In 2000, 150 million people lived in cities with perennial water
shortage (i.e., annual water availability <100 L per person perday) within their urban extent. Many more people—886 million—
lived in cities with seasonal water shortage (i.e., monthly water
availability <100 L per person per day), with insufficient flows
occurring in at least 1 mo of the year. If water in buffer zones
adjacent to an urban extent is considered available to the extent’s
residents, fewer urban dwellers are under perennial or seasonal
water shortage. For instance, a 100-km buffer reduces the number
of urban dwellers facing perennial or seasonal shortage to 24
million and 312 million, respectively.
Population growth will greatly increase the number of urban
dwellers who live with water shortage (Fig. 1). In 2050, we forecast
that 993 million people will live in cities with perennial water
shortage within their urban extent. At the 100-km buffer distance,
this number would fall to 145 million. In 2050, similarly, 3.1 billion
urban residents would confront seasonal water shortage within
their urban extent, or 1.3 billion at the 100-km buffer distance.
There is little difference between our two demographic scenarios
(indicated by the lines on the bars in Fig. 1), with numbers for
perennial and seasonal water shortage in the Basic Demographic
scenario being approximately 3.3% and 0.7% less, respectively,
than those in the Ecological Factors scenario.
Climate and land use change (hereafter referred to simply as
“climate change”) will further increase the number of urban
dwellers facing water shortage (Fig. 1). In some cities, water
availability will decrease owing to climate change, whereas other
cities will see increases, with more cities having less water than
having increased flows. Averaging across all climate change
scenarios, ≈100 million more urbanites will live under perennial
shortage under climate change conditions than under current
climate. At the 100-km buffer difference, the equivalent figure is
22 million. Climate change does not greatly change the aggregate
number of urban residents facing seasonal shortage, although the
effect for particular cities may be large. Our hydrologic model
does not fully account for water storage through glaciers and
snowpack, an important source of water for many cities (17–19),
and aggregate effects might emerge if this issue could be investigated
in greater depth. Interestingly, there is relatively little
variation among the impacts on water shortage of our four scenarios
of climate change. At the 100-km buffer distance, the
coefficient of variation of perennial and seasonal water shortage
across all four climate change scenarios and both demographic
scenarios was only 2.7% and 0.3%, respectively.
Fig. 2 shows water shortage status for cities of more than 1
million people in 2000. Perennial water shortage is generally
confined to cities in the Middle East and North Africa. Seasonal
shortage is much more geographically widespread, occurring on
all continents and in many different climates. Rapidly urbanizing
China and India will have a large number of cities with seasonal
water shortage by 2050.
Urban population can be usefully divided into three categories
reflecting their perennial water shortage status (Table 1). A total
of 162 million people will live in cities that will have perennial
water shortage in 2050. The majority of people in this category
will be in Asia (94 million), although Africa will have a greater
percentage (7.7%) of total urban dwellers under perennial water
shortage. A second category is people in cities that will not have
perennial water shortage by 2050 at the 100-km buffer distance,
but the buffer distance needed to avoid perennial water shortage
will increase from 2000 to 2050. This potentially implies infrastructure
investment to enable short-scale (<100 km) water
transport to satisfy the needs of a projected 2050 population of
720 million. Again, the majority of people in this category will be
in Asia (338 million), but Africa will have a greater percentage
(36.3%) of total urban dwellers in this category. Finally, a third
category is people in cities that do not seem likely to have
problems with perennial water shortage, whose population will
grow from 1.0 billion to 2.9 billion from 2000 to 2050. Many city
dwellers who fall into this residual category will, however, face
seasonal water shortage.
One useful way to visualize the cumulative impact of urban
water consumption by the thousands of cities in our study area is
to examine them relative to freshwater ecoregions. Freshwater
ecoregions are areas with similar ecological characteristics and
are often defined relative to major hydrologic units (20). Freshwater
ecoregions with high numbers of urbanites with insufficient
water (Fig. 3, Upper) will potentially have flows inadequate to
maintain biodiversity, because by our definition of water shortage
there will be at least 1 mo per year in which some rivers in
an ecoregion have essentially all water withdrawn for urban use.
These freshwater ecoregions with substantial urban water shortage
populations vary widely in ecological context, from wet river
basins like the Ganges Delta and Plain (119 million people in
water shortage by 2050) to the endorheic basins of the Arabian
Interior (40 million people). Even some of West Africa’s tropical
river basins with substantial precipitation will have extensive
water shortage by 2050, including the Bight Drainages of Nigeria,
Benin, and Togo (92 million people).
Potential ecological impacts vary depending on the freshwater
ecoregion and the taxonomic group considered. Fig. 3 (Lower)
shows the number of species of freshwater fish in each freshwater
ecoregion, a taxonomic group especially vulnerable to water
withdrawals. The Arabian Interior has fewer than 50 freshwater
species, perhaps not surprising given its dry climate, whereas the
Ganges Delta and Plain has more than 250 species of fish (20).
Of particular conservation concern is the Western Ghats of India,
which will have 81 million people with insufficient water by
2050 but also houses 293 fish species, 29% of which are endemic
to this ecoregion and occur nowhere else in the world.
It is difficult to quantitatively predict how many fish species
globally will be imperiled by increased future urban water use,
simply because the current relationship between urban water use
and imperilment is unknown, because the current conservation
status of many fish species has not been assessed. One exception
Fig. 1. Number of people living in cities with either perennial or seasonal
water shortage (<100 L per person per day). Shortage numbers are shown
for current conditions (ca. 2000), with projected population growth (2050),
and with both population growth and climate change (2050). Errors bars are
the range across various scenarios of population growth and climate change.
Shortage numbers are shown for water available within the urban extent
(0 km) as well as varying buffer distances, with a large spatial area
การแปล กรุณารอสักครู่..