0 100 200 300 400 500
Los Angeles, U.S.
Dar es Salaam, Tanzania
San Diego, U.S.
Pôrto Alegre, Brazil
Santa Ana, U.S.
Long Beach, U.S.
If cities have first
other water-using sectors
If cities are last
Parched cities By 2050, some
3. 5 billion to 4. 4 billion people
around the world will live with
limited access to water, more than
1 billion of them in cities. Among
482 cities, more than a quarter will
face demands that outpace supply,
according to a study that analyzed
water sources and demands.
In general, urban growth is the
main driver of cities’ future water
deficits. Los Angeles tops the list
because its population is expected
to boom even as climate change
dries up its water sources. Cities
will be worse off if other sectors
get priority for water access.
SOURCE: M. FLÖRKE, C. SCHNEIDER AND
R. MCDONALD/NATURE SUSTAINABILITY 2018
As greenhouse gases build
up in Earth’s atmosphere,
trapping heat and altering
the planet’s weather and
climate, water will become
more precious. This map
shows how water stress —
the ratio of water use to
water supply — is expected
to look by the year 2040.
It assumes a “
business-as-usual” scenario in which
carbon emissions rise
steadily. The highest stress
is expected in areas where
water supply is vulnerable
because of already arid
climates and growing
year, it looked as if the more than 4 million people living in Cape
Town, South Africa, were going to run out of water. Officials
calculated a “Day Zero” in April when the taps would run dry.
Only through belated and desperate conservation measures,
such as slashing the amount of water for irrigating crops, did
city residents eke through until the rainy season began in May.
That Cape Town crisis is almost certainly the first of many.
In the face of such inexorable changes, it’s easy to despair. But
science offers hope, in the form of alternative paths forward.
Computer modelers at MIT, for example, find that policies to
fight climate change, such as the 2015 Paris agreement that the
United States announced its intention to pull out of last year
(SN Online: 6/1/17), can reduce the severity of future water
shortages. If nations follow commitments similar to those in
the agreement, 60 million people across Asia could avoid dire
water scarcity by 2050, the team wrote in June in Environmental
But the Paris agreement is not enough. As research increasingly makes clear, there are trade-offs and decisions to be
made. Cape Town’s experience shows how governments need
to better prepare for the competing demands on water supplies. Municipalities may need to raise the cost of water to the
point where people value it enough to conserve it.
“We can address the problem by thinking about technological solutions, but we also have to think about changing our
behavior,” says Martina Flörke, a hydrologist and environmental scientist at the University of Kassel in Germany. “If we can
make clear ... that water has value, that it’s an ecosystem service
that we use and have to take care of — then we are really thinking about how to adapt.” s
s United Nations. “SDG 6 Synthesis Report 2018 on Water
and Sanitation.” bit.ly/UNwater2018
Top 20 cities with largest urban water deficits in 2050
Ratio of withdrawal
Low to medium (10–20%)
Medium to high (20–40%)
Extremely high (>80%)