Biology 12

Chapter 15 Human Ecology • MHR 525

decisions. Such decisions require information
about the costs and benefits of human practices,
as well as about the details of how ecosystems
function. This is a big undertaking. Looking at each
of the renewable resources shown in Figure 15.15
on page 523 is a good place to begin.

Clean Air

Earth’s atmosphere is made up of a mixture of gases
— normally about 79 percent nitrogen, 21 percent
oxygen, and 0.03 percent carbon dioxide. It also
contains varying amounts of pollutants— thousands
of chemicals that adversely affect living things. Our
production of these pollutants currently exceeds the
ability of Earth’s sinks to absorb them or convert
them into harmless forms. Four of the most
important categories of air pollution today include
excess greenhouse gases, acid precipitation,
photochemical smog, and the production of
chemicals that deplete the ozone layer. As
Figure 15.17 shows, there may be several types of
pollutants contributing to each of these problems,
which often makes it difficult to determine who is

responsible and what steps can be taken to remedy

the situation.

Greenhouse Gases and Global Warming

Several of the gases in the atmosphere produce

what is called the greenhouse effect; that is, they

allow sunlight to pass through the atmosphere and

hit Earth. As illustrated in Figure 15.18 on the

following page, they then absorb and re-radiate most

of the heat that bounces back. Some of the most

important gases involved in this process include

carbon dioxide, methane (CH 4 ), and nitrous oxide

(NOx). The trapping of heat is actually natural and

normal. Without it, most of the Sun’s heat would

radiate away from Earth and average temperatures

would be below freezing. Most of the carbon

dioxide component of greenhouse gases has

accumulated over millions of years as a result of

the cellular respiration of living things; nitrous

oxide and methane are also naturally produced,

chiefly by the process of digestion in animals.

In recent decades, however, human activities

have added new gases to the list of those that

produce the greenhouse effect. These include nitrous

oxide from a variety of sources (such as burning

coal and nitrogen fertilizer), chlorofluorocarbons

(commonly referred to as CFCs and used primarily

as refrigerants), and halons (a group of gases used

in fire extinguishers).

Even more significantly, these activities have

dramatically increased the amounts of normal

gases being produced, particularly carbon dioxide.

In other words, humans have added anthropogenic

The Population Reference Bureau estimated in 1998 that it

would cost $6 billion a year to provide basic education for

all humans in the developing world and $9 billion a year for

clean water and basic sanitation. The amount spent annually

on cosmetics in the United States is $8 billion; approximately

$11 billion is spent on ice cream in Europe every year.

BIO FACT

Figure 15.17Each human activity may

produce more than one type of air

pollution, and each type of air pollution

may have more than one effect. Which of

the human activities shown contributes

to the most types of air pollution?

ozone layer destruction

global warming

acid precipitation

smog

Source:

vehicle exhaust

fossil fuel burning

paints

deforestation

farms

refrigerants

fire extinguishers

plastic foam

pesticides

Air pollutants:

nitrogen oxides ( )

hydrocarbons (HC)

carbon monoxide (CO)

sulfur dioxide ( )

NOx

SO 2

halons (contain bromine – Br)

chlorofluorocarbons (CFCs)

methane ( )

carbon dioxide ( )CO 2

CH 4