the threat of global warming in perspective
J. A. A. JONES
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK
e-mail: [email protected]
Abstract Predicted climate change over the coming decades is likely to add
measurable stress to water resources in many regions of the world, including some
areas that are currently well endowed. The stresses are likely to involve changes in the
frequency of extreme events as well as gradual changes in mean annual net resources.
The paper analyses these predictions. It also attempts to place them in context, first,
comparing their impact with other major barriers to sustainability, such as increasing
demand, wastage, poor water resources assessment and international conflict, and,
secondly, considering the limitations of current predictive techniques.
Modification climatique et durabilité des ressources en eau: dans la
perspective d'une menace de réchauffement planétaire
Résumé Les modifications climatiques devant se produire au cours des prochaines
décennies devraient aggraver le stress hydrique dans de nombreuses régions du
monde, y compris dans certaines d'entre elles actuellement bien pourvues en eau. On
doit s'attendre à des modifications concernant aussi bien la fréquence des événements
extrêmes que les modules annuels. Cet article s'intéresse à ces prédictions et s'efforce
de les remettre en perspective, en comparant leurs impacts à ceux d'autres obstacles à
la durabilité comme la croissance de la demande, les rejets, les incertitudes pesant sur
l'évaluation des ressources et les conflits internationaux, mais aussi en prenant en
considération les limites actuelles capacités de prédiction.
INTRODUCTION
The International Commission on Water Resources Systems of IAHS has convened a
series of symposia and published a Special Issue of this journal on the sustainable
management of water resources, which have sought to define aims and measures for
future sustainability (Simonovic et ai, 1995; Rosbjerg et ai, 1997; Simonovic, 1997).
They have inevitably also identified some of the barriers to sustainability. Falkenmark
(1997) noted two such threats: increasing food needs and increasing pollution. This
paper is concerned with a third potential threat, climate change, and to adjudge its
relative significance.
Many recent publications have attempted to predict the impact of an enhanced
greenhouse effect upon regional water resources in the twenty-first century (e.g. Arnell
& Reynard, 1993; Arnell, 1996; IPCC,1996; Jones et al, 1996). A recent release from
the UK Hadley Centre (1997) focuses on the global impact of the climatic predictions
from the HadCM2 high resolution coupled atmosphere-ocean General Circulation
Model (GCM) transient run on vegetation, agriculture and water resources. In this,
Arnell & King (1997) compare the impact on water resources, modelled by inputting
Open for discussion until I February 2000
542
J. A. A. Jones
the climatic parameters into a daily runoff simulation model, with the calculated
increase in demand, based on recent estimates of national population growth rates
(WMO, 1997).
Three general observations on the results of this recent work seem pertinent. First,
GCMs are far less consistent and reliable in their predictions of regional patterns of
precipitation and the factors controlling évapotranspiration than they are with
temperatures. It is notable that the global patterns of increased or decreased
precipitation produced by the recent generation of high resolution transient coupled
atmosphere-ocean General Circulation Models developed by the UK Hadley Centre,
e.g. the transient run of the original high resolution model (UKTR) and HadCM2, are
very different from those of the first generation of low resolution, equilibrium GCMs
in which the ocean was represented by a static surface "slab" (Jones, 1996a). The
problem for this author is that many of the predictions of the simpler models seem to
be generally more climatologically believable than those coming from the newer
scientifically superior models. To take two examples, enhanced monsoonal rainfall in
Southeast Asia and India might be expected from higher sea surface temperatures
(SSTs), higher land surface temperatures in the continental interior and a poleward
shift in the Ferrel Jetstream in summer. This often appears in the earlier models, but not
in the latest GCMs. Similarly, early American GCMs tended to predict marked
reductions in annual rainfall and increased summer drought in the Canadian prairies
and Great Plains. This caused major political concern that a reduction in food aid from
North America could destabilize many African countries (e.g. Tickell, 1991; Jones,
1997). Yet the problem seems to have been substantially reduced or has even
disappeared in the latest GCM output (cf. Hadley Centre, 1995; Jones, 1996a; Arnell &
King, 1997).
Secondly, whilst analyses of recent instrumental records suggest trends that are
consistent with some predicted by the GCMs, there are also many cases of opposite
trends. Jones (1996b) collated evidence from recent instrumental analyses in Europe
that was broadly in line with the tendencies indicated by most GCMs, in particular, a
drying trend in southern Europe, increased water resources in northern Europe and
long-term reduction in snowfall, snow and ice cover, and snowmelt floods. However,
most of the trends observed in the recent worldwide collection of historical analyses in
Jones et al. (1996) are at variance with the HadCM2 climate change scenario for 2080
(Table 1). Three conclusions are possible: (a) the recent hydrometric data do not yet
Table 1 Comparison of HadCM2 derived runoff with recent observed trends.
Recent observations
drying in north and west,
lower discharges,
declining glaciers and lakes
USA
increase in southeast, decrease in northeast
Japan
decrease in discharges
Australia
increase discharge in east, if at all
Europe
decreased discharge, if at all,
including Scandinavia
Argentina
increased precipitation in north
Source of observed trends: Jones et al (1996).
China
HadCM2 2080
+10-50% runoff
decrease in south, increase in north
+10% discharge
decreased discharge everywhere
-10-25% discharge in south,
increases in Scandinavia and UK
decreased precipitation in north
Climate change and sustainable water resources
543
indicate a significant global warming signal; (b) the model scenario is misleading; and
(c) the progress of greenhouse gas driven climate change may not be unidirectional in
specific regions. The latter possibility is climatologically feasible in many regions, for
example, if higher SSTs result in greater oceanic evaporation and land downwind
receives more precipitation before a general shift in storm trajectories, e.g. mid-latitude
depressions, carries that precipitation to a different latitude.
DEMAND AND WASTAGE AS BARRIERS TO SUSTAINABILITY
The threat of climate change in the twenty-first century aggravates, and is also
aggravated by, projected trends in population growth and demand. The single most
fundamental barrier to sustainable use of water resources is population growth. UN
(1991) estimates suggest a "most likely" rise of about 50% in world population by 2050.
Unfortunately, most of this expansion will be in the lesser developed countries,
which as a group are also more prone to water shortage. The WMO's (1997)
assessment of global freshwater resources calculated that, whereas a third of the
world's population currently lives in countries suffering moderate to high water stress,
by 2025 this will have risen to two thirds of a much larger population.
Key sources of demand
The trend to urbanization further aggravates the problem by increasing per capita
demand. Between 1990 and 2025, the percentage of the world population living in
urban areas could rise by 50%, accounting for nearly two thirds of the total world
population in 2025. Whereas most rural populations consume no more than 100200 1 per capita per day, and within the last decade nearly two thirds of the world
population survived on less than 50 1, cities typically consume 300-600 1.
Centralized sewage systems, industrial processing and the accoutrements of modern
urban living are largely to blame. Perhaps fortunately for water demand, but sadly
not for human health, much of the new urbanization in the developing world is
unplanned and often unsupported by central supply systems and sewerage. Nevertheless, it is estimated that by 2025, 79% of the world urban community could be in
the least developed countries (LDCs), compounding the stress on the least resilient
economies.
A very crude but conservative estimate of the "multiplier effect" of urbanization
suggests that by 2025 the world will have over two billion more city dwellers than in
the early 1990s, almost double the 1990 urban community, each consuming over twice
the current rural average.
To this must be added the demand for irrigation, which is rising in direct response
to the need to feed the growing population. The peak wave of expansion may have
passed, but irrigation remains the largest consumer of water worldwide, consumption
has increased by nearly 20% in the last two decades, and a further 20% expansion in
the area of irrigated agriculture is still regarded as environmentally feasible (Jones,
1997, p. 14).
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J. A. A. Jones
Inefficiency and wastage
To the effects of expanding urbanization and irrigation must be added the problems of
inefficiency and wastage in all spheres of consumption. Here the problems are very
much a question of water quality as well as quantity. Lvovich's (1979) doomsday
scenario of pollution essentially destroying global water resources by 2000 may have
been averted, but the legacy of the decades of neglect that influenced Lvovich's
viewpoint was clearly apparent in much of former communist eastern Europe in the
early 1990s. Current fears focus on urban wastewater pollution in Southeast Asia,
where rapid urbanization is likely to triple the amount of urban wastewater during the
1990s, and where increasing amounts receive little or no treatment. The current
financial crises in the "tiger economies" will only exacerbate urban squalor and delay
the provision of treatment plants.
As tragically illustrated during 1998 in Bangladesh, China and Central America,
floods combined with poor urban sanitation can be the source of major threats to
human health from water-borne diseases. Similar health problems followed the River
Oder floods in Poland in 1997, although petroleum products and industrial chemicals
were possibly more important ingredients than human sewage in those floodwaters.
There is a potentially important interaction here with climate change, if flood
frequencies increase.
Pollution will remain an important barrier to global sustainability for the
foreseeable future. Even in the most advanced countries, where point sources of
pollution, like sewage and industrial outfalls, are now largely under control, nonpoint
sources such as agriculture, urban drainage and landfills are not. Nearly two thirds of
all pollutants entering US rivers come from nonpoint sources, of which agriculture
accounts for 60%.
Again, water efficiency is improving rapidly in the most advanced economies, but
there is little evidence of this in the developing world, where per capita wastage
remains high and rising in most of the main areas of increasing population (Jones,
1997, p. 15ff). In part, this is due to the nature of their industries and the affordable
level of technology, including irrigated agriculture. In part, it is also due to the level of
public education and the dissemination of information which agencies like WMO and
UNESCO are still fighting to improve.
To take just one example from the developing and two from the developed world:
modern methods of drip or sprinkler irrigation can reduce evaporative losses by up to
75%. In the UK, leakage rates of 30-40% from public water supply systems are rapidly
being reduced following the National Leakage Control Initiative of 1994, but the best
values are still 10-20% and therefore comparable to the probable average effect of
climate change next century. Experiments in England have also indicated that domestic
metering may save up to 20% of public water supply (Jones, 1997, p. 322).
Finally, an important point must be made on the efficiency of management
systems. Wastage, inefficiency and even damage to the fundamental resource have
occurred all too frequently, especially in developing countries, as a result of piecemeal
developments and fragmented institutional responsibilities (Hufschmidt & Tejwani,
1993). At the grandest scale, the era of rapid dam-building, especially in the 1960s and
1970s, resulted in numerous cases where lack of understanding of environmental
Climate change and sustainable water resources
545
impacts or of the socio-political infrastructure has either impaired the resources or
reduced the estimated benefits (Takeuchi, 1997; Jones, 1997). In many instances, the
repayments on loans taken out to fund such schemes have added unnecessary financial
stress to developing countries. In the worst instances, they have led to probably
irrecoverable damage to the environment and human health, as witnessed in the
irrigation schemes of the Aral Sea basin (Glazovsky, 1990).
The UN is actively promoting "integrated water resource management" (IWRM) to
link all resources within a drainage basin under one overarching management structure.
Bonell et al. (1993) note the importance of this approach for management in the humid
tropics. The vertical reorganization of water industries to combine all stages of water
use and reclamation began in the West in the 1970s as recommended by the UN (1970),
but the concepts are now being extended to cover ever broader aspects of environmental management on the land as well as within the water cycle. This trend recognizes
the intimate linkage between land management and runoff (Kundzewicz, 1997).
Among the biggest obstacles to adoption of basinwide IWRM are international
boundaries. Whilst international cooperation is reasonably well established in North
America and western Europe, the greatest number of international basins occur in the
developing world where collaboration has been sadly lacking (McCaffrey, 1992). Over
60% of Africa and Asia is covered by shared basins. The internationalization of the
Aral Sea basin since the demise of the USSR is already an obstacle to environmental
rehabilitation.
Recent but qualified successes may be found in the international accords on the
Mekong (1994), in southern Africa (1995) and on the Ganges (1996), although the
Mekong agreement excludes the important headwater nation, China. In contrast, the
Middle East and North Africa is essentially an agreement-free zone. The Middle East is
also a region under considerable stress, where the "demographic explosion" is likely to
add 34 million to the population by the 2020s and push demand well above the
conventionally available resources (Anderson, 1991). It is also a region where shared
aquifers and "groundwater mining" can be as important as shared rivers, where there is
severe inter-community enmity, and where the prospect of "water wars" is still real
(Jones, 1997, p. 330ff).
CLIMATE CHANGE AS A BARRIER TO SUSTAINABILITY
Effective water management requires a knowledge of the likelihood of extreme events
as well as averages. Current GCMs are beginning to give usable future scenarios for
averages, but the estimation of what might happen to patterns of extreme events over
the coming decades is still very much a research frontier.
Results from the HadCM2 GCM
The second high resolution coupled atmosphere-ocean GCM developed by the UK
Hadley Centre showed a significant improvement in the hindcasting experiment over
its predecessor (Johns et al, 1997) and has formed the basis for a wide-ranging study
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J. A. A. Jones
of global impacts (Meteorological Office, 1997). Amell & King's (1997) estimation of
the impact of greenhouse gas warming (the HadCM2GHG scenario) on global runoff
by 2080 (Fig. 1 ) suggests increases in equatorial regions, peaking at over 50% in parts
of East Africa and eastern Brazil, much of Southeast Asia, the USA (except the Great
Plains) and the south-central Asian republics of the CIS (Commonwealth of Independent States), as well as generally north of 60°N. The main areas with reduced surface
water resources occur in the sub-tropical and Mediterranean climates, especially
between the North Atlantic and India and from Argentina, across southern Africa to
Australia. In general, the reductions are under 25%, but in India and the Southern
Hemisphere there are large areas with deductions over 25% and even over 50%.
The critical feature of these calculations is that most of the increase will occur in
regions that are not currently stressed and most of the decreases will occur in those that
are already stressed under the present climate. The main beneficiaries, countries
currently using more than 20% of their potential resources and therefore considered
"under stress", that will experience fewer problems by the 2020s, include the USA, the
broad belt from the Caspian to China and Mongolia, the Sudan, Somalia and Peru. The
main losers include Mexico, the belt from Mauritania through Egypt, Turkey and the
Middle East to India and Bangladesh, South Africa and western continental Europe
from Spain to Poland and Italy.
Table 2 shows the combined effects of population increase and climate change
over the coming century, assuming that per capita demand remains static. Although
Table 2 Predicted trends in global precipitation, runoff and per capita water resources based on the
output from HadCM2GHG, with some additions from HadCM2SUL.
2020s
Present
2050s
+3.0%
+4.5%
Precipitation*(HadCM2GHG)
Precipitation* (HadCM2SUL)
+2.0%
+2.5%
Runoff (HadCM2)+
+2.9%
+4.0%
Additional runoff (km3) (HadCM2GHG)
1126
1553
Population increase
9759
Population in millions*
5266
8121
+54%
+85%
% increase
Per capita resources
-35%
-46%
Due to population increase only
-44%
Due to population increase + climate
-34%
change
Population in stress
Billions using >20% of available water
1.9
5.1
5.9
resources (AWR)
+ 168%
+211%
% increase
66
extra millions due to climate change*
3.1
Billions using >40% of available water
0.454
2.4
resources (AWR)
% increase
+429%
+583%
1]7!t)170«)
extra millions due to climate change
26
* Data from Hadley Centre and Climatic Research Unit, University of East Anglia, LINK
' Data from graphs and text in Arnell & King (1997).
* UN ( 1991 ) "most likely" estimates.
5
Arnell (1998).
2080s
+6.25%
+4.2%
+6.5%
2524
10 672
+ 103%
-51%
^8%
6.5
+242%
3.6
+693%
98
Programme.
Climate change and sustainable water resources
547
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J. A. A. Jones
population growth is seen to be the overwhelming source of stress, climate change
alone could bring at least 100 million extra people within the realm of "extreme water
stress", without allowing for any increase in demand due to climate change. Estimates
for the 2050s vary from 117 million (graphs in Arnell & King, 1997) to 170 million
(Arnell, 1998), but perhaps with a slight reduction by 2080.
Results from HadCM2SUL
The rerun of HadCM2 with sulphate aerosols produced significantly cooler
temperatures than the "greenhouse gas only" run over large areas downwind of the
main industrial sources of sulphates in North America, Europe, Russia and Japan
(Mitchell et al., 1995; Hadley Centre, 1995). It also reduced the global increase in
precipitation by 2080 by one third (Table 2).
There is still considerable debate over whether sulphates will be such an important
factor by 2080. The issue centres on the probable reduction in sulphate emissions from
fossil fuel burning and on the relatively short residence time of the aerosols in the
atmosphere. Increased rainout and washout in a higher rainfall environment in the
future could complicate the already difficult task of transforming emissions into actual
atmospheric concentrations. It remains, nevertheless, a scenario worth examining,
especially given the predicted high level of increases in sulphate emissions in
Southeast Asia over the next half century.
The broad pattern of change in HadCM2SUL is similar to HadCM2GHG, but there
is a markedly lower increase in global precipitation and runoff by 2080, and a commensurate reduction in the degree of change in regional water resources (Fig. 2;
Table 2). The area of negative trend in surface water resources is reduced, but many of
the same areas of reduction can be identified, e.g. the Middle East to Bangladesh, the
Ukraine, Argentina, south-central Africa, east-central Australia and parts of the Sahel.
The only area of significant increase outside the Greenland icecap lies in east-central
China.
There is also some evidence for "seesaw" trends in precipitation patterns, for
example, a drier Britain and wetter Spain in 2020, reversing by 2080 (Fig. 3).
A few notes of caution are needed here. First, the Hadley Centre emphasize that
these estimates are based on a single experiment and cannot therefore incorporate the
range of uncertainties. Hence, they are not "predictions" so much as possible
realizations of how the climate system may respond to a given forcing. They do,
nevertheless, represent the results from one of the few transient experiments with
coupled atmosphere-ocean models, with 19 atmospheric and 20 oceanic levels.
Second, the runoff in Fig. 2 is derived via the "on-line" runoff calculation within
HadCM2, whereas that used in Fig. 1 was derived "off-line" by inputting HadCM2
hydrometeorology into a separate water balance model. Thirdly, there is some concern
about the model itself and the emissions scenario used. Although the sulphate model
produced a hindcast over the last 100 years that fitted the observed temperature trend
better than with greenhouse gases only (Hadley Centre, 1995), the scenario used
overestimates present-day sulphate emissions and may well overestimate future
emissions (Conway, 1998). The Hadley Centre has recently been running HadCM3
Climate change and sustainable water resources
549
550
J. A. A. Jones
Annual precipitation change (mm/day)
%
t//
',/,%
4*~
.,,
)
M///
•f,
Vf //,.
20*0 2069
, . *7-sr.
/ x
y „,
% ^
<,A<
^W^yWé$^%^^
2070 2099
Fig. 3 C hanges in mean annual daily precipitation for the years around 2020, 2030 and
2080 (mmday~') according to the HadCM2SUL transient simulation. Redrawn with
permission from a map provided by the LINK programme.
Climate change and sustainable water resources
551
which assumes only half the sulphate emissions of the previous model and these results
are eagerly awaited. Conversely, HadCM2SUL did not consider the indirect effects of
sulphate aerosols on clouds and rainfall, only the radiative effects, and, in this respect,
the net effects are likely to be underestimated from the hydrological viewpoint.
Effects on snowmelt and extreme events
Of all the hydrological extreme events, the trend in snowmelt floods is likely to be
most readily and accurately predicted. This will centre on the seasonal altitudinal and
latitudinal shift in the "freezing line". The result is likely to be generally decreased
snowfall over most regions, combined with more rapid melting at lower altitudes and
latitudes, reducing snow cover durations. Flood risk is likely to increase on rivers like
the Rhine issuing from the Alps due to more rapid melting, despite some reduction in
snowfall and snow cover in France and Switzerland (Kwadijk, 1993). Higher temperatures could also maintain spring flood levels on the Danube despite less snow cover,
but the floods will occur slightly earlier (Gauzer, 1993).
Hence, earlier spring melt events, more rain-on-snow events and more mid-winter
melt events are to be expected. Thinner ice covers on rivers and lakes could also
reduce the frequency of ice jam floods.
Despite a general rise in snowlines, however, not all basins will respond in the
same way. Collins (1989) found that basins with high levels of glacierization are more
sensitive to temperature changes, whereas those with less glacier ice respond more to
changes in the total snowfall receipts. Some increase in snowfall at higher altitudes is
possible given increased oceanic evaporation, increased air temperatures and vapour
pressures and increased instability, and some larger ice masses could grow (e.g. Woo,
1996). In other cases, meltwater floods could reduce after an initial increase, as ice
masses contract (e.g. Quintela et al, 1996).
Although actual snowfall amounts are likely to be lower, the dominance of the
Bergeron precipitation-forming process is likely to be maintained in mid to high
latitudes as convective instability and higher vapour pressures will probably outweigh
the rise in the altitude of the zero isotherm (Jones, 1996b). This could raise rainfall
intensities and increase local summer floods.
This leads to a consideration of storminess and rain-generated floods. There is a
general perception that storm frequency and intensity will increase globally as a result
of increases in air temperatures, SSTs, vapour pressure and relative humidity,
convection and land-sea contrasts. In northern Europe this could be intensified during
the earlier phases of global warming by a temporary increase in the poleward thermal
gradient in high mid-latitudes, whilst higher SSTs coexist with Arctic sea ice in the
North Atlantic. But this phase should pass as the ice retreats and the equilibrium
scenario should see a net reduction in the poleward thermal gradient worldwide.
The limited number of studies that exist in this field, tend to support these
predictions. They remain largely statistical estimates rather than physically-based
predictions (cf. Wigley, 1989; Gregory et al, 1990; Beven, 1993; Holt & Jones,
1996a), although Gellens (1991) found an increase in winter floods and summer
droughts in Belgium using a conceptual hydrological model.
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J. A. A. Jones
Drought and low flow frequencies are of major concern to water managers and
there are widespread fears that these may increase. Many regions are predicted to
experience a reduction in summer precipitation combined with increased évapotranspiration losses. Holt & Jones (1996b) found that the scenarios from the UKTR
transient run year 75 would cause regular autumn droughts in Wales, with flows
perhaps reduced by 25%. Pilling et al. (1998) calculated that the HadCM2GHG
scenario for 2080 is likely to reduce summer rainfall by 14% in central Wales, and
Pilling & Jones (1999) predict that large areas of southeast England could see late
summer flows reduced by 50% or more. In Greece, Mimikou & Baltas (1997)
concluded that lower reservoir inflows under the UKTR scenario would require
increases of 25-50% in reservoir storage volumes to maintain the current level of risk,
and 12-38% under the Hadley Centre high resolution equilibrium scenario, UKHI.
Lower and earlier meltwater discharges are likely to increase summer drought
problems in regions like the Canadian prairies and central Europe, although the exact
response may change over time and will be different for nival runoff regimes than for
pluvial and mixed regimes (cf. Gauzer, 1993; Kwadijk, 1993; Woo, 1996).
UNCERTAINTIES AND RESEARCH PRIORITIES FOR IMPROVING
PREDICTIONS
Major advances have been made in the last few years in the physical basis and spatial
resolution of GCMs and we are beginning to emerge from the era of "sensitivity
studies" and approach the stage of true prediction. Crucial improvements have been
made in introducing and modelling oceanic circulation and atmosphere-ocean
interactions, in cloud physics, and in vertical and horizontal resolution. The resolution
of the Alpine Low in the high resolution European window in UKTR was an important
advance (Hadley Centre, 1995). Valuable progress is also being made outside GCMs in
upscaling and downscaling procedures to link GCMs with hydrological and biological
models, especially through the IGBP's BAHC project (Biospheric Aspects of the
Hydrological Cycle) (Bass et al, 1996). Future experiments introducing variations in
solar activity and volcanic eruptions planned for the new HadCM3 model will further
improve simulations of the atmospheric system.
Nevertheless, there are still several difficult and critical scientific problems
currently occupying the modellers that the "user" hydrologist needs to appreciate.
First, both temperature and precipitation are very sensitive to cloud processes,
especially upon the altitude, thickness, updraft intensity, temperature and density of ice
crystals. Experiments at the Hadley Centre have shown reductions of up to 2°C in
global warming if ice crystals are allowed to fall out of clouds preferentially
(Rowntree, 1990; Mitchell, 1991). However, increased convective activity and vapour
pressure in a warmer climate should increase the vertical development of clouds and
probably boost the Bergeron process and increase rainfall intensities. According to the
Clausius-Clapeyron theory, vapour pressure should increase by 6% per °C of warming.
It is thought that just under half of this is likely to be converted into precipitation. This
percentage seems to be confirmed by the average results from HadCM2. However,
Probable Maximum Precipitation formulae suggest that extreme rainfall events use
Climate change and sustainable water resources
553
vapour content more efficiently, i.e. a higher percentage of vapour is transformed into
rainfall, leading to higher rainfall intensities.
Secondly, both precipitation and temperatures on land, especially in maritime
regions, are sensitive to SSTs and ocean currents. Doubts have been expressed about
the future stability of the ocean thermohaline circulation or "conveyor belt", especially
in the North Atlantic, given an expected increase in the discharge of fresh meltwater
into the Arctic Ocean. This could stifle the production of dense "deep water" that
drives the circulation and so cause cooling in western Europe. The latest AOGCMs are
now capable of testing this hypothesis. Interestingly, experiments with the new Hadley
Centre model suggest that any associated southward shift in the Gulf Stream and
slowdown in the rate of circulation will still cool western Europe less than the amount
of direct warming from greenhouse gases (Meteorological Office, 1998). Further
experiments of this nature have become realizable with the increased oceanic
resolution (1.25° x 1.25°) in the new HadCM3 model.
Thirdly, there is still a tremendous amount to be learnt about interactions with the
biosphere. This includes the oceans. It has been known for some years that the role of
the oceans in the carbon balance is not simply one of chemical solution and release.
Phytoplankton are an important carbon sink. More recently, phytoplankton have
emerged as an important source of dimethylsulphide (DMS) which can form into
atmospheric aerosols that act as condensation nuclei and encourage cloud formation,
just like the sulphate aerosols derived from SO2 emissions. More DMS is likely to be
produced in a warmer world, which would mean more condensation nuclei competing
for moisture and smaller droplets. Since the reflectivity of clouds is inversely proportional to the mean droplet radius, these changes could have a marked effect on the heat
balance (Jones, 1993). Unfortunately, there is still a lot to be learnt about how phytoplankton will react to global warming and how much their genetic material might be
damaged as a result of the ozone hole.
On land, estimating evapotranspirational losses is still difficult. Far more
variables are involved than with precipitation: indeed, the complexity increases from
temperature through precipitation to évapotranspiration. Radiation, temperature, soil
moisture content, wind, relative humidity and atmospheric CO2 all affect the physiological responses of plants. Carbon dioxide-fertilization is likely to increase biomass
and absorb C0 2 , but also increase interception and evapotranspirational losses. The
type of plant carbon fixation biochemistry also affects response: C3 plants like wheat
will tend to grow more than C4 plants like maize and rice, but their response is
particularly sensitive to soil moisture content. Indeed, soil moisture itself is still a
difficult parameter to estimate, yet it could have important climatic feedback
especially on rainfall. Since all trees have C3 metabolisms, their accelerated growth
is likely to increase interception and canopy evaporation losses. This alone could
substantially reduce runoff, as indicated by the results of the UK Institute of
Hydrology paired catchment experiment in mid-Wales, where yields from the
forested basin are 15-20% lower mainly because of interception losses (e.g. Hudson
& Oilman, 1993).
There is also evidence that temperature and CO2 concentrations may act in
opposite directions upon stomatal resistance. Increased temperature should reduce
stomatal resistance and increase transpiration, but CO2 may stimulate closure or even
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J. A. A. Jones
cause a long-term reduction in the number of stomata (Woodward, 1987; Jones,
1996b). The balance could be difficult to resolve.
Finally, two observations on the actual output from recent GCMs: actual
evapotranspirational losses and soil moisture are sensitive to the frequency and
intensity of rainfall, but current GCM output is not giving this information. Also,
current GCMs are giving important information on the seasonal pattern of warming.
They are indicating that most warming will occur at night and in the winter in Europe,
in which case the effects upon actual evapotranspirational losses could be less than
expected from the average increase in temperature.
These complexities suggest some obvious research priorities, not least for hydrologists (Jones, 1996c). More information is needed on the magnitude/frequency distributions of events. Stochastic Weather Generators seem to offer a promising way forward
here (cf. BAHC, 1993; Bârdossy et al, 1993; Wilby et al, 1998; Pilling et al, 1998).
More needs to be done to link GCM output with hydrological simulation models at
all scales, and this will involve more work on physically-based models for large basins,
e.g. the Rhine and Mississippi (cf. Parmet & Mann, 1993). The interactive responses of
vegetation and soils to climate change need to be built into hydrological models, and
this must include the important question of including the effects of changes in land use
and agriculture, which may be partly in response to climate change.
CONCLUSIONS
There are likely to be reduced resources in the subtropics and mid-latitudes
equatorwards of 50°N or 40° S, and an increase in resources polewards of 55°N or
40°S. Seasonal shortages are likely even where there is a net annual increase, e.g. in the
UK. Most of the loss in resources will be due to reduced precipitation. Increases in
precipitation are likely to be mainly in rainfall and the percentage in snowfall will be
reduced. This means reduced volumes and durations for glaciers, snow cover and
freshwater ice.
Extreme events are likely to occur more frequently in most climatic regions,
though meltwater and ice jam floods could eventually decrease.
Some areas show current trends that fit well with the predicted climate change
trends, but many do not. Few regions show widespread and consistent trends in
discharge (cf. Chiew & McMahon, 1996). However, some confusion may be introduced by different local sensitivities to climatic variability and cycles, e.g. El Nino, and
to human modification of catchments.
Even so, there is some GCM evidence that precipitation trends may not be as
unidirectional as temperature trends.
Placed in perspective, climate change is only one of a wide range of barriers to
sustainability, many of which are likely to be problematical long before there is real
pressure from global warming. Population growth, urbanization and profligate use of
resources are the main hindrances. Nevertheless, the effects of climate change are
likely to become first apparent in extreme events, which could hit "net winners" as
badly as, if not worse than, regions where average water stress will increase. In western
Europe, increased seasonality in precipitation, with wetter winters and drier summers,
Climate change and sustainable water resources
555
will provide a background for increased flooding and reservoir spillage in winter and
drought and low flow events in summer (Holt & Jones, 1996a). At the very least, this
will require substantial increases in the sizes of reservoirs to maintain public water
supply (Cole et al, 1991) and it may even be a critical factor in decisions to undertake
long-distance inter-basin transfer (NRA, 1994).
There are still substantial uncertainties in climate change prediction and the best
informed water management authorities are opting at most to keep a "watching brief
on the predictions, perhaps considering the possible future need to raise the height of
dams and dykes and designing new works accordingly, rather than taking immediate
constructive action. This is reasonable: forewarned is forearmed.
Meanwhile, there is considerable work to be done in curbing demand, increasing
public awareness, controlling leakage and protecting surface water and groundwater
from pollution. There is also an urgent need to improve national planning and water
resources assessment over most of the globe (WMO/UNESCO, 1991) and to maintain
the effectiveness of its foundation, hydrological observations, within a general context
of declining networks (Burn, 1997; Kundzewicz, 1997). Beyond this, there is a
widespread requirement for water agencies, and the governments that give them their
powers and their briefs, to appreciate more the intimate relationships between water
resources and the wider environment (cf. Falkenmark, 1997). This may be summarized
in two dicta:
(a) Water should be managed in such a way as to minimize the interference with
nature and to maximize the benefits for nature.
(b) The environment should be managed in such a way as to minimize adverse impacts
on and maximize benefits for water resources or flood hazard.
Without following these dicta, there is no hope of achieving true sustainability.
Acknowledgements The author wishes to thank Dr David Viner and Ruth Dogherty of
the University of East Anglia and The Hadley Centre for kindly providing the rainfall
and runoff maps from HadCM2SUL, under the Climate Link Programme, and Dr Nigel
Arnell of the University of Southampton for permission to reproduce the runoff map
from HadCM2.
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