438 CHAPTER 12: Gases and the Kinetic–Molecular Theory
CC The Environment
HEMISTRY IN USE
The Greenhouse Effect
During the twentieth century, the great increase in our use
of fossil fuels caused a significant rise in the concentration of
carbon dioxide, CO 2 , in the atmosphere. Scientists believe
that the concentration of atmospheric CO 2 could double by
early in the 21st century, compared with its level just before
the Industrial Revolution. During the last 100 to 200 years,
the CO 2 concentration has increased by 25%. The curve in
Figure (a) shows the recent steady rise in atmospheric CO 2
concentration.
Energy from the sun reaches the earth in the form of light.
Neither CO 2 nor H 2 O vapor absorbs the visible light in sun-
light, so they do not prevent it from reaching the surface of
the earth. The energy given off by the earth in the form of
lower-energy infrared (heat) radiation, however, is readily
absorbed by both CO 2 and H 2 O (as it is by the glass or plas-
tic of greenhouses). Thus, some of the heat the earth must
lose to stay in thermal equilibrium can become trapped in
the atmosphere, causing the temperature to rise (Figure b).
This phenomenon, called the greenhouse effect,has been
the subject of much discussion among scientists and the topic
of many articles in the popular press. The anticipated rise in
average global temperature by the year 2050 due to increased
CO 2 concentration is predicted to be 2 to 5°C.
An increase of 2 to 5°C may not seem like much. How-
ever, this is thought to be enough to cause a dramatic change
in climate, transforming now productive land into desert and
altering the habitats of many animals and plants beyond their
ability to adapt. Another drastic consequence of even this
small temperature rise would be the partial melting of the
polar ice caps. The resulting rise in sea level, though only a
few feet, would mean that water would inundate coastal cities
such as Los Angeles, New York, and Houston, and low-lying
coastal areas such as southern Florida and Louisiana. On a
global scale, the effects would be devastating.
The earth’s forests and jungles play a crucial role in main-
taining the balance of gases in the atmosphere, removing CO 2
and supplying O 2. The massive destruction, for economic
reasons, of heavily forested areas such as the Amazon rain
forest in South America is cited as another long-term con-
tributor to global environmental problems. Worldwide, more
than 3 million square miles of once-forested land is now bar-
ren for some reason; at least 60% of this land is now unused.Environmental scientists estimate that if even one quarter of
this land could be reforested, the vegetation would absorb
1.1 billion tons of CO 2 annually.
Some scientists are more skeptical than others about the
role of human-produced CO 2 in climate changes and, indeed,(a) A plot of the monthly average CO 2 concentration in
parts per million (ppm), measured at Mauna Loa
Observatory, Hawaii, far from significant sources of CO 2
from human activities. Annual fluctuations occur because
plants in the Northern Hemisphere absorb CO 2 in the
spring and release it as they decay in the fall.Seasonal
fluctuationsCarbon dioxide concentration (parts per million)348344340336332328324320316312Year352356MeanSummer (low)Winter (high)36060 64 68 72 76 80 84 88 9236496