Geoscience Reference
In-Depth Information
5
Longwave radiation
absorbed by greenhouse
gases heats Earth's surface
(counterradiation)
4
Longwave radiation
absorbed by greenhouse
gases escapes to space
3
Longwave radiation
absorbed by
greenhouse gases
2
Some longwave
radiation escapes
directly to space
Figure 4.7 The greenhouse effect.
Earth absorbs shortwave radiation
from the Sun and emits it as long-
wave energy. Some of this emitted
longwave radiation flows directly to
space, but most is absorbed by car-
bon dioxide (CO
2
) and water vapor
in the atmosphere. Subsequently,
this longwave radiation either es-
capes into space or reflects back to
the Earth's surface through the pro-
cess of counterradiation.
1
Shortwave radiation
from the Sun absorbed
at the surface
e
d
cerned, is that the balance is currently just right for the planet
to be neither too hot nor too cold. To see the significance of this
balance, consider the planets Mars and Venus. In both places,
CO
2
is the dominant component of the atmosphere, comprising
about 96% of the volume on each planet. On Mars, the overall
density (measured by pressure) of the atmosphere at the surface
is about 1000 times less than that of Earth; thus, there is compar-
atively much less CO
2
on Mars than on Earth. As a result, Mars
is much colder than Earth, with an average temperature of -63°C
(-81°F). With respect to Venus, the atmosphere there is about
90 times denser at the surface than Earth's atmosphere; thus, the
concentration of CO
2
on Venus is much higher than on Earth.
Consequently, Venus is considered to have a runaway green-
house effect, with an average temperature of 500°C (932°F)!
Although Venus provides an extreme example of the green-
house effect, it is a context through which we can view the con-
cerns that many scientists have regarding the ongoing human
impact on the greenhouse process through the combustion of
fossil fuels for energy. Since the middle of the 19th century, lev-
els of atmospheric CO
2
have steadily increased largely through
our consumption of fossil fuels. To see how these levels have
changed, examine Figure 4.8, which shows CO
2
measurements
obtained from Mauna Loa in Hawaii. These data indicate that
atmospheric CO
2
increased approximately 90 parts per million
(ppm) between 1958 and 2010. Most climatologists believe this
increase is a primary cause of global climate change and is the
negative side of the greenhouse effect that many people associ-
ate with the relationship. This important issue will be covered
in greater detail in Chapter 9.
390
Monthly mean atmospheric
carbon dioxide at the
Mauna Loa Observatory, Hawaii
380
370
360
350
340
330
320
310
Year
Sources: 1958 -1974 Scripps Institute of Oceanography; NOAA
1974 -2012 National Oceanic and Atmospheric Administration; NOAA
Figure 4.8 Changes in atmospheric carbon dioxide from 1958
to 2010 as measured at mauna loa, Hawaii.
The vertical axis is
the concentration of CO
2
in parts per million (ppm). Note the steady
increase during this period of time, which has been attributed to
human consumption of fossil fuels. Peaks and valleys in the curve
represent seasonal fluctuations, with low points taking place dur-
ing the Northern Hemisphere summer when plants take up CO
2
.
(
Sources
: C. D. Keeling and T. P. Whorf. Atmospheric CO
2
records
from sites in the SIO Air Sampling Network. In
Trends: A Com-
pendium of Data on Global Change
. U.S. Department of Energy;
NOAA; NASA.)
