Environmental Engineering Reference
In-Depth Information
We are in the middle of a large, uncontrolled experiment on
the only planet we have.
D
ONALD
K
ENNEDY
16-1 PAST CLIMATE CHANGE
AND THE NATURAL GREENHOUSE
EFFECT
Mount Pinatubo's eruption temporarily altered the
temperature of the entire earth. This chapter discusses
the considerable body of evidence indicating that the
earth's troposphere is warming (partly because of hu-
man activities), how our activities are depleting ozone
in the stratosphere, and what we can do about these
threats. It addresses the following questions:
Science: Past Historic Changes
in the Earth's Temperature
The earth has experienced prolonged periods of
global warming and global cooling.
Changes in the earth's climate are neither new nor un-
usual. Over the past 4.7 billion years, the planet's cli-
mate has been altered by volcanic emissions, changes
in solar input, continents moving as a result of shifting
tectonic plates (Figure 4-8, p. 72), strikes by large mete-
ors, and other factors.
Over the past 900,000 years, the average tempera-
ture of the troposphere has experienced prolonged pe-
riods of
global cooling
and
global warming
(Figure 16-2,
top left). These alternating cycles of freezing and thaw-
ing are known as
glacial and interglacial
(between ice
ages)
periods.
During each cold period, thick glacial ice covered
much of the earth's surface for about 100,000 years.
For roughly 12,000 years, we have had the good for-
tune to live in an interglacial period characterized by
a fairly stable climate and average global surface tem-
perature (Figure 16-2, top, right). In other words,
since agriculture began, the global climate has been
favorable to life as we know it. However, even during
this generally stable period, regional climates have
changed significantly.
Past temperature changes such as those depicted
in Figure 16-2 are estimated by analysis of radioiso-
topes in rocks and fossils, plankton and radioisotopes
in ocean sediments, ice cores from ancient glaciers
(Figure 16-3), temperature measurements taken at dif-
ferent depths from boreholes drilled deep into the
earth's surface, pollen from lake bottoms and bogs,
tree rings, historical records, and temperature mea-
surements (since 1860).
How have the earth's temperature and climate
changed in the past?
■
How might the earth's temperature change in the
future?
■
What factors influence the earth's average
temperature?
■
What are some possible beneficial and harmful
effects of a warmer earth?
■
How can we slow or adapt to projected increases in
the earth's temperature?
■
How have human activities depleted ozone in the
stratosphere, and why should we care?
■
How can we slow and eventually reverse ozone
depletion in the stratosphere caused by human
activities?
■
KEY IDEAS
■
During its long history, the earth has experienced
prolonged periods of global warming and global cool-
ing. Considerable evidence indicates that the earth's
troposphere is warming, partly because of human ac-
tivities, and that this trend will lead to significant cli-
mate change during this century.
■
Global warming is one of the most serious environ-
mental threats faced by humanity.
■
Although we cannot stop climate change, we
can use existing technological and policy options to
help slow the rate of climate change and adapt to its
effects.
■
Widespread use of certain long-lived chemicals has
reduced ozone levels in the stratosphere, which al-
lows more harmful ultraviolet radiation to reach the
earth's surface.
■
If countries adhere to its terms, an international
treaty phasing out ozone-depleting chemicals should
return ozone concentrations in the stratosphere to
1980 levels by 2050 and to 1950 levels by 2100.
Science: The Natural Greenhouse Effect
Certain gases in the atmosphere absorb heat and
warm the lower atmosphere.
In addition to incoming sunlight, a natural process
called the
greenhouse effect
(Figure 5-5, p. 82) warms the
earth's lower troposphere and surface. Swedish chem-
ist Svante Arrhenius first recognized this natural tro-
pospheric heating effect in 1896. Since then, numerous
laboratory experiments and measurements of atmo-
spheric temperatures at different altitudes have con-
firmed this relationship. It is now one of the most
widely accepted theories in the atmospheric sciences.
