CHAPTER 20 Global Climate Change
522
Key Terms
aerosols (p. 500)
climate-feedback mechanisms
(p. 514)
climate system (p. 495)
wavelengths of the energy being transmitted, as well as the size
and nature of the absorbing or reflecting substance, determine
whether solar radiation will be scattered and reflected back to
space or absorbed.
Radiant energy that is absorbed heats Earth and eventually is
reradiated skyward. Because Earth has a much lower surface
temperature than the Sun, its radiation is in the form of longwave infrared radiation. Because the atmospheric gases, primarily
water vapor and carbon dioxide, are more efficient absorbers of
terrestrial (long-wave) radiation, the atmosphere is heated from
the ground up. The transmission of short-wave solar radiation by
the atmosphere, coupled with the selective absorption of Earth
radiation by atmospheric gases, results in the warming of the
atmosphere and is referred to as the greenhouse effect.
Several explanations have been formulated to explain climate
change. Current hypotheses for the “natural” mechanisms (causes
unrelated to human activities) of climate change include (1) plate
tectonics, rearranging Earth’ s continents closer to or farther from
the equator, (2) variations in Earth’ s orbit, involving changes in
the shape of the orbit (eccentricity), angle that Earth’ s axis makes
with the plane of its orbit (obliquity), and/or the wobbling of the
axis (precession), (3) volcanic activity, reducing the solar radiation
that reaches the surface, and (4) changes in the Sun’ s output
associated with sunspots.
Humans have been modifying the environment for thousands
of years. By altering ground cover with the use of fire and the
overgrazing of land, people have modified such important climatological factors as surface reflectivity (albedo), evaporation rates,
and surface winds.
By adding carbon dioxide and other trace gases (methane,
nitrous oxide, and chlorofluorocarbons) to the atmosphere,
modern humans are contributing to global climate change in a
significant way.
When any component of the climate system is altered, scientists must consider the many possible outcomes, called climatefeedback mechanisms. Changes that reinforce the initial change are
called positive-feedback mechanisms. For example, warmer surface
temperatures cause an increase in evaporation, which further
increases temperature as the additional water vapor absorbs more
radiation emitted by Earth. On the other hand, negative-feedback
mechanisms produce results that are the opposite of the initial
change and tend to offset it. An example would be the negative
effect that increased cloud cover has on the amount of solar
energy available to heat the atmosphere.
Global climate is also affected by human activities that contribute to the atmosphere’ s aerosol (tiny, often microscopic, liquid
and solid particles that are suspended in air) content. By reflecting sunlight back to space, aerosols have a net cooling effect. The
effect of aerosols on today’ s climate is determined by the amount
emitted during the preceding couple of weeks, while carbon
dioxide remains for much longer spans and influences climate
for many decades.
Because the climate system is so complex, predicting specific
regional changes that may occur as the result of increased levels
of carbon dioxide in the atmosphere is speculative. However,
some possible consequences of greenhouse warming include
(1) altering the distribution of the world’ s water resources,
(2) a probable rise in sea level, (3) a greater intensity of tropical
cyclones, and (4) changes in the extent of Arctic sea ice and
permafrost.
Due to the complexity of the climate system, not all future
shifts can be foreseen. Thus, “surprises” (relatively sudden
unexpected changes in climate) are possible.
greenhouse effect (p. 504)
negative-feedback mechanism
(p. 514)
oxygen isotope analysis (p. 497)
paleoclimatology (p. 496)
positive-feedback mechanism
(p. 514)
proxy data (p. 496)
sunspots (p. 508)
522
Key Terms
aerosols (p. 500)
climate-feedback mechanisms
(p. 514)
climate system (p. 495)
wavelengths of the energy being transmitted, as well as the size
and nature of the absorbing or reflecting substance, determine
whether solar radiation will be scattered and reflected back to
space or absorbed.
Radiant energy that is absorbed heats Earth and eventually is
reradiated skyward. Because Earth has a much lower surface
temperature than the Sun, its radiation is in the form of longwave infrared radiation. Because the atmospheric gases, primarily
water vapor and carbon dioxide, are more efficient absorbers of
terrestrial (long-wave) radiation, the atmosphere is heated from
the ground up. The transmission of short-wave solar radiation by
the atmosphere, coupled with the selective absorption of Earth
radiation by atmospheric gases, results in the warming of the
atmosphere and is referred to as the greenhouse effect.
Several explanations have been formulated to explain climate
change. Current hypotheses for the “natural” mechanisms (causes
unrelated to human activities) of climate change include (1) plate
tectonics, rearranging Earth’ s continents closer to or farther from
the equator, (2) variations in Earth’ s orbit, involving changes in
the shape of the orbit (eccentricity), angle that Earth’ s axis makes
with the plane of its orbit (obliquity), and/or the wobbling of the
axis (precession), (3) volcanic activity, reducing the solar radiation
that reaches the surface, and (4) changes in the Sun’ s output
associated with sunspots.
Humans have been modifying the environment for thousands
of years. By altering ground cover with the use of fire and the
overgrazing of land, people have modified such important climatological factors as surface reflectivity (albedo), evaporation rates,
and surface winds.
By adding carbon dioxide and other trace gases (methane,
nitrous oxide, and chlorofluorocarbons) to the atmosphere,
modern humans are contributing to global climate change in a
significant way.
When any component of the climate system is altered, scientists must consider the many possible outcomes, called climatefeedback mechanisms. Changes that reinforce the initial change are
called positive-feedback mechanisms. For example, warmer surface
temperatures cause an increase in evaporation, which further
increases temperature as the additional water vapor absorbs more
radiation emitted by Earth. On the other hand, negative-feedback
mechanisms produce results that are the opposite of the initial
change and tend to offset it. An example would be the negative
effect that increased cloud cover has on the amount of solar
energy available to heat the atmosphere.
Global climate is also affected by human activities that contribute to the atmosphere’ s aerosol (tiny, often microscopic, liquid
and solid particles that are suspended in air) content. By reflecting sunlight back to space, aerosols have a net cooling effect. The
effect of aerosols on today’ s climate is determined by the amount
emitted during the preceding couple of weeks, while carbon
dioxide remains for much longer spans and influences climate
for many decades.
Because the climate system is so complex, predicting specific
regional changes that may occur as the result of increased levels
of carbon dioxide in the atmosphere is speculative. However,
some possible consequences of greenhouse warming include
(1) altering the distribution of the world’ s water resources,
(2) a probable rise in sea level, (3) a greater intensity of tropical
cyclones, and (4) changes in the extent of Arctic sea ice and
permafrost.
Due to the complexity of the climate system, not all future
shifts can be foreseen. Thus, “surprises” (relatively sudden
unexpected changes in climate) are possible.
greenhouse effect (p. 504)
negative-feedback mechanism
(p. 514)
oxygen isotope analysis (p. 497)
paleoclimatology (p. 496)
positive-feedback mechanism
(p. 514)
proxy data (p. 496)
sunspots (p. 508)