Memorandum submitted by Professor Nigel
Arnell, Department of Geography, University of Southampton
THE POTENTIAL
IMPACTS OF
CLIMATE CHANGE
ON WATER
RESOURCES IN
THE DEVELOPING
WORLD
— Climate change poses a threat to
water resources and their management in developing countries.
An important component of this threat is the uncertainty over
the magnitude of climate change impacts at a location.
— The effects of climate change on
river flows and recharge in developing countries are largely dependent
on changes in rainfall amount and timing.
— Changes in the variability of rainfall
from year to year—and changes in patterns such as El Niño—may
have as important an effect on water resources as changes in mean
climate.
— Under most climate change scenarios,
southern and western Africa and the Middle East are projected
to see reduced river flows and recharge in the future. Possible
changes in south Asia are less clear: some scenarios suggest reduced
runoff (greater drought risk) whilst others show increased runoff
(greater flood risk).
— Climate change is likely to have
a substantial effect on people living in areas that are currently
water-stressed, most of which are in developing countries.
— Different aspects of water resources
will be affected by climate change in different ways. Whilst access
to safe water will probably be influenced by climate change, especially
in rural areas, access to sanitation is likely to be relatively
unaffected.
— The actual impacts of climate change
will depend not only on the change in climate, but also on changes
in how water resources are managed. The adoption of flexible management
approaches and policies which can cope with current climatic variability
(such as increased access to safe water) will reduce vulnerability
to climate change.
— Water resources are also influenced
by many other pressures, such as population growth, economic development
and land use change, which will affect exposure to climate change.
— One practical implication is that
all water resources development proposals should consider the
effects of climate change on the ability of the proposed scheme
to deliver its objectives, and whether the proposed scheme will
affect the vulnerability of others to climate change.
1. INTRODUCTION:
CLIMATE CHANGE
AND WATER
RESOURCES
1.1 It is a cliché that human society
relies upon adequate water resources. Rivers and groundwater provide
drinking water, water for agriculture in general and irrigation
in particular, and water for industry and for cooling. River flows
generate hydropower and enable navigation, and provide a means
for the disposal of waste. Many economically-important ecosystems,
such as wetlands, are dependent on inflows of water from upstream.
Floods bring damage and disruption and threaten lives, and rivers
and water bodies can harbour water-borne disease. The sustainable
development of societies in developing countries relies on the
sustainable management of water resources in general.
1.2 However, in the mid 1990s approximately
a third of the world's population lived in countries deemed by
the United Nations to be water stressed, 20 per cent lacked access
to safe drinking water and about 50 per cent lacked adequate sanitation[6].
Pressures on water resources over the next few decades will be
strongly influenced by economic developments, attitudes towards
resource use and population growth—and climate change will
add to these pressures. In some areas climate change may improve
the water resources position, but in others it is likely to make
matters considerably worse.
1.3 This memorandum summarises the potential
impacts of climate change on water resources in developing countries.
The information is largely taken from the Third Assessment Report
of the Intergovernmental Panel on Climate Change[7]
and climate change impacts research funded by the UK Department
of the Environment, Food and Rural Affairs (DEFRA)[8].
2. EFFECTS OF
CLIMATE CHANGE
ON RIVERS
AND GROUNDWATER
2.1 Climate change has three potential effects
on river flows and groundwater recharge. First, it may affect
the volume of flow or recharge at a particular time of year. Second,
it may affect the timing of flows or recharge, either through
the year or from year to year. Third, it may affect the quality
of river flows or groundwater through changes in the volume and
timing of water, and through changes in water temperature.
2.2 Although climate change will generally
result in increased evaporation, largely due to increased temperature,
changes in the volume and timing of river flow and recharge are
primarily dependent on changes in the amount and timing of rainfall.
A map of changes in river flows due to climate change therefore
closely follows the map of changes in rainfall (see attached figure),
although the percentage changes are generally greater and if the
increase in rainfall over a catchment is less than the increase
in evaporation, river flows decrease despite the increase in rainfall.
The actual effect of a change in climate on river flows and recharge
in a catchment depends on the current climate of the catchment
and its geology. For example:
— a given percentage change in rainfall
produces a greater percentage change in river flow or recharge,
the more arid the climate: river flows and recharge in dry areas
are more sensitive to climate change than river flows in wetter
areas;
— changes in river flows in a catchment
with large amounts of storage—in lakes, wetlands or groundwater,
for example—will be determined by changes in the cumulative
rainfall over a season, whilst changes in river flows in more
responsive catchments will be determined by changes in the intensity
of storm rainfall.
2.3 The timing of river flows through the
year in areas receiving snowfall—such as regions receiving
water from the Himalaya—reflects the melting of snow in
spring. In general, higher temperatures mean that more precipitation
during winter falls as rain rather than snow, and runs off quickly
to the river. Winter flows are therefore increased, and spring
snowmelt flows are reduced. Rivers feeding a small number of communities
in the developing world are fed from glaciers: higher temperatures
will result in increased glacier melt, and therefore higher flows
in the short term, but may result in the glacier disappearing.
2.4 The sensitivity of river flows and recharge
to changes in rainfall means that quantitative estimates of the
actual effects of climate change in a catchment are uncertain,
because climate model projections of changes in rainfall are uncertain.
At the regional scale, however, there are some consistencies.
Most scenarios result in a decrease in river flows in southern
Africa, much of west Africa, the Middle East and parts of South
America. Most scenarios result in an increase in river flows in
east Africa and south east Asia. There are conflicting projections
for south Asia and central Africa.
2.5 Changes in flood and drought frequency
are dependent not only on changes in mean climate, but also on
how variability in climate from year to year will change. This
is currently not well known, and the indications so far are that
changes in flood and drought flows are similar in direction—but
greater in relative magnitude—to changes in average flows.
It is possible that a particular location could see both an increase
in floods and an increase in droughts, if the seasonal pattern
of rainfall is significantly exaggerated. Changes in the pattern
of variability in climate from year to year—such as an altered
El Niño—are likely to have particular effects in
tropical and sub-tropical regions.
2.6 There have been very few detailed studies
of the potential effects of climate change on river flows and
groundwater recharge in dry or tropical regions generally, and
developing countries in particular. It is possible that changes
in climate may result in "surprising" changes in hydrological
processes and therefore river flows and groundwater recharge in
some catchments.
3. WHAT ARE
THE POTENTIAL
IMPACTS ON
WATER RESOURCES?
3.1 Changes in river flows and groundwater
recharge will affect the availability of water resources and the
water-related hazards of floods and ill-health, but so too will
population change, economic development and alterations in the
way in which water resources and hazards are managed.
3.2 A crude, but widely used, indicator
of water resources pressure is the ratio of water withdrawals
to the amount of available water: if a country is using more than
20 per cent of its available runoff and recharge, then it is likely
to be suffering water-related limitations on development. The
table below summarises the number of people living in countries
withdrawing more than 20 per cent of their resources in the absence
of climate change, together with the numbers of people who would
experience a reduction in water availability due to climate change
and hence an increase in water resources stress. Most of these
people live in south Asia, central Asia and southern Africa. The
table shows that resource pressures are likely to increase for
very large numbers of people, and also that the effects of climate
change are likely to be reduced substantially only by aiming for
a low CO2 stabilisation target.
Global population at risk of increased
water scarcity
|
| Population in countries using more than 20 per cent of their resources, with no climate change (millions)
| Population with an increase in stress
(millions)
|
| | Unmitigated emissions
| Stabilisation at 750ppmv
| Stabilisation at 550ppmv
|
|
| 1990 | 1,710
| | | |
| 2025 | 5,022
| 338-623 | 242
| 175 |
| 2050 | 5,914
| 2,209-3,195 | 2,108
| 1,705 |
| 2085 | 6,405
| 2,831-3,436 | 2,925
| 762 |
|
Source: Arnell et al. (2002) Climatic Change in
press
Note that other climate change scenarios could give different
quantitative results
3.3 Such global and regional figures provide only one
indication of the potential implications of climate change for
water resources. In practice, different aspects of water resources
in developing countries are likely to be differently affected
by climate change. The attached table summarises the implications
of climate change for water resources in developing countries:
there are unfortunately very few published quantitative case studies
which have looked at these impacts. Many of the users of water
resources will be more affected by changes in variability over
time and extreme conditions than by change in mean climate.
3.4 It is likely that the impacts of climate change on
water resources will be greatest on the poorest parts of the community,
who may be least able to recover from flood damage or obtain access
to scarcer water resources. There has, however, been no empirical
research on this issue.
3.5 Whilst climate change has the potential to adversely
affect water resources, it is likely that in some cases it may
lessen some water resources pressures. For example, an increased
frequency of flooding in desert wadis would increase flood risk,
but could also increase the recharge of local floodplain aquifers.
3.6 The actual impacts of climate change on water resources
in a particular location will depend not just on the change in
climate, but also on the procedures used to manage those water
resources. In general, unmanaged resources—such as many
rural water supplies—are more exposed to climate change
than managed resources, because these in principle have some means
of intervention. A corollary of this is that as management procedures
and infrastructure change, then vulnerability to climate change
alters. As a general rule, measures which are taken to lessen
exposure to current water-related problems will also reduce the
potential impacts of climate change.
3.7 An increasing scarcity of water in some regions has
the potential to increase conflict between different users of
water within an area and between upstream and downstream users.
The extent to which such conflict develops will depend on other
pressures within the region, although it is likely that resource
pressures may become more frequent.
3.8 The primary uncertainties in the potential impacts
of climate change on water resources in developing countries are:
— uncertainty in the potential change in climate,
and hence river flows and groundwater recharge;
— uncertainty in the future development of water
management policies and practices, which will affect exposure
to climate change; and
— uncertainty in the evolution of other pressures
on water resources, including population growth, economic development
and land cover change (which also affects river flows and recharge).
4. IMPLICATIONS
4.1 The main implications of climate change for water
resources are firstly that there is an additional pressure on
water resources in many developing countries, and secondly that
the conventional assumption, widely used in project planning and
assessment, that the past is a good guide to the future is no
longer tenable.
4.2 One practical implication is that all water resources
development proposals and strategies should explicitly consider
the effects of climate change on the ability of the proposed plan
to deliver its objectives, and whether the proposed scheme will
affect the vulnerability of others to climate change.
4.3 The effects of climate change on water resources
are uncertain, and it is this uncertainty, rather than climate
change per se, that poses the greatest challenge to scheme
design and assessment. This uncertainty suggests that flexible
schemes are to be prefered over fixed long-term schemes that are
difficult to upgrade to reflect altered circumstances.
Water resources in developing countries: impacts of
climate change
|
| Aspect of water resources | Description
| Implications of climate change |
|
| Municipal water supply | Generally derived either directly from large rivers or from reservoirs
| Source reliability depends on changing frequency or duration of low flows in the catchment supplying the source. Increased high flows could lead to increased sedimentation of reservoirs. Changes in the price of urban water due to increased scarcity will affect the poorest most significantly
|
| Rural water supply | Generally derived from local streams or groundwater
| Availability of source will depend on changes in local rainfall
|
| Irrigation supply | Large-scale from reservoirs, small scale from local streams or groundwater. Higher temperatures will increase demand
| As above, but coupled with possible effects of increased demands
|
| Access to safe water | see municipal/rural water supply
| As above Quality of available water largely determined by non-climatic factors around source
|
| Effluent disposal | Into small water courses or large rivers
| Dilution and assimilation capacity influenced by the amount of water in the receiving waters, as well as water temperature (higher temperatures generally lead to degradation of water quality)
|
| Access to sanitation | Related to effluent disposal
| Unlikely to be affected by climate change |
| Hydropower generation | Generally from large schemes supported by reservoirs or lakes
| Change in performance dependent on changes in the timing of inflows to reservoirs and lakes and the frequency and duration of droughts
|
| Riverine and lake navigation | Commercial activities focused on large rivers and lakes
| River navigation affected by frequency and duration of high and low flows; lake navigation influenced by lake levels which affect port facilities
|
| Water-borne disease | Largely, but not exclusively, associated with standing water bodies, which may be local
| Changes in local rainfall will influence occurrence of standing water bodies
|
| Flood management | Distinction between flooding on major rivers and localised flash flooding
| Flooding on major rivers affected by changes in long-duration rainfall upstream; flash flood frequency affected by changes in local storm rainfall frequency
|
| Environmental conservation | Wetlands are dependent on river flows and/or recharge, and provide valuable services
| Changes in wetlands dependent on changes in incoming river flows or local recharge
|
|
Professor Nigel Arnell, Department of Geography and Tyndall Centre for Climate Change Research,
University of Southampton
January 2002
|
6
UNEP (1999) Global Environment Outlook 2000. Earthscan:
London. Back
7
Arnell, N W & Liu, C (2001) Hydrology and water resources.
in: J McCarthy et al. (Editors), Climate Change 2001:
Impacts, Adaptations and Vulnerability. Contribution of Working
Group II to the Third Assessment Report of the Intergovernmental
Panel on Climate Change. Cambridge University Press, Cambridge.
pp 191-233. Back
8
Arnell, N W (1999) Climate change and global water resources.
Global Environmental Change 9, S31-S49. and Arnell, N W
et al. (2002) The consequences of CO2 stabilisation for
the impacts of climate change. Climatic Change. in press. Back
|
