Chapter 15 Human Ecology • MHR 529on smokestacks. Nitrogen oxide emissions have
been reduced by design improvements in cars.
Acid precipitation is still occurring, however, and
is having damaging effects on ecosystems.
Photochemical Smog
Nitrogen oxides are also involved in other types of
air pollution. In the presence of sunlight and water
vapour, they react with another group of gases
called hydrocarbons to produce low-atmosphere
ozone and PANs (peroxyacyl nitrates). These are
the major constituents of photochemical smog—
the brown or grey haze that tends to hang over the
area where it was produced. See Figure 15.20 for
an example. Both of these constituent substances
irritate eyes and lungs and may lead to long-term
respiratory illnesses, such as emphysema. Smog may
also contain particles of ash, asbestos, lead, dust, and
other materials. It is more common in areas that
have undergone rapid growth and industrialization.
Many cities have now reached the stage where
clean-up measures, often involving the use of
cleaner-burning fuels in automobiles, have reduced
the frequency and severity of smog. However, the
problem is still increasing in some centres.
Figure 15.20Have you ever experienced photochemical
smog like that shown in this photo?
Drinking Water
There are two main issues relating to human impact
on water supply. The first is that the supply is not
limitless. Although water is a renewable resource,
it is possible to use up the supply faster than it
can be replenished. Second, human activities
negatively affect the supply that does exist,
reducing the useable portion still more.
Over 97 percent of the water on Earth is ocean
water (which is salty and therefore undrinkable)
and approximately two percent is frozen in glaciers
and polar ice caps. This leaves less than one percent
as fresh water, contained in bodies of surface water
(lakes, rivers) and in aquifers(reservoirs of ground
water held below Earth’s surface). In addition, this
relatively scarce supply of water is unequally
distributed. While some parts of the world (for
example, Canada) have a lot, desert and semiarid
regions — which make up large portions of Africa,
for example — have little.
The history of human population has, in large
part, been dependent on water supply or on how we
have manipulated it. Irrigation of crops made the
first human cities possible. One of the most amazing
features of Rome was its system of elevated
aqueducts — channels that carried water great
distances to supply the city. During the Industrial
Revolution and the period of rapid population
growth that followed it, water demand increased
dramatically. The total amount of water taken out
of rivers, lakes, and aquifers in 2000 was nine
times greater than the amount withdrawn in 1900.
This is mostly due to population growth, as annual
per capita usage has only doubled since that time
and has actually declined slightly in recent years.Population Growth and the Water Supply
Despite this increase in efficiency, many experts
worry that we will still soon be faced with a water
shortage. Projections combining future population
size with water availability suggest that by 2025,
40 percent of the world’s population may experience
serious difficulties in agriculture, industry, or
human health if they have to rely on natural water
sources. Twenty countries were already suffering
from water stress in 1998, having less than
1000 cubic metres of water per person per year.
(Per capita usage in Canada was 1622.7 cubic
metres in 2000.) In addition, when humans divert
water for their own purposes it is often at the cost
of supplies for other species. In portions of the
western United States, for example, choices already
have to be made between irrigating crops or saving
water for rivers used by salmon for spawning.
Human activities often reduce supplies of fresh
water further as a result of pollution. Non-gaseous
waste materials including untreated sewage, solid
and liquid waste from factories, and run-off from
agricultural fields and city streets (containing