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Recent years have witnessed an increase in global average air temperatures as well as ocean temperatures, as documented by the Intergovernmental Panel on Climate Change (IPCC). The rise in temperature is considered irrefutable evidence of climate change, and this has already started to have serious consequences for water resources and will have even more dire consequences in the future. Compounding these consequences are population growth, land-use changes and urbanization, increasing demands for water and energy, rising standards of living, changing dietary habits, changing agricultural practices, increasing industrial activities, increased pollution, and changing economic activities. All these will likely have adverse effects on water resources. This article briefly discusses climate change and its causes and impacts on water resources.
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11
Climate Change and Its Impact on Water Resources
Vijay P. Singh, Ashok K. Mishra, H. Chowdhary,
and C. Prakash Khedun
CONTENTS
INTRODUCTION
CLIMATE CHANGE
EVIDENCE OF CLIMATE CHANGE
IMPACTS OF CLIMATE CHANGE ON WATER RESOURCES
CONTINENTAL-SCALE IMPACT OF PROJECTED CLIMATE CHANGES ON WATER RESOURCES
ADAPTATION TO CLIMATE CHANGE
CONCLUSIONS
REFERENCES
Abstract Recent years have witnessed an increase in global average air temperatures as well
as ocean temperatures, as documented by the Intergovernmental Panel on Climate Change
(IPCC). The rise in temperature is considered irrefutable evidence of climate change, and this
has already started to have serious consequences for water resources and will have even more
dire consequences in the future. Compounding these consequences are population growth,
land-use changes and urbanization, increasing demands for water and energy, rising standards
of living, changing dietary habits, changing agricultural practices, increasing industrial
activities, increased pollution, and changing economic activities. All these will likely have
adverse effects on water resources. This article briefly discusses climate change and its causes
and impacts on water resources.
Key Words Climate change Extreme events Ecosystem Water quality Groundwater
Agriculture Transboundary water problems Adaptation to climate change.
From: Handbook of Environmental Engineering, Volume 15: Modern Water Resources Engineering
Edited by: L.K. Wang and C.T. Yang, DOI 10.1007/978-1-62703-595-8_11, ©Springer Science+Business Media New York 2014
525
1. INTRODUCTION
Water is vital for all forms of life and survival. Freshwater resources are limited, and
therefore their protection and management are of utmost importance. Sustainable manage-
ment of freshwater resources depends on an understanding of how climate, freshwater, and
biophysical and socioeconomic systems are interconnected at different spatial scales: at
watershed scales, at regional scales [1], and at a global scale [2]. Recently documented
activities contributing to climate change can be a major challenge to the availability of
freshwater quantity (too much or too less) or quality. These activities will play a critical
role in sectorial and regional vulnerability to water resource mismanagement. Examples of
vulnerabilities include multiyear drought in the USA and southern Canada, flood disasters in
Bangladesh, ecosystem damage due to reduced stream flow in the Murray-Darling basin in
Australia, and reduced water supply to reservoirs in northeastern Brazil. The use of water has
increased manifold over recent decades due to the increase in population, industrialization,
economic growth, energy production, changes in life style, and irrigation demand as global
irrigated land has increased approximately from 140 million ha in 1961–1963 to 270 million
ha in 1997–1999 [3]. On a global scale, basins are generally called water-stressed if they have
a per-capita water availability below 1,000 m
3
/year (based on long-term average runoff), and
such water-stressed basins are located in Northern Africa, the Mediterranean region, the
Middle East, the Near East, southern Asia, Northern China, Australia, the USA, Mexico,
northeastern Brazil, and the west coast of South America [4,5], as shown in Fig. 11.1.
Fig. 11.1. IPCC’s Fourth Assessment Report shows the range of vulnerabilities that may be affected
by future climate change, superimposed on a map of water stress (Source: IPCC [5], Fig. 3.2).
526 V.P. Singh et al.
Therefore, the relationship between climate change and freshwater resources is of fundamen-
tal concern for the well-being of society.
Climate change leads to changes in the hydrologic cycle since different components of
the climatic system, including the atmosphere, hydrosphere, cryosphere, land surface, and
biosphere, are involved. Therefore, climate change affects water resources both directly and
indirectly. The objective of this chapter is to highlight the impact of climate change on water
resources. The chapter is organized as follows. Following a brief introduction to climate
change in Sect. 2, Sect. 3presents an overview of the evidence for climate change, followed
by a discussion on the impact of climate change on different water resources sectors in
Sect. 4.Section5reviews continental-scale impacts of projected climate change on water
resources, and Sect. 6discusses adaptation to climate change. The article is concluded
in Sect. 7.
2. CLIMATE CHANGE
2.1. What Is Climate Change?
Natural ecosystems are generally driven by climatic patterns of a region that can be
quantified by understanding the patterns in hydrometeorological variables, such as tempera-
ture, precipitation, humidity, and wind. Climate change is defined by the Intergovernmental
Panel on Climate Change (IPCC) as changes in the state of the climate that can be identified
by changes in its properties and that persist for an extended period, typically decades or
longer, due to natural internal processes or external forcing or to persistent anthropogenic
changes in the composition of the atmosphere or in land use. Another definition, this one by
the United Nations Framework Convention on Climate Change (UNFCCC), is as follows: “a
change of climate which is attributed directly or indirectly to human activity that alters the
composition of the global atmosphere and which is in addition to natural climate variability
observed over comparable time periods.” The American Meteorological Society glossary
(AMS Glossary) defines climate change as “any systematic change in the long-term statistics
of climate elements (such as temperature, pressure, or winds) sustained over several decades
or longer. Climate change may be due to natural external forcings, such as changes in solar
emissions or slow changes in the Earth’s orbital elements; natural internal processes of the
climate system; or anthropogenic forcing.”
2.2. Causes of Climate Change
The causes of climate change can be regarded as a complex interaction between Earth,
atmosphere, ocean, and land systems; so the changes in any of these systems can be
both natural and anthropogenic, based on changes in atmospheric concentrations of
greenhouse gases (GHG), aerosol levels, land use and land cover, and solar radiation
affecting the absorption, scattering, and emission of radiation within the atmosphere and
at the Earth’s surface. Some of the important factors responsible for climate change are
discussed in what follows.
Climate Change and Its Impact on Water Resources 527
2.2.1. Greenhouse Gases
One of the main causes of climate change are changes in Earth’s atmosphere due to
changes in the amounts of greenhouse gases, aerosols, and cloudiness. The anthropogenic
increase in greenhouse gas emissions, not natural variability, is responsible for most of the
warming in recent decades [6]. Since the start of the industrial era (ca. 1750), the overall effect
of human activities on climate has been one of warming. The major greenhouse gases, for
example, carbon dioxide (CO
2
), methane (CH
4
), nitrous oxide (N
2
O), and the halocarbons,
are the result of human activities, and they accumulate in the atmosphere, and the concentra-
tion increases with time, as shown in Fig. 11.2 [7]. The major causes of the increase in CO
2
are
the increased use of fossil fuel use in transportation, building heating and cooling, and the
manufacture of cement and other goods. Human activities, such as agriculture, natural gas
distribution, and landfills, result in increases in CH
4
, whereas the use of fertilizer and the
burning of fossil fuels leads to increases in N
2
O. The increasing use of the principal
halocarbons (chlorofluorocarbons) as refrigeration agents and in other industrial processes
has been found to cause stratospheric ozone depletion.
2.2.2. Radiative Forcing
The energy balance of the Earth-atmosphere system can be measured based on radiative
forcing, which is usually quantified as the rate of energy change per unit area of the globe as
measured at the top of the atmosphere. The Earth-atmosphere system gets warmer when
radiative forcing is positive; for negative radiative forcing, the energy will ultimately
Fig. 11.2. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000
years [7]. Concentration units are parts per million (ppm) and parts per billion (ppb).
528 V.P. Singh et al.
decrease, leading to a cooling of the system. The major causes of forcing include increases in
greenhouse gases; tropospheric ozone increases contributing to warming; stratospheric ozone
decreases contributing to cooling; the influence of aerosol particles through reflection and
absorption processes; the nature of land cover around the globe principally through changes in
croplands, pastures, and forests; and persistent linear trails of condensation due to aircraft in
regions that have suitably low temperatures and high humidity [7].
2.2.3. Natural Processes
The human impact on climate during this era greatly exceeds that due to known changes in
natural processes, such as solar changes and volcanic eruptions. The original Milankovitch
theory [8] identifies three types of orbital variation that could act as climate-forcing mecha-
nisms: the obliquity or tilt of the Earth’s axis (which affects the distribution of insolation in
space and time), the precession of the equinoxes, and the eccentricity of the Earth’s orbit
around the Sun. The other natural processes are volcanic eruptions that release huge amounts
of gases by reducing the amount of solar radiation reaching the Earth’s surface, lower
temperatures, and change atmospheric circulation patterns, whereas tectonic movements
generate both atmospheric circulation changes and greenhouse feedback, directly or
indirectly.
2.3. Debate on Climate Change
The increase in the average surface temperature by the end of the twentieth century due to
emissions of anthropogenic greenhouse gases (GHGs) have already reached the critical
threshold for many elements of the climate system [9]. Current climate policy emphasizes
undertaking sustainable measures on long-term reductions of CO
2
, and even when CO
2
emissions end, climate change is largely irreversible for 1,000 years [10]. A number of
forums and institutions have been established to develop actions for mitigating the adverse
impact of climate change. For example, the UNFCCC was created in 1992 to provide a
framework for policymaking to mitigate climate change by the stabilization of atmospheric
greenhouse gases at a sufficiently low level to prevent dangerous anthropogenic effects on the
climate. The countries that are parties to the Kyoto Protocol—excluding the countries of the
former Soviet Union whose economies are in transition—had increased their emissions by
9.9 % above the 1990 levels by 2006 [11].
The Kyoto Protocol, formed in 1997 and considered to be a milestone document in
international climate change policy, for the first time established legally binding limits for
industrialized countries on emissions of carbon dioxide and other greenhouse gases [12]. For
example, national targets range from 8 % reductions for the European Union and some others
to 7 % for the USA, 6 % for Japan, and 0 % for Russia; it permits increases of 8 % for
Australia and 10 % for Iceland. International initiatives where the threat of climate change is
considered to be the greatest challenge facing humanity include the Montreal Protocol,
considered to be the most successful environmental treaty, which calls for reducing almost
100 ozone-depleting chemicals by 97 % [13], and recent forums, including the Bali Road Map
of 2007 and the 2009 Copenhagen Accord.
Climate Change and Its Impact on Water Resources 529
3. EVIDENCE OF CLIMATE CHANGE
The following section discusses the changing patterns of temperature and precipitation
during the twentieth century as evidence of climate change.
3.1. Increases in Temperature
In its fourth assessment report, the IPCC unequivocally states that the Earth’s climate is
warming [14]. During the last century, the Earth warmed by roughly 0.6 C, with most of the
warming occurring during the period 1920–1940 and during the last 30 years. An increase has
been noted in the Earth’s surface and atmospheric temperature but this warming is not evenly
distributed across the globe. Land masses are warming faster than oceans. Higher northern
latitudes have seen larger increases, and the average temperature of the Arctic has risen by
almost twice the global average rate in the past 100 years [14]. Several places, including
mountaintops, have seen losses in ice cover. Expressed as a global average, surface temper-
atures have increased by approximately 0.74 C over the past 100 years (between 1906 and
2005), with an increase (0.35 C) occurring in the global average temperature from the 1910s
to the 1940s, followed by a slight cooling (0.1 C), and then a rapid warming (0.55 C) up to
the end of 2006 (Fig. 11.3)[15]. It is worth noting that for shorter recent periods, the slope is
greater, indicating accelerated warming.
A number of research groups around the world have produced estimates of global-scale
changes in surface temperature [16], for example, the retreat of mountain glaciers on every
continent [17], reductions in the extent of snow cover, earlier blooming of plants in spring,
Fig. 11.3. Annual global mean observed temperatures (black dots) along with simple fits to the data;
left-hand axis: anomalies relative to 1961–1990 average; right-hand axis: estimated actual temperature
(C) [15].
530 V.P. Singh et al.
and increased melting of the Greenland and Antarctic ice sheets [18]. Many studies are in
general agreement with long-term temperature variations [15], including the operational
version of the Global Historical Climatology Network (GHCN), the National Climatic Data
Center (NCDC) [19], the National Aeronautics and Space Administration’s (NASA’s)
Goddard Institute for Space Studies (GISS) [20], and improved analysis of CRU/Hadley
Centre gridded land-surface air temperature version 3 (CRUTEM3) [21,22].
The spatial pattern of changes in the annual surface temperature for 1901–2005 and
1979–2005 is shown in Fig. 11.4 [15], where differences in trends between locations can be
large, particularly for shorter time periods; based on the century-long period, warming is
statistically significant over most of the Earth’s surface. Based on data compiled from
20 sources, including Global Historical Climatology Network, and two editions of world
weather records, from 1950 to 2004, the annual trends in minimum and maximum land-
surface air temperature averaged over regions were 0.20 C per decade and 0.14 C per
decade, respectively, with a trend in the diurnal temperature range of 0.07 C per decade
[23]. Based on the reconstructed annual Northern Hemisphere mean temperature series, the
warmth of the 1990s (3 years in particular: 1990, 1995, and 1997) was unprecedented in at
least the past 600 years [24], taking into account the self-consistently estimated uncertainties
in the reconstruction back to AD 1400.
3.2. Changes in Precipitation Patterns
The increase in evaporation due to the rise in temperature has led to more precipitation [7],
which generally increased over land located north of 30N from 1900 to 2005 but has mostly
declined over the tropics since the 1970s, and globally there has been no statistically
significant overall trend in precipitation over the past century, with a wide variability in
patterns by region and over time. Spatial patterns of trends in annual precipitation (as a
percentage per century or per decade) during the periods 1901–2005 and 1979–2005 are
shown in Fig. 11.5 [15]; the observations include the following: (1) in most of North America,
Fig. 11.4. Linear trend of annual temperatures for 1901–2005 (left)(
C per century) and 1979–2005
(right)(
C per decade) [15].
Climate Change and Its Impact on Water Resources 531
and especially over high-latitude regions in Canada, annual precipitation has increased, and
the primary exception is over the southwestern parts of the USA, northwestern Mexico, and
the Baja Peninsula, where drought has prevailed in recent years; (2) across South America,
increasingly wet conditions were observed over the Amazon Basin and southeastern South
America, including Patagonia, while negative trends in annual precipitation were observed
over Chile and parts of the western coast of the continent; and (3) the largest negative trends in
annual precipitation were observed over West Africa and the Sahel.
Global annual mean precipitation is constrained by the energy budget of the troposphere,
and extreme precipitation is constrained by the atmospheric moisture content [25]; changes in
extreme precipitation are greater than those in mean precipitation. Similarly, based on the
physical mechanisms governing changes in the dynamic and thermodynamic components of
Fig. 11.5. Trend of annual land precipitation amounts for 1901–2005 (top) (percentage per century)
and 1979–2005 (bottom) (percentage per decade) [15].
532 V.P. Singh et al.
mean and extreme precipitation, a greater percentage increase in extreme precipitation versus
mean precipitation was observed in models [26]. Several other findings include the following:
a higher increase in tropical precipitation intensity due to an increase in water vapor, while
mid-latitude intensity increases are related to circulation changes that affect the distribution of
increased water vapor [27]; the most intense precipitation occurring in warm regions [28];
higher temperatures leading to a greater proportion of total precipitation in heavy and very
heavy precipitation events with no changes in total precipitation [29]; increases in total
precipitation, with a greater proportion falling in heavy and very heavy events if the frequency
remains constant, as demonstrated empirically [30] and theoretically [31]; rises in tempera-
ture that are likely to increase the moisture content faster than the total precipitation, which is
likely to lead to an increase in the intensity of storms [32]; increases in observed extreme
precipitation over the USA, where the increases are similar to changes expected under
greenhouse warming (e.g., [33,34]).
4. IMPACTS OF CLIMATE CHANGE ON WATER RESOURCES
Water resources are sensitive to variations in climatic patterns. It is believed that there will
be changes in the water resources sector due to climate change, which is discussed in the
following section.
4.1. Runoff
Current observations and climate projections suggest that one of the most significant
impacts of climate change will likely be on the hydrological system and, hence, on river
flows and regional water resources [5,35]. Variability in climate causes flooding patterns in
space and time. During the twentieth century several studies examined potential trends in
measures of river discharge at different spatial scales, some detected significant trends
in several indicators of flow, and some demonstrated statistically significant links with trends
in temperature or precipitation [5]. In addition, human interventions have affected flow
regimes in many catchments at the global scale, and there is evidence of a broadly coherent
pattern of change in annual runoff, with some regions experiencing an increase in runoff (e.g.,
high latitudes and large parts of the USA) and others (such as parts of West Africa, southern
Europe and southernmost South America) experiencing a decrease in runoff [5,36].
Some of the changes in runoff described in studies during the twentieth century include
widespread increases in runoff largely due to the suppression of evapotranspiration by
increasing CO
2
concentrations [37]; in addition, based on a 30-year running averaged
streamflow, a linearly increasing pattern in four major river basins in southeastern South
America was observed after the mid-1960s, but not the same in all rivers [38]. Increasing
streamflow has been observed in the USA since 1940, though these increases have not been
uniform across the range of annual streamflows, nor have they been uniform geographically or
seasonally, as reported by the USGS using a variety of approaches [3941]. Regions that have
experienced the most widespread increases are the Upper Mississippi, Ohio Valley, Texas-
Gulf, and the Mid-Atlantic. Fewer trends were observed in the South Atlantic Gulf region,
Climate Change and Its Impact on Water Resources 533
Missouri, and regions of the far West. The Pacific Northwest and the South Atlantic Gulf
actually had a number of streamflow decreases, particularly in the lowest percentiles.
Long-term shifts in the timing of streamflow have been observed for snowmelt-dominated
basins throughout western North America since the late 1940s [42,43]. These shifts represent
an advance to earlier streamflow timing by 1–4 weeks in recent decades relative to conditions
that prevailed in the 1950s through the mid-1970s, and the evidence for the shift includes
earlier snowmelt onsets and advances in the center of mass of the annual hydrograph, which is
called center timing [44]. Several researchers have highlighted an increasing runoff trend,
especially in winter and spring seasons, over the past several decades in most northern rivers,
including the largest arctic rivers in Siberia [4547]. The causes of spring discharge increase
in Siberian regions are primarily due to earlier snowmelt associated with climate warming
during the snowmelt period [47,48], reduction in permafrost area extent, and an increase in
active layer thickness under warming climatic conditions [48,49].
Based on the IPCC Special Report on Emissions Scenario (SRES) A1B from an ensemble
of 24 climate model runs, the mean runoff change until 2050 is shown in Fig. 11.6 [36]. Major
observations include runoff change in the high latitudes of North America and Eurasia, with
increases of 10–40 % and of decreasing runoff (by 10–30 %), and in the Mediterranean,
southern Africa, and western USA/northern Mexico. In general, between the late twentieth
century and 2050, the areas of decreased runoff will expand [36].
4.2. Floods
Floods cause significant damage to the economies of affected areas, and this is considered
to be one of the commonly occurring natural hazards around many parts of the world due to
the impact of climate change [50]. The causes of flooding are many and include heavy rainfall,
torrential rain, and snowmelt; their spatial locations since 1985 are shown in Fig. 11.7. Severe
floods from high rainfall (of long or short duration) have occurred in almost all humid regions
of the world, as well as some semiarid zones. Tropical storms (known as hurricanes, cyclones,
Fig. 11.6. Change in annual runoff by 2041–2060 relative to 1900–1970, in percentage, under SRES
A1B emissions scenario [36,54].
534 V.P. Singh et al.
or typhoons) are more concentrated in distribution, with hotspots around the western Pacific
coasts, the Caribbean, southeastern USA, and the Bay of Bengal. The common cause of
flooding in India is the tremendous impact of monsoonal rain, which causes levy breaks.
Intense rainfall of long duration induced extreme flooding in five countries of central and
Eastern Europe in August 2002 [51].
Fig. 11.7. Spatial distribution of extreme floods from three different causes listed by Dartmouth Flood
Observatory since 1985. (a) Heavy rain (b) Brief torrential rain (c) Snowmelt (Source:http://www.
dartmouth.edu/~floods/archiveatlas/floodcauselocation.htm).
Climate Change and Its Impact on Water Resources 535
Detecting anthropogenically forced changes in flooding is difficult due to the substantial
natural variability; land-use changes on flow regime further complicate the issue. For exam-
ple, changes in the risk of great floods, that is, floods with discharges exceeding a 100-year
return period from basins larger than 200,000 km
2
using both streamflow measurements and
numerical simulations of anthropogenic climate change, and the frequency of great floods
increased substantially during the twentieth century [52]. On the global scale the number of
great inland flood catastrophes during the last 10 years (between 1996 and 2005) is twice as
large, per decade, as between 1950 and 1980, while economic losses have increased by a
factor of 5 [53]. However, a warmer climate, with its increased climate variability, will
increase the risk of flooding [54].
4.3. Drought
Of twentieth century natural hazards, droughts have had the greatest detrimental impact
[55,56], and large-scale intensive droughts have been observed on all continents in recent
decades affecting large areas in Europe, Africa, Asia, Australia, South America, Central
America, and North America [57].
During the past two centuries, at least 40 long-duration droughts have occurred in Western
Canada. In southern regions of Alberta, Saskatchewan, and Manitoba, multiyear droughts
were observed in the 1890s, 1930s, and 1980s [58]. The drought situation in many European
regions has already become more severe [59]. It is observed that during the past 30 years,
Europe has been affected by a number of major drought events, most notably in 1976
(Northern and Western Europe), 1989 (most of Europe), 1991 (most of Europe), and, more
recently, the prolonged drought over large parts of Europe associated with the summer heat
wave in 2003 [60]. The impacts of droughts in the USA has increased significantly with the
increased number of droughts or increase in their severity [61,62]. Based on the data available
from the NCDC, USA (2002), nearly 10 % of the total land area of the United States
experienced either severe or extreme drought at any given time during the last century.
Frequent severe droughts during 1997, 1999, and 2002 in many areas of Northern China
caused significant economic and societal losses [63]. The severe drought of 1997 in Northern
China resulted in a period of 226 days with no streamflow in the Yellow River, which is the
longest drying-up duration on record. There has also been an increased risk of droughts since
the late 1970s as global warming progresses and produces both higher temperatures and
increased drying [64]. In addition, drought is a recurring theme in Australia, with the most
recent, the so-called millennium drought, now having lasted for almost a decade [65].
Several investigators have highlighted more drought episodes in the twenty-first century.
Using 15 coupled models from the IPCC AR4 simulations under the SRES A1B scenario, the
general drying over most of the planet’s land, except parts of the northern mid- and high-
latitude regions during the nongrowing season, points to a worldwide agricultural drought by
the late twenty-first century [66]. All of the eight AR4 models show a decrease in soil
moisture for all scenarios, with a doubling of both the spatial extent of severe soil moisture
deficits and frequency of short-term (4–6 months in duration) droughts from the
mid-twentieth century to the end of the twenty-first century, while long-term (>12 months)
536 V.P. Singh et al.
droughts will become three times more common [67]. Based on the Palmer drought severity
index calculated using the Penman–Monteith potential evapotranspiration (PDSI-PM) as the
drought index, the spatial distribution of drought was obtained by calculating the trend in the
PDSI-PM per decade at each point using the A2 ensemble (Fig. 11.8)[68]. The observation
predicts drying over Amazonia, the United States, Northern Africa, Southern Europe, and
Western Eurasia and wetting over Central Africa, Eastern Asia, and high northern latitudes.
There is an overall drying trend with a decrease in the global average PDSI of 0.30/decade
projected for the first half of the twenty-first century, while the rate of drying over the second
half of the twenty-first century increases, with the PDSI-PM decreasing by 0.56/decade. There
is a projected increase in the proportion of the land area under drought over the twenty-first
century, and this increase continues throughout the twenty-first century (Fig. 11.9)[68].
The reader may consult [56] to gain a better understanding of drought concepts and [69] for
drought under climate change scenarios.
Fig. 11.8. Trend in PDSI-PM per decade for (a) ensemble mean of first half of twenty-first century and
(b) ensemble mean of second half of twenty-first century projected by SRES A2 [68].
Fig. 11.9. Proportion of land surface in drought for twenty-first century based on results from A2
emissions scenario [68].
Climate Change and Its Impact on Water Resources 537
4.4. Snowmelt and Glacier Melt
Many major river systems around the world are fed by snowpack and melting glaciers, and
global warming is likely to have an appreciable impact on snowmelt and associated runoff.
Glaciers are sensitive to every hydrological variable, including precipitation, humidity, and
wind speed, but mostly to temperature, and hence are a good indicator of global warming.
There is clear evidence of glacier retreat on every continent, with global warming having a
noticeable influence [70]. High-latitude and high-altitude rivers may experience an increase in
discharge despite a decrease in precipitation resulting from the melting of glaciers [71,
72]. The change in volume and timing of discharge may cause significant fluvial geomorpho-
logical changes in rivers, including channel enlargement and incision, higher sinuosity,
increased bank erosion, and faster channel migration [73].
The European Alps have lost between 30 and 40 % in surface area and around half of its
volume since the Little Ice Age, and this loss has accelerated in the late twentieth century
[70]. The Bolivian Andes lost two-thirds of their volume and 40 % in average thickness
between 1992 and 1998 and may disappear within 15 years if the current trend persists [74]. In
North America, varying gains and losses have been observed in three glaciers—South
Cascade Glacier in the Pacific Northwest and Wolverine and Gulkana glaciers in Alaska
[75]. In the Himalayas, depending on the location and time period, glaciers have been
observed both retreating and advancing at varying rates. Nevertheless, the system has suffered
a net overall decrease in glacier area and thickness during the last century [76]. The Gangotri
Glacier, for example, which forms the headwaters of the Ganges River, has shrunk by 1.5 km
over 69 years but is currently retreating at a slower rate [77].
The Alaskan glacier has been shrinking by 52 15 km
3
/year, adding about 0.14 0.04
mm/year to sea level during the period from the mid-1950s to the mid-1990s [78]. For the
period from the mid-1990s to 2000–2001 the Alaskan glaciers have been thinning at a faster
rate, equating to a volume loss of 96 35 km
3
/year and a sea level rise of 0.27 0.10 mm/
year. Subpolar glaciers, for the period 1961–1997, have shrunk by 147 mm/year on average,
representing a total volume of 3.7 10
3
km
3
[79]. Associated sea level rise has been
estimated at 0.51 mm/year for the period 1961–2003, with the rate for 1994–2003 being
0.93 mm/year, signifying faster melting [80].
In the alpine regions, studies based on scenarios project further glacier retreat [81] and loss
in thickness, while small glaciers may disappear in the near future [74]. A similar dreadful
future has been predicted for the Rockies [82], Himalayas [83,84], Andes [85], and Australian
Alps [86]. A more recent study utilizing a global glacier model coupled with a land surface
and hydrological model showed large-scale glacier mass loss in Asia, Europe, Canadian Artic
islands, and Svalbard [87], and the mass loss has increased dramatically since 1990, resulting
in an increase in sea level by 0.76 mm/year.
The societal impacts of retreating glaciers are hard to quantify. Major rivers fed by glacial
melt sustain agriculture, domestic water needs, and water stored in dams for hydroelectric
production. If the appropriate infrastructure is not put in place, the additional water issuing
from melting glaciers may lead to devastating floods, or droughts as the timing of the runoff
changes. The costs for additional power capacity to cope with retreating glaciers in Peru are
estimated to be around US$1 billion per gigawatt [88].
538 V.P. Singh et al.
4.5. Water Quality
There has been an increase in temperature as observed in the past century, and climate
change models also predict increasing temperatures. Warmer temperatures can affect water
quality in several ways, including decreased dissolved oxygen levels, increased contaminant
load to water bodies, reduced stream and river flows, increased algal blooms, and an
increased likelihood of saltwater intrusion near coastal regions. Acid rain is one of the
primary reasons for degrading water quality, and the principal cause of this is sulfuric and
nitrogen compounds from human actions, and variations in streamflow characteristics will
alter the transport of chemical loads in rivers. Due to low flows in rivers, the dilution process
between water and waste will be affected, which can increase the mineralization of organic
nitrogen in soil.
Warmer water holds less oxygen, so global warming would lead to lower dissolved oxygen
contents. An increase in runoff and erosion due to greater precipitation intensity will result in
increased pollutant transport. Several other factors influencing water quality include soil,
geological formations and terrain in catchment areas (river basins), surrounding vegetation,
human activities, precipitation and runoff from adjacent land, and biological, physical, and
chemical processes in water.
Studies reveal a rise in surface water temperature since the 1960s in Europe, North
America, and Asia of between 0.2 and 2 C, which is mainly due to atmospheric warming
in relation to solar radiation increases [5,89]. For example, after the severe drought of
2003, there was an average increase in water temperature of around 2 C, in the Rhine
and Meuse Rivers causing a decrease in a decrease in dissolved oxygen (DO) [90].
The stratified period has lengthened by 2–3 weeks due to a rise in temperature of
0.2–1.5 C in several lakes in Europe and Northern America, which affects thermal stratifi-
cation and lake hydrodynamics [89].
Many factors that are influenced by climate change, including air temperature, rainfall
intensity, atmospheric CO
2
(increase) and acid deposition (decrease) levels, have shown
significant dissolved organic carbon (DOC) increases in Northern Europe [9193], Central
Europe [94], and North America [92].
Changes in weather patterns have a significant impact on nutrient loading [95]. A warmer
climate affects water bodies, leading to increased nutrients loads in surface and groundwater
[90]. Rainfall patterns, including seasonality and intensity, along with increased air temper-
atures, are the main drivers for changing the fate and behaviors of pesticides [96].
Several studies investigated the impact of climate change on water quality in future
decades. An increase in water temperature of around 2 C by 2070 in European lakes,
depending on the lake characteristics and the season, would place shallow lakes at the highest
risk of being adversely affected by climate change [97,98]. In addition, the residence time of
lakes would probably increase in summer by 92 % in 2050 for lakes with short residence times
[97]. The deepest lakes are most sensitive to climate warming over a long period of time due
to their greater heat storage capacity and will consequently show the highest winter temper-
atures [97]. An increase in water temperature would also affect lake chemical processes with
Climate Change and Its Impact on Water Resources 539
increases in pH and greater in-lake alkalinity generation [99]. Higher nitrate concentration in
rivers can occur due to an increase in summertime frequency, which might lead to a gradual
mobilization of nitrates in soils that would be flushed into streams at the beginning of the wet
seasons [100]. Other studies have pointed to increases in nitrate concentration in the Seine
basin aquifer layers for the years 2050 and 2100 due to an increase in precipitation and,
consequently, in soil leaching [101] and a 40–50 % increase in nitrate flux by 2070–2100 in a
Norwegian river basin due to an increase in precipitation and, consequently, in soil
leaching [102].
4.6. Groundwater
Global warming will likely affect groundwater resources by altering precipitation and
temperature patterns, which will likely be further aggravated by overexploitation. Based on
NASA’s Gravity Recovery and Climate Experiment (GRACE) twin satellites [103], the
groundwater in the states of Rajasthan, Punjab, and Haryana in India is declining at a rate
of 33 cm/year. Similar studies in the United States showed that groundwater in the San
Joaquin Valley in California has been dropping by 60–150 cm over the last 5 years
[104]. Groundwater recharge is affected by land-use and land-cover change, urbanization,
loss in forest cover, changes in cropping patterns and rotation, and changes in soil properties
occurring over a long period of time that may affect infiltration capacity.
The recharge dynamics of the Edwards Balcones Fault Zone Aquifer in Texas, USA, based
on climate change, coupled with water requirements for the year 2050, are such that climate
change would result in an increase in the springflow for that area, but growing demand will be
the major factor of concern for the aquifer [105]. A study on the effect and resulting change in
the hydrological cycle in Bie
´vre-Valloire in Grenoble [106], due to a doubling in CO
2
and its
ensuing effect on groundwater recharge, it was found that global warming would not cause a
major change in rainfall patterns but may result in a large increase in evaporation and a
decrease in recharge. Further, changes in land use land cover and changes in precipitation
regimes (flash rainfall instead of drizzles) could alter runoff patterns and further reduce
recharge.
Based on the four climate scenarios, the computed groundwater recharge decreases dra-
matically by more than 70 % in northeastern Brazil, southwest Africa, and along the southern
rim of the Mediterranean Sea (Fig. 11.10)[54,107]. Regions with groundwater recharge
increases of more than 30 % by the 2050s include the Sahel, the Near East, Northern China,
Siberia, and the western USA. Understanding the future concerns of groundwater resources,
UNESCO-IHP (International Hydrological Programme) has established the Groundwater
Resources Assessment under the Pressures of Humanity and Climate Changes (GRAPHIC)
project to study the interaction between groundwater and the global water cycle, how it
supports ecosystems and humankind, and the threats posed by a growing population and
climate change [108]. The study will be carried out on every continent and will make use of
various recent technologies in assessing and monitoring changes to groundwater, including
GRACE.
540 V.P. Singh et al.
4.7. Transboundary Problems
A remote sensing study showed that 261 river basins, covering 45.3 % of the land surface
of the Earth (excluding Antarctica), extends beyond political boundaries and are shared by
two or more countries [109]. These basins carry around 60 % of the surface water flows and
are home to 40 % of the world’s population. The sharing of a watercourse has, due to its
sensitive nature, given rise to several instances for concern and has created opportunities for
either conflict or cooperation. Several nations have conceded that it is not in their interest to
wage war over water and have, even in the event of hostility associated with other issues,
managed to successfully work toward agreements and treaties. There exist more than
300 treaties dealing with a wide array of nonnavigational uses of water in international basins
[109]. In the last 50 years alone, no less than 157 treaties have been negotiated [110].
Nevertheless, the potential for conflict has not yet been eliminated.
Four general theories have inspired legislators in debates around the nonnavigational
uses of international watercourses [111], namely, absolute territorial sovereignty, absolute
Fig. 11.10. Simulated impact of climate change on long-term average annual diffuse groundwater
recharge based on percentage changes of 30-year average groundwater recharge between present day
(1961–1990) and 2050s (2041–2070) based on four different climate change scenarios [54,107].
Climate Change and Its Impact on Water Resources 541
territorial integrity, limited territorial sovereignty, and the concept of community of interests.
Global warming is perceived by many as a new threat to relations over transboundary basins
because it interferes with the hydrological cycle, influencing the timing, quantity, and even
quality of both surface and groundwater. The IPCC, in its Third Assessment Report [112],
states that
...where there are disputes, the threat of climate change is likely to exacerbate, rather than ameliorate, matters
because of uncertainty about the amount of future resources that it engenders. One major implication of climate
change for agreements between competing users (within a region or upstream versus downstream) is that
allocating rights in absolute terms may lead to further disputes in years to come when the total absolute amount
of water available may be different.
The following section highlights some examples of the impact of climate change on
transboundary rivers.
4.7.1. Nile River Basin
The Nile is the world’s longest river and is transboundary to 11 countries in Africa. It drains
approximately 3.3 million km
2
and is home to 160 million people. The river has been subject to
a number of treaties, some dating back to colonial times [113]. The Nile Basin Initiative (NBI),
a relatively new effort among 9 of the 11 riparian countries, puts the concept of community
of interests into action. The basin has been the subject of a number of papers on the effect of
climate change. Based on a water balance model for 12 subcatchments of the Nile, the effect
of climate change for five different General Circulation Model (GCMs) [114] highlights
the higher flows in equatorial Africa and the expansion of the Sudd swamps, and, depending
on the GCM used, the response on the Ethiopian highlands of the Blue Nile and Atbara
basins varied.
4.7.2. Danube River Basin
The Danube River Basin is the second longest river in Europe and is one of the most
international river basins. It is shared among ten countries and is home to 85 million people.
The basin is already threatened because it faces deteriorating water quality and ecological
problems due to its heavy use of electricity generation [116]. With global warming, the flow in
the Danube may decrease by 16 % toward the end of the century [117]. A reduction in flow
will exacerbate the water quality problem. A Danube River Basin Management Plan has been
formulated to address the impact of climate change and water quality issues in the river
to ensure satisfactory socioeconomic development without further threatening the ecology
of the river [118].
4.7.3. Rio Grande Basin
The Rio Grande or Rı
´o Bravo del Norte is a transboundary river between three states within
the Unites States and between the United States and Mexico. The river is fed by snow melt
from the Rockies and runs through arid and semiarid areas. The water in the river is
overallocated, and on several instances it has not made it to the Gulf of Mexico. By the
542 V.P. Singh et al.
time the river reaches the Presidio, along the Texas Mexico border, the flow is almost
negligible and is restored by water from the Rı
´o Conchos, a Mexican tributary. Predictions
from climate models suggest that this region will become drier and droughts will be more
frequent. The effect of climate change using five GCMs and two emission scenarios (A2 and
A1B) on the Rı
´o Conchos will be a decrease in precipitation, which will result in a reduction
in flow at the basin outlet in Ojinaga by approximately 18 % [119]. Winter and summer flows
are expected to decrease by 25 % toward the end of the century.
4.8. Agriculture
The agricultural sector is a contributor to greenhouse gas emissions due to land conversion,
including deforestation, tillage and burning practices, volatilization of organic and inorganic
fertilizers, and methane emission from ruminant livestock and paddy rice cultivation.
Globally, carbon dioxide emissions from land-use change (approximately 1,600 million
tons C/year), largely driven by agricultural expansion, grew most rapidly in the period
1950–1970. Also, agriculture is the major anthropogenic source of methane, a gas with
very high global warming potential [120,121].
Approximately 70 % of the world’s freshwater use goes to agriculture, and already some
30 developing countries are facing water shortages; by 2050 this number will increase to some
55 countries, the majority in the developing world [122]. This water scarcity, together with the
degradation of arable land, could become the most serious obstacle to increasing food
production. The vulnerability of agricultural sectors to climate change is mainly due to
changes in precipitation and temperature, and, consequently, the likely impacts on the
agricultural sector have prompted concern over the magnitude of future global food produc-
tion [123]. Many countries are agriculture dependent; therefore, climate change could create a
linkage between the agricultural sector and poverty and is likely to affect many developing
countries. For example, in Africa, it is estimated that nearly 60–70 % of the population is
dependent on the agricultural sector for employment, and this sector contributes on average
nearly 34 % to the gross domestic product (GDP) per country, whereas in the case of the West
African Sahel alone, more than 80 % of the population is involved in agriculture and stock
farming in rural areas [124]. Findings of several investigations highlight that the degree of
vulnerability of the agricultural sector depends on local biological conditions, moisture
content, cropping patterns, extent of knowledge, and awareness of expected changes in the
climate, and the increased uncertainty of climate effects represents an additional problem that
farmers must address.
Human-induced climate change has the potential to substantially alter agricultural systems
[125127]. Climate change will affect agricultural productivity characterized mainly by five
factors, including changes in precipitation, temperature, carbon dioxide (CO
2
) fertilization,
climate variability, and surface water runoff [128]. However, precipitation and temperature
will have direct effects as these combinations are useful for determining the availability of
freshwater and the level of soil moisture, which are critical inputs for crop growth. Also,
higher precipitation leads to a reduction in yield variability [129], and higher precipitation
will reduce the yield gap between rainfed and irrigated agriculture, but it may also have a
Climate Change and Its Impact on Water Resources 543
negative impact if extreme precipitation causes flooding [130]. The combination of temper-
ature and soil moisture determines the length of the growing season and controls crop
development and water requirements; however, in arid and semiarid areas, higher tempera-
tures will shorten the crop cycle and reduce crop yields [131].
Several studies have been carried out to assess crop yield based on climate change impacts.
Based on a statistical model using 22 climate models and three IPCC global emissions
scenarios (A1B, A2, and B1), temperature increases of 1–3 C would reduce yields of
three temperate-zone California perennials (almond, walnut, and table grapes) by 2050, even
without consideration of possible impacts on irrigation water availability [132]. Similarly,
climate-change-induced drought episodes affect agricultural productivity; for example, peri-
odic drought in the Yakima basin will lead to substantial reductions in crop yields and
increases in economic risk both in dry years under the current climate scenario and in a future
climate with 2 C warming and no change in annual precipitation [133].
Some studies have investigated crop productivity for Europe, where an overall increase in
crop productivity is anticipated as a result of climate change and increased atmospheric
carbon dioxide (CO
2
); however, because of technological developments, wheat yields will
increase by 37–101 % by the 2050s, depending on the scenario [134]. However, air pollution
could also reduce crop yields since tropospheric ozone has negative effects on biomass
productivity [135,136]. Also, annual temperature increases may lead to a longer crop (and
grass) growing season and vegetative growth and cover, particularly in Northern Europe
[137]. Negative impacts in Northern Europe could include increased pest and disease pres-
sures and nutrient leaching and reduced soil organic matter (SOM) content [138]. There could
be an increasing demand for water for crop irrigation (up to 10 %, depending on the crop
type), especially in southern and Mediterranean regions [139], and for fruit and vegetable
production in Northern Europe [140].
4.9. Ecosystems
Physical processes and biological systems on many scales have been altered due to global
climate change caused by the sudden increase in the energy balance of the planet, which
affects ecosystems that support human society [7]. This may cause sudden, irreversible effects
that fundamentally change the function and structure of the ecosystem, with potentially huge
impacts on human society [141]. The global ecosystem is sensitive to many components
[142], including large variability and extremes of CO
2
and climate throughout geological
history [143]; anthropogenic changes, such as land use, nitrogen deposition, pollution, and
invasive species [144]; natural disturbance regimes (e.g., wildfire); and subtle changes in
management practices within a given land-use type, e.g., intensification of agricultural
practices [145]. Land-use changes, habitat loss, and fragmentation have long been recognized
as important causes of ecosystem change, particularly changes in biodiversity [146]. A regime
shift occurs in ecosystems, such that even small changes in physical conditions can provoke a
regime shift that may not be easily or symmetrically reversed [7,147], and most initial
ecosystem responses appear to dampen change [148], although ecosystems are likely to
respond to increasing external forcing in a nonlinear manner.
544 V.P. Singh et al.
Deserts, one of the largest terrestrial biomes, are likely to experience more episodic climate
events, and interannual variability may increase in the future, although there is substantial
disagreement among GCM projections and across different regions [149]. Vulnerability to
desertification will likely be exacerbated due to increases in the incidence of severe drought
globally [68]; for example, in the Americas, temperate deserts are projected to expand
substantially under doubled CO
2
climate scenarios [150]. Further, desert biodiversity is likely
to be affected by climate change [144]; for example, 2,800 plant species in the Succulent Karoo
biome of South Africa as a bioclimatically suitable habitat could be reduced by 80 % with
global warming of 1.5–2.7 C above preindustrial levels [142]. Rainfall change and variability
are very likely to affect vegetation in tropical grassland and savanna systems [151]. Changing
amounts and variability of rainfall may also strongly control temperate grassland responses to
future climate change [152]. The Mediterranean Basin regions, however, could see increased
occurrence of fires [153], and doubled CO
2
climate scenarios could increase wildfire events by
40–50 % in California [154] and double the fire risk in Cape Fynbos [155].
Inland aquatic ecosystems will be affected by climate change directly with rises in
temperature and CO
2
concentrations and indirectly through alterations in the hydrology
resulting from changes in regional or global precipitation regimes [156]. Microorganisms,
benthic invertebrates, and many species of fish will be negatively affected by higher temper-
atures [157]; invertebrates, waterfowl, and tropical invasive biota are likely to shift poleward
[158] with some potentially going extinct [159]. Other major changes will likely occur in
species composition, seasonality, and the production of planktonic communities and their
food web interactions [160]. Enhanced UV-B radiation and increased summer precipitation
will significantly increase dissolved organic carbon concentrations, altering major biogeo-
chemical cycles [161]. Wetland plants and animals at different stages of their life cycle are
affected by small increases in the variability of precipitation [162]. In monsoonal regions,
increased variability risks that diminish wetland biodiversity and prolonged dry periods
promote terrestrialization of wetlands, for example, in Keoladeo National Park, India
[163]. There is evidence of riparian ecosystems among many rivers around the world
experiencing additional pressure due to changes in climate and land-use practices [164],
whereas the pattern of freshwater flows will affect coastal wetlands due to changes in salinity,
sediment inputs, and nutrient loadings [165].
5. CONTINENTAL-SCALE IMPACT OF PROJECTED CLIMATE
CHANGES ON WATER RESOURCES
Future climate projections are made based on four IPCC SRES [166] storylines considering
a range of plausible changes in population and economic activity over the twenty-first century.
The scenarios include: A1 storyline and scenario family (a future world of very rapid
economic growth, global population that peaks in mid-century and declines thereafter, and
rapid introduction of new and more efficient technologies); A2 storyline and scenario family
(a very heterogeneous world with continuously increasing global population and regionally
oriented economic growth that is more fragmented and slower than in other storylines);
Climate Change and Its Impact on Water Resources 545
B1 storyline and scenario family (a convergent world with the same global population as in
the A1 storyline but with rapid changes in economic structures toward a service and infor-
mation economy); B2 storyline and scenario family (a world in which the emphasis is on local
solutions to economic, social, and environmental sustainability, with continuously increasing
population but lower than A2 and intermediate economic development).
Due to advances in modeling as well as in our understanding of the physical processes
involved, it has been possible to make more reliable climate projections. The development of
Atmosphere-Ocean General Circulation Models (AOGCMs) remains the foundation for pro-
jections, though they cannot provide information at scales finer than their computational
grids. The climatic patterns are region-specific due to several reasons [167], which include an
uneven distribution of solar heating; individual responses of the atmosphere, oceans, and land
surface; interactions among these; and physical characteristics of the regions. Therefore, it
will be useful to study the projected climate for different regions. The following region-
specific discussions are based on the IPCC Fourth Assessment Report: Climate Change 2007,
where the information is drawn from AOGCM simulations, downscaling of AOGCM-
simulated data using techniques to enhance regional detail, physical understanding of the
processes governing regional responses, and recent historical climate change [167].
5.1. Africa
During the twenty-first century all of Africa is very likely to warm, and the warming is very
likely to be larger than the global, annual mean warming throughout the continent and in all
seasons, with drier subtropical regions warming more than the moister tropics. Annual rainfall
is likely to decrease in much of Mediterranean Africa, the northern Sahara, and southern
Africa during winter, whereas there is likely to be an increase in annual mean rainfall in
East Africa [167].
Climate change is expected to exacerbate critical water stress conditions before 2025 due to
rises in water demand and in population. Based on the full range of SRES scenarios, the
population to be affected is projected to be 75–250 million and 350–600 million people by the
2020s and 2050s, respectively [168]. Groundwater is the most common primary source of
drinking water in Africa, particularly in rural areas, and its recharge is projected to decrease
with decreased precipitation and runoff, resulting in increased water stress in those areas
[5]. Similarly, projections of hydroelectric power generation, conducted based on projections
of future runoff, indicate that hydropower generation would be negatively affected by climate
change, particularly in river basins situated in subhumid regions [169].
Several studies have linked climate change with health issues on the continent. For
example, results from the Mapping Malaria Risk in Africa project indicate changes in the
distribution of climate-suitable areas for malaria by 2020, 2050, and 2080 [170]. From an
agricultural perspective, the net crop revenues would likely fall by as much as 90 % based on
the three scenarios [171]. The changes in freshwater flows and greater intrusion of saltwater
into lagoons would affect species that form the basis of inland fisheries, which is considered to
be an important source of revenue [172]. The reduction in soil moisture could affect natural
systems in several ways [5], for example, significant extinctions in both plant and animal
546 V.P. Singh et al.
species as over 5,000 plant species could be impacted by climate change, mainly due to the
loss of suitable habitats. In addition, the Fynbos biome is projected to lose 51–61 % of its
extent due to decreased winter precipitation.
5.2. Europe
Overall, the annual mean temperature is likely to increase more than the global mean.
Northern Europe is likely to experience the greatest warming during winter, whereas in
summer the most dramatic warming will likely occur in the Mediterranean area. In addition,
the increase is more likely in the lowest winter temperature in comparison to average
temperature in northern Europe, and in southern and central Europe, the highest summer
temperatures are likely to increase more than the average summer temperature. Annual
precipitation is very likely to increase in most of northern Europe and decrease in most of
the Mediterranean area. Based on seasons, precipitation will likely increase in winter and
decrease in summer in central Europe, whereas the annual number of precipitation days is
very likely to decrease in the Mediterranean area. The daily precipitation extremes are very
likely to increase in northern Europe, and summer drought is likely to affect central Europe
and the Mediterranean area more. The duration of the snow season and snow depth are likely
to decrease in most of Europe [167].
European countries will face a range of impacts on water resources due to climate change
[5]. These impacts are summarized in this section. Annual average runoff is projected to
increase in northern Europe (north of 47N) by approximately 5–15 % up to the 2020s and by
9–22 % up to the 2070s, under the A2 and B2 scenarios and climate scenarios from two
different climate models [173], whereas in southern Europe (south of 47N), runoff is
projected to decrease by 0–23 % up to the 2020s and by 6–36 % up to the 2070s (for the
same set of assumptions). There is a possibility of a reduction in groundwater recharge in
central and eastern Europe [174], with a larger reduction in valleys [175] and lowlands, e.g., in
the Hungarian steppes [176].
An increase in irrigation water demand will likely make regions more prone to the drought
risk in the Mediterranean and some parts of central and eastern Europe [177]. Also, irrigation
needs will substantially increase in countries where they hardly exist today [178]. The risk of
flooding is projected to increase throughout the continent [5], and the regions most prone to
increases in flood frequencies are eastern Europe, then northern Europe, the Atlantic coast,
and central Europe, while projections for southern and southeastern Europe show significant
increases in drought frequency. In some regions, the risks of both floods and droughts are
projected to increase simultaneously. Increases in the intensity of daily precipitation events
are likely to be observed even in areas with a decrease in the mean precipitation, such as
central Europe and Mediterranean regions [179]. The Mediterranean regions and even much
of eastern Europe may experience an increase in dry periods by the late twenty-first century
[180]. The hydropower potential for all of Europe is expected to decline by 6 %, which
translates into a 20–50 % decrease around the Mediterranean, a 15–30 % increase in northern
and eastern Europe, and a stable hydropower pattern for western and central Europe
[181]. Extreme rainfall and droughts can increase the total microbial loads in freshwater
Climate Change and Its Impact on Water Resources 547
and have implications for disease outbreaks and water-quality monitoring [5]. The predicted
increase in extreme weather events is projected to increase yield variability [182] and reduce
average yield [183].
5.3. Asia
It is very likely that Asia will warm during the twenty-first century, and warming could
vary by region, for example, temperatures could be well above the global mean in central
Asia, the Tibetan Plateau, and northern Asia be above the global mean in East and South Asia,
and be similar to the global mean in Southeast Asia. Similarly, changes will likely be observed
in precipitation patterns; boreal winter precipitation is very likely to increase in northern Asia
and the Tibetan Plateau and likely to increase in eastern Asia and the southern parts of
Southeast Asia; summer precipitation is likely to increase in northern Asia, East and South
Asia, and most of Southeast Asia, but it is likely to decrease in central Asia.
Changes will likely be observed in extreme events, and the frequency of intense
precipitation events in parts of South Asia and in East Asia will likely increase; summer
heat waves/hot spells in East Asia will be of longer duration, more intense, and more frequent;
and tropical cyclones are likely to increase in East, Southeast, and South Asia.
Several impacts are likely to be observed in the water resources sector in Asian countries
[5]. One of the major changes is in seasonality and the amount of water flows in river systems.
That could significantly alter the variability of river runoff such that extremely low runoff
events might occur much more frequently in the crop-growing regions of the southwest parts
of Russia [184]. The availability of surface water from major rivers, such as the Euphrates and
Tigris, might be affected by the alteration in river flow. In comparison to 1961–1990, the
maximum monthly flow of the Mekong River is projected to increase more in the basin in
comparison to the delta, with a lower value estimated for the years 2010–2038 and a higher
value for the years 2070–2099 [5].
The central part of Asia is expected to witness an increased probability of events such as
mudflows and avalanches due to the rise in temperature [185]. There is likely to be a 27 %
decline in glacier extent, a 10–15 % decline in frozen soil area, an increase in flood and debris
flow, and more severe water shortages by 2050 compared with 1961–1990 in Northwest China
[186]. There is likely to be a reduction of 20–40 % in runoff per capita in Ningxia, Xinjiang,
and Qinghai Provinces by the end of the twenty-first century [187]. Based on the SRES A1B
scenario, there is likely to be an increase of 1.1–1.2 times in flood risk in Tokyo (Japan)
between 2050 and 2300 compared with present risk levels [188]. The gross per capita water
availability in India is projected to decline from approximately 1,820 m
3
/year in 2001 to as
little as 1,140 m
3
/year in 2050 as a result of population growth [189], which is likely to be
affected by spatiotemporal precipitation variability. Severe water stress will be one of the
most pressing environmental problems in South and Southeast Asia in the future. It is
estimated that under the full range of SRES scenarios, from 120 million to 1.2 billion and
from 185 to 981 million people will experience increased water stress by the 2020s and the
2050s, respectively [168]. The variability in runoff will have a significant effect on
hydropower-generating countries, such as Tajikistan [190]. The increases in water demand
548 V.P. Singh et al.
and soil-moisture deficit, along with a projected decline in precipitation, could lead to water-
related challenges in future rainfed crops in the plains of northern and northeastern China
[191]. In addition, in northern China, irrigation from surface water and groundwater sources is
projected to meet only 70 % of the water requirement for agricultural production due to the
effects of climate change and increasing demand [186].
5.4. North America
The annual mean warming of all of North America is very likely to exceed the global mean
warming in most areas, and the warming is likely to be greatest in winter in northern regions.
Based on the lowest winter temperature, northern North America is likely to witness an
increase in temperature that will be greater than the average winter temperature, whereas the
highest summer temperatures are likely to increase more than the average summer temper-
ature in the southwestern parts of the USA. The northeastern parts of the USA and Canada are
very likely to witness an increase in annual precipitation, whereas the American Southwest is
likely to witness a decrease in precipitation. Winter and spring precipitation is likely to
increase in southern Canada but decrease in summer. The snow depth in the northernmost
part of Canada will likely increase, whereas the snow season length and snow depth are very
likely to decrease in most of North America [192].
Annual mean precipitation is projected to decrease in the southwestern USA but increase
over most of the remainder of North America up to 2100, whereas increases in precipitation in
Canada are projected to be in the range of +20 % for the annual mean and +30 % for winter,
under the A1B scenario [5]. Some studies project widespread increases in extreme precipi-
tation but also droughts associated with greater temporal variability in precipitation.
The variability in runoff affects hydropower production significantly; for example, the
impact of lengthy droughts on the Great Lakes in 1999 significantly affected hydropower
production [193]. With an increase of 2–3 C in warming in the British Columbia Hydro
service areas, the hydroelectric supply under worst-case water conditions for winter peak
demand will likely increase; in addition, Colorado River hydropower yields will likely
decrease significantly [194], as will Great Lakes hydropower [195].
Waterborne diseases and degraded water quality are very likely to increase with heavier
precipitation. Waterborne diseases are likely to be clustered in key watersheds due to many
factors, including heavy precipitation in the USA [196] and extreme precipitation and warmer
temperatures in Canada [197]. Moreover, heavy runoff following severe rainfall can also
contaminate recreational waters through higher bacterial count [198]. Several investigations
highlighted that moderate climate change will likely increase yields of North American
rainfed agriculture, but with smaller increases and more spatial variability than in earlier
estimates [199]; however, many climate projections indicate decreasing yields (currently
under climate threshold) in terms of quality, or both, with even modest warming [200]. In
another example, water availability will be the major factor limiting agriculture in southeast
Arizona [201]. The changes in rainfall patterns and drought regimes could lead to ecosystem
disturbances [202] and cause the areal extent of drought-limited ecosystems to increase 11 %
per 1 C warming in the continental USA [203].
Climate Change and Its Impact on Water Resources 549
5.5. Central and South America (Latin America)
Central and South America will likely be warmer during the twenty-first century, and
annual mean warming will be larger than the global mean warming in most of parts, whereas
warming similar to that of the global mean is likely to be observed in southern South America.
Annual precipitation is likely to decrease in most of Central America, with the relatively dry
boreal spring becoming drier; in the southern Andes, relative precipitation changes will be
greatest in summer. Increasing precipitation is likely in Tierra del Fuego during winter and in
southeastern South America during summer.
According to different climate models, the projected mean temperature for Latin America
ranges from 1 to 4 C for the B2 emissions scenario and from 2 to 6 C for the A2 scenario for
2100 [5]. In the absence of climate change, the number of people living in already water-
stressed watersheds is estimated at 22.2 million (in 1995), whereas under the SRES scenarios,
this number is estimated to increase to between 12 and 81 million in the 2020s and to between
79 and 178 million in the 2050s [168]. The potential vulnerabilities in many regions of Latin
American countries are likely to increase as a result of rising populations and their concom-
itant increased demands on water supplies and irrigation and as a result of the expected drier
conditions in many basins. Glacial retreat is projected to impact the generation of hydroelec-
tricity in countries such as Colombia and Peru [204], whereas some small tropical glaciers
have already disappeared, which is likely to affect hydropower generation [74].
Approximately 31 % of the Latin American population lives in areas at risk of malaria
(i.e., tropical and subtropical regions) [205], and, based on SRES emissions scenarios and
socioeconomic scenarios, some projections indicate that additional numbers of people will
be at risk in areas around the southern limit of the disease distribution in South America
[206]. The reason for this is the decrease in the length of the transmission season of
malaria due to the reduction in precipitation, such as the Amazon and Central America.
There is also the possibility of a substantial increase in the number of people at risk of
dengue due to changes in the geographical limits of transmission in Mexico, Brazil, Peru,
and Ecuador [207].
Based on several studies using crop simulation models, under climate change, for com-
mercial crops, the number of people at risk of hunger under SRES emissions scenario A2 is
projected to increase by one million in 2020, while it is projected that there will be no change
for 2050 and that the number will decrease by four million in 2080 [5]. The biodiversity of the
region is likely to be affected due to a complex set of alterations comprising modifications in
rainfall and runoff, and a replacement of tropical forest by savannas is expected in eastern
Amazonia and in the tropical forests of central and southern Mexico, along with replacement
of semiarid by arid vegetation in parts of northeastern Brazil and most of central and northern
Mexico due to the synergistic effects of both land-use and climate changes [5].
5.6. Australia and New Zealand
All of Australia and New Zealand are very likely to warm during this century, comparable
overall to the global mean warming. Precipitation is likely to decrease in southern Australia in
winter and spring and in southwestern Australia in winter, whereas there is likely to be an
550 V.P. Singh et al.
increase in precipitation in the western part of the South Island of New Zealand. The extremes
based on daily precipitation are very likely to increase; increased risk of drought in southern
areas of Australia is also likely to occur.
Australia’s largest river basin, Murray-Darling, accounts for approximately 70 % of
irrigated crops and pastures [208], and according to the SRES A1 and B1 emissions
scenarios and a wide range of GCMs, annual streamflow in the basin is projected to fall
10–25 % by 2050 and 16–48 % by 2100, with salinity changes of 8to+19%and25 to
+72 %, respectively [209]. Similarly, water security problems are very likely to increase by
2030 in southern and eastern Australia and in parts of eastern New Zealand that are far
from major rivers [5]. The runoff in 29 Victorian catchments is projected to decline
by 0–45 % [210].
Energy production is likely to be affected in Australia and New Zealand in regions
where climate-induced reductions in water supplies lead to reductions in feed water for
hydropower turbines and cooling water for thermal power plants. Cropping and other
agricultural industries are likely to be threatened where irrigation water availability is
reduced [211].
Heavier rainfall events are likely to affect mosquito breeding and increase the variability in
annual rates of Ross River disease, particularly in temperate and semiarid areas [212]. Aus-
tralia faces a threat from dengue, and outbreaks of dengue have occurred with increasing
frequency and magnitude in far northern Australia over the past decade. Possible changes are
likely to occur in a range of geographical regions and in the seasonality of some mosquito-
borne infectious diseases, e.g., Ross River disease, dengue, and malaria. The other major
problem in water quality is eutrophication [213], and toxic algal blooms could increase in
frequency and be present for a longer time due to climate change. They can pose a threat to
human health for both recreation and consumptive water use and can kill fish and livestock
[214]. Alterations in the composition of species of freshwater habitats, with consequent
impacts on estuarine and coastal fisheries, are likely to occur due to multiple factors, including
saltwater intrusion as a result of sea-level rise, decreases in river flows, and increased drought
frequency [215,216].
6. ADAPTATION TO CLIMATE CHANGE
Climate change poses a new threat to adaptation policies, even though human being are
known to adapt and survive and develop methodologies to overcome many water-related
problems. The major problem of adaptation to climate change might be related to the multiple
dimensions involved, which include [217], for example, various sectors (water resources,
agriculture, industrial); the spatial scale (local, regional, national); type of action (physical,
technological, investment, regulatory, market); and climatic zones (dryland, floodplains,
mountains, and arctic regions). The major focus on climate adaptation has emerged since
the third assessment report of IPCC highlighted several strategies, including adaptations to
observed climate changes, planned adaptations to climate change in infrastructure design and
coastal zone management, the variable nature of vulnerability, and adaptive capacity [218].
Climate Change and Its Impact on Water Resources 551
For several decades measures have been in place to reduce climate impacts by understanding
their variability on decadal, annual, and seasonal scales so as to develop climate forecasting
methods, risk analysis based on disasters, and crop and livelihood diversification.
Various types of adaptations have been implemented around the world [217], and what
follows are some examples based on water-related sectors:
1. Drought: enhancing the use of traditional rainwater harvesting and water-conserving techniques,
building of shelter belts and windbreaks to improve the resilience of rangelands and setting up of
revolving credit funds (e.g., for Sudan, Africa) [219]; inclusion of drought-resistant plants,
adjustment of planting dates and crop variety, accumulation of commodity stocks as economic
reserves, creation of local financial pools (for Mexico and Argentina) [220]; creating employment
program options following drought in national government programs and assistance to small
subsistence farmers to increase crop production (e.g., in Botswana; [221]).
2. Sea-level rise: acquisition of land with a view to climate change to acquire coastal lands damaged/
prone to damage by storms or buffering other lands (e.g., New Jersey Coastal Blue Acres land
acquisition program in the USA) [222]. Installation of hard structures in areas vulnerable to coastal
erosion and adoption of a national climate change action plan integrating climate change concerns
into national policies (e.g., in Egypt) [223]. Introduction of participatory risk assessment, capacity
building for shoreline defense system design, construction of cyclone-resistant housing units, and
review of building codes (e.g., in Philippines) [224]. Adoption of Flooding Defense Act and
Coastal Defense Policy as precautionary measures allowing for the incorporation of emerging
trends in climate and building of a storm surge barrier taking a 50-cm sea-level rise into account
(e.g., in the Netherlands) [225].
6.1. Assessment of Adaptation Costs and Benefits
A few studies have investigated adaptation costs and benefits in the context of climate
change [217] and some of the studies these issues as they pertain to Bangladesh [226], Fiji and
Kiribati [227], and Canada [228]. Based on individual issues, some studies address sea-level
rise [229], agriculture [129], and water resource management [230].
The greatest number of studies have been carried out for costs and benefits related to
sea-level rise. Some of the important findings are as follows: (a) almost 100 % of coastal cities
and harbors in the countries that make up the Organisation for Economic Cooperation and
Development (OECD) should be protected, while the optimal protection for beaches and open
coasts would vary between 50 and 80 % [231]; (b) the total cost of sea-level rise could be
reduced by around 20–50 % for the US coastline if real estate market prices adjusted
efficiently as land became submerged [232]; (c) based on the IPCC SRES [166] the A1FI,
A2, B1, and B2 scenarios, with the exception of certain Pacific Small Island States, coastal
protection investments comprise a very small percentage of gross domestic product (GDP) for
the 15 most-affected countries by 2080 [217]; (d) based on a global-scale assessment,
uncertainties surrounding endowment values could lead to a 17 % difference in coastal
protection, a 36 % difference in the amount of land protected, and a 36 % difference in the
direct cost globally [233].
Adaptation studies for the agricultural sector may be based on increases in yield or on the
welfare of people at risk of hunger, which can be considered at the farm level or on the level of
552 V.P. Singh et al.
international trade [234,235]. For many countries located in tropical regions, low-cost
adaptation measures, such as modifying crop mixes and changing planting dates, will likely
be insufficient to offset the significant damages that will result from climate change
[235,236]. In another study, the benefits to the American economy from adaptation
measures increased from US$3.29 billion (2000 values) to US$4.70 billion (2000 values)
[234,237,238]. Adaptation measures could reduce the variability in welfare by up to 84 % in
the case of Mali [235].
Only a few studies have investigated adaptation costs and benefits in water resource
management; for example, the reliability of the water supply in the Boston metropolitan area
under climate change scenarios is estimated to be 93 % by 2100 on account of the expected
growth in water demand [230]. The costs of adaptation to climate change for storm water
management by water utilities in Canada, where the potential adaptation strategies include
building new treatment plants, improving the efficiency of existing plants, or increasing
retention tanks, were considered, and the results indicated that the adaptation costs for
Canadian cities could be as high as Canadian $9,400 million for a city like Toronto if
extreme events are considered [228]. The other major impact of climate change includes the
increase in energy demand due to increases in temperature. In one study, the global energy
costs related to heating and cooling would increase by US$2 billion to US$10 billion (1990
values) for a 2 C increase in temperature by 2100 and by US$51 billion to US$89 billion
(1990 values) for a 3.5 C increase [239]. The other major impact is sea-level rise, whose
associated global protection costs have been estimated at US$1,055 billion for a 1-m
sea-level rise [240].
6.2. Limitations in Adaptation to Climate Change
To overcome the impact of climate change, systems need to adapt or respond successfully
to climate variability and changes; therefore, the presence of an adaptive capacity has been
shown to be a necessary condition for the design and implementation of effective adaptation
strategies. Many factors that act as a limitation on adaptation to climate change [217] are
discussed in the following sections.
6.2.1. Threshold Level of Ecosystem
Several investigations lead to findings that the resilience of coupled socioecological
systems to climate change will depend on the rate and magnitude of climate change, and
when climate change persists beyond critical thresholds, some systems may not be able to
adapt to changing climate conditions without radically altering their functional state and
integrity [217]. The physical environment can change dramatically in response to changing
climatic patterns, for example, the resilience of kelp forest ecosystems, coral reefs,
rangelands, and lakes affected both by climate change and other pollutants beyond a
certain threshold level [241,242]. Persistent below-average rainfall and recurrent droughts
in the late twentieth century in the Sudano-Sahel region of Africa, have led to constricted
physical and ecological limits by contributing to land degradation, diminished livelihood
opportunities, food insecurity, and internal displacement of people [243]. Loss of sea ice
Climate Change and Its Impact on Water Resources 553
in Arctic sea threatens the survival of polar bears [244]. Climate change also significantly
affects the economy due to its impact on ecosystems, such as fisheries and agricultural
systems. This leads to significant challenges with respect to resource management
from ecosystem shifts, but such challenges are often outside the experience of
institutions [245].
6.2.2. Technological Limits
Adapting to new technologies serves as a potential means of adapting to climate variability
and changes, and the transfer of appropriate technologies to developing countries forms an
important component of the UNFCCC [246]. There are also potential limits to technology as
an adaptation response to climate change [217]; for example, technology is developed and
applied in a social context, and decision making under uncertainty may inhibit adaptation to
climate change [247], and although some adaptations may be technologically possible, they
may not be economically feasible or culturally desirable [248]. New technology is unlikely to
be equally transferable to all contexts and to all groups or individuals, regardless of the extent
of country-to-country technology transfer [249], and adaptations that are effective in one
location may be ineffective in other places.
6.2.3. Financial Barriers
The estimated total costs for the implementation of adaptation measures are quite high, and
they face a number of financial barriers; for example, preliminary estimates are that climate
proofing development could be as high as US$10 billion to US$40 billion/year [250]. Simi-
larly, institutions at the local level and individuals can be similarly constrained by the lack of
adequate financial resources; for example, farmers often cite the lack of adequate financial
resources as an important factor that constrains the use of adaptation measures, such as
irrigation systems, improved or new crop varieties, and diversification of farm operations
[251]. The lack of resources is likely to reduce the ability of low-income groups to afford
proposed adaptation mechanisms by raising the actuarial uncertainty in catastrophe risk
assessment, placing upward pricing pressure on insurance premiums and possibly leading to
reductions in risk coverage [252].
6.2.4. Informational and Cognitive Barriers
Risks associated with climate change are context specific [253], and adaptation responses
to climate change can be limited by human cognition [254]. Therefore, knowledge of the
factors responsible and their impacts and possible solutions do not necessarily lead to
adaptation. Perceptions of climate change risks vary, and the psychological dimensions of
evaluating long-term risk focus on changes in relation to climate change mitigation policies.
Some studies have explored the behavioral foundations of adaptive responses [217,255].
For example, thresholds of rapid climate change may induce different individual responses
influenced by trust in others, resulting in adaptive and nonadaptive behaviors [256]. In another
study [255] aimed at human cognition and adaptive capacity in populations living in
554 V.P. Singh et al.
flood-prone areas in Germany and farmers struggling with drought in Zimbabwe, the authors
note that divergence between perceived and actual adaptive capacity is a real barrier to
adaptive actions.
6.2.5. Social and Cultural Barriers
There are social and cultural limits to climate change adaptation where people and groups
experience, interpret, and respond to impacts [217]. Differences in understanding and prior-
itizing climate change issues across different social and cultural groups can limit adaptive
responses [256]. In addition, societies are responsible for changes to their environments, and
they alter their own vulnerability to climate fluctuations, as illustrated, for example, by the
development of the Colorado River Basin in the face of environmental uncertainty [257].
Several case studies have revealed that there exists a diversity of traditional practices for
ecosystem management under environmental uncertainty based on social regulation, mech-
anisms for cultural internalization of traditional practices, and the development of appropriate
worldviews and cultural values [258].
7. CONCLUSIONS
Various observations, including increases in global average air and ocean temperatures and
the melting of snow and ice, confirm that the climatic system is getting warmer. Impacts of
this warming have been observed on global and local scales in different water resource
sectors. The following conclusions are drawn from this study:
1. Overall changes in large-scale hydrologic cycles are observed based on spatiotemporal scales. The
noted observations include increased precipitation in high northern latitudes since the 1970s,
increased frequency in heavy precipitation events, runoff patterns, reductions in snow cover,
and shifts in the amplitude and timing of glacial runoff.
2. Based on global climate models, precipitation is likely to increase in high latitudes and decrease in
lower mid-latitude regions during the twenty-first century. The risks of flooding and drought are
likely to increase in many parts due to increases in precipitation intensity and variability. Water
availability and annual river runoff are likely to increase at high latitudes by the middle of the
twenty-first century, whereas many arid and semiarid areas are projected to suffer a decrease in
water resources.
3. The basic food sector (i.e., availability, stability, and access), which will likely be affected by
changes in the quantity and quality of water resources, will be vulnerable in the arid and semiarid
tropics and Asian and African mega deltas.
4. Water quality, which will likely be affected by climate change due to higher water temperatures,
precipitation extremes, flood and drought events, and pollution, will decrease because of imbal-
ances in several factors, including sediments, nutrients, dissolved organic carbon, pathogens,
pesticides, salt, and thermal pollution.
5. The quantity and quality of water resources will have a severe impact on ecosystems and human
health, which will be further impacted by sea-level rise that will extend areas of salinization of
groundwater and estuaries, resulting in a decrease in freshwater availability for humans and
ecosystems in coastal areas.
Climate Change and Its Impact on Water Resources 555
6. Climate change can aggravate freshwater systems, which are already stressed due to increases in
population and rising water demand in various sectors, changing economic activity, alterations in
land use, and urbanization. The global population rise will have repercussion on the global scale,
whereas demand in various affected sectors will impact regional water demand.
7. To ensure proper management of water resources, various strategies should be adopted based on
water availability and water demand depending on the different needs at local, regional, and
continental levels. Some of the strategies include water conservation, improved water use effi-
ciency by recycling water, development of water markets and implementation of virtual water
trade, and improved storage facilities to act as lifelines during drought periods. There is also a need
to improve decision making based on the efficient modeling of climate change related to the
hydrologic cycle through a better understanding of the uncertainties likely to exacerbate the
impacts of climate change.
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Chapter
The Colorado River flows 2300 km (about 1400 mi) from the high mountain regions of Colorado through seven basin states to the Sea of Cortez in Mexico (Figure 1). The river supplies much of the water needs of seven U.S. states, two Mexican states, and 34 Native American tribes. These represent a population of 25 million inhabitants, with a projection of 38 million by the year 2020. Approximately 2% of the basin is in Mexico. The Colorado does not discharge a large volume of water. Because of the scale of impoundments and withdrawals relative to its flow, the Colorado has been called the most legislated and managed river in the world. It has also been called the most “cussed” and “discussed” river in the United States. About 86% of the Colorado’s annual runoff originates within only 15% of the area, in the high mountains of Colorado and the Wind River Range in Wyoming. In the semiarid Southwest, even relatively small changes in precipitation can have large impacts on water supplies. The coefficient of variation for the Colorado is about 33%.
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A simple statistical model of daily precipitation based on the gamma distribution is applied to summer (JJA in Northern Hemisphere, DJF in Southern Hemisphere) data from eight countries: Canada, the United States, Mexico, the former Soviet Union, China, Australia, Norway, and Poland. These constitute more than 40% of the global land mass, and more than 80% of the extratropical land area. It is shown that the shape parameter of this distribution remains relatively stable, while the scale parameter is most variable spatially and temporally. This implies that the changes in mean monthly precipitation totals tend to have the most influence on the heavy precipitation rates in these countries. Observations show that in each country under consideration (except China), mean summer precipitation has increased by at least 5% in the past century. In the USA, Norway, and Australia the frequency of summer precipitation events has also increased, but there is little evidence of such increases in any of the countries considered during the past fifty years. A scenario is considered, whereby mean summer precipitation increases by 5% with no change in the number of days with precipitation or the shape parameter. When applied in the statistical model, the probability of daily precipitation exceeding 25.4 mm (1 inch) in northern countries (Canada, Norway, Russia, and Poland) or 50.8 mm (2 inches) in mid-latitude countries (the USA, Mexico, China, and Australia) increases by about 20% (nearly four times the increase in mean). The contribution of heavy rains (above these thresholds) to the total 5% increase of precipitation is disproportionally high (up to 50%), while heavy rain usually constitutes a significantly smaller fraction of the precipitation events and totals in extratropical regions (but up to 40% in the tropics, e.g., in southern Mexico). Scenarios with moderate changes in the number of days with precipitation coupled with changes in the scale parameter were also investigated and found to produce smaller increases in heavy rainfall but still support the above conclusions. These scenarios give changes in heavy rainfall which are comparable to those observed and are consistent with the greenhouse-gas-induced increases in heavy precipitation simulated by some climate models for the next century. In regions with adequate data coverage such as the eastern two-thirds of contiguous United States, Norway, eastern Australia, and the European part of the former USSR, the statistical model helps to explain the disproportionate high changes in heavy precipitation which have been observed.