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The Impacts and Costs of Climate Change
variable rainfall increase the risk of drought. The implications for water supply are an increase in
potential regions of water stress and water poverty. Above 2 to 2.50C global average temperature
increase it is projected that an additional 2.4 to 3.1 billion people will be at risk of water stress20. The
regional area subject to water stress under some scenarios is 10% of the Earth's land surface.21
Water quality is also sensitive to higher temperatures, lower river flows, saline intrusion with sea level
rise and increased storminess. Low flows and higher temperatures are likely to decrease the dissolved
oxygen in lakes and slow moving rivers, increasing stresses on fish. Low flows are already a problem
in southern Europe, and this could be exacerbated by climate change. The many local controls on
water quality have hindered a global assessment of potential climate change damages.
The impacts of projected climate change on water resources appear to be significant. The IPCC's
Third Assessment Report highlights that existing water stressed regions are likely to be more stressed
in the future as a consequence of climate change. Water stress is a key impact projected to affect large
numbers of people. The effects will be exacerbated by threshold behaviour caused by the interplay
between climate change effects, socio-economic trends and limits to adaptation capacity (Arnell 2000;
Jones 2000). For many regions under water stress, a global mean temperature increase above around
1.5C would lead to a decrease in water supply. The table below from Arnell et al. (2002),
summarizes the risks of water shortage with the associated increase in global mean temperature above
1861-1990 for three emission scenarios. The increase in water stress is presented for unmitigated
emissions, and stabilization at 550 and 750 ppm CO2 for the 2020s, 2050s and 2080s.
Table 7. Population with potential increase in water stress for three emission scenarios.
Year or aNo climate Unmitigated S750 S550 (Additional
period change (Millions) Emissions (Additional (Additional Millions
Millions) Millions)
2020s 5022 338-623 242 175
2050s 5914 2209-3195 2108 1705
2080s 6405 2831-3436 2925 762
Sources: Arnell et al. (2002)
aNumber of people in countries using more than 20% of their water resources. Increase in stress means a
reduction in resource availability by more than 10%.
One of the main messages from this is that after the 2020s the number at risk rises rapidly with
temperature and that reduction of the increase in temperature, at lower stabilization levels reduces the
risk substantially (Hare 200322).
There is a major increase in risk of water stress in the 2080s. The shape of the temperature response
curves in the 2050s is quite different from that in the 2080s. Risk rises rapidly with any temperature
increase in the 2050s, whilst in the 2080s, risk initially rises quite slowly. A loC increase in the 2050s
is associated with an impact almost ten times larger than in the 2080s, whereas the level of risk is
comparable in both periods for a 20C or higher warming. As temperature increases in the 2080s period
from around 1.00C above 1861-1990 to around 20C, the number at risk increases about five fold. One
of the major reasons for this is the increased water scarcity problem for major mega-cities in Asia
estimated for this time period (Hare 2003).
One of the major future risks identified in the Parry et al. (2001) and Arnell et al. (2002) work is that
of increased water demand from megacities in India and China in the 2080s. It is not clear whether or

20 Source: Parry et al, 2001
21 Alcamo, J. and Henrichs, T. (2002) Critical regions: A model-based estimate of world water regions sensitive to global changes. Aquatic
Science 64: 352-362.
2 Source: Hare, W. (2003) Assessment of knowledge on impacts of climate change contribution to the specification of art. 2
of the UNFCCC: Impacts on ecosystems, food production, water and socio-economic systems (see

AEA Technology Environment, August 2005


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Watkiss, Paul; Downing, Tom; Handley, Claire & Butterfield, Ruth. The Impacts and Costs of Climate Change, text, September 2005; Oxford, England. ( accessed September 23, 2017), University of North Texas Libraries, Digital Library,; .