Encyclopedia of Evolution.pdf - Online Reading Center

0 group selection
used the carbon to produce calcium carbonate shells, which
have often become fossils (see fossils and fossilization)
and in some cases have produced thick deposits of limestone,
chalk, or diatomaceous earth. In some regions, such as the
tundra, much partially decomposed organic material (humus)
remains frozen, which also represents storage of carbon that
had previously been atmospheric carbon dioxide. The inorganic
production of limestone (calcium carbonate) in the
oceans, in which carbon dioxide reacts with calcium silicate
to produce limestone and silicon dioxide, can also remove
carbon dioxide from the atmosphere.
It is possible that the relatively cool conditions of the
modern Earth are partly the result of photosynthesis, followed
by the entrapment of carbon in the Earth as fossil
fuels, limestone, and humus, in addition to the configuration
of the continents. It has been suggested that excessive photosynthesis
by oceans full of green scum and stromatolites was
a cause of Snowball Earth, when the planet nearly froze
over. Scientists know from studies of oxygen isotopes that
the temperature of the Earth has fluctuated over the course of
many millions of years. They cannot directly measure atmospheric
carbon dioxide levels any further back than several
hundred thousand years. They can, however, estimate atmospheric
carbon dioxide levels from carbon isotope ratios of
fossilized organic molecules. During many of the geological
periods of the past, warm temperatures have been associated
with worldwide forests, which would suggest that the greenhouse
effect was a minor contributor to climatic fluctuations
at those times. Scientists are fairly certain of the importance
of the greenhouse effect to the climate of the past several hundred
thousand years, during which time continental movements
have been slight.
Carbon dioxide makes up only 0.035 percent of the
atmosphere of the Earth. Yet this tiny amount of carbon
dioxide makes life possible on the Earth. Both Mars and
Venus have tremendous amounts of carbon dioxide in their
atmospheres. This is why Venus has a surface temperature
much warmer than Mercury, even though it is farther from
the Sun, and why Mars actually has a few days with equatorial
temperatures above the freezing point of water. In the
past, the Martian atmosphere was much thicker and Mars
had water. Today the Martian atmosphere is too thin for the
greenhouse effect to help it very much, but four billion years
ago the greenhouse effect may have made Mars a warm little
pond of bacterialike life-forms (see Mars, life on).
The greenhouse effect is part of the set of global processes
that some scientists believe regulates the climatic and
atmospheric conditions of the Earth. According to the Gaia
hypothesis, the activity of organisms affect the atmosphere
of the Earth and help to maintain it in a state in which life
can continue to function.
Human activities such as industry and transportation are
releasing a tremendous amount of carbon dioxide into the
atmosphere. At the same time, humans have been destroying
forests that absorb carbon dioxide. The net effect is that
carbon dioxide levels in the atmosphere have been increasing
more rapidly than at any time in the last 400,000 years
or perhaps ever. Exactly parallel to this has been a rapid
increase in the average temperature of the Earth, as indicated
by the studies of ice cores and tree rings. The average temperature
of the Earth has vacillated wildly (but, ever since
Snowball Earth, not wildly enough to threaten the existence
of life), whether as a result of wild changes in carbon dioxide
or not. For example, the warming trend at the end of the
most recent ice age was interrupted by a period of cooling
known as the Younger Dryas period (named after a species
of arctic wildflower), which began, and ended, over a period
of decades or centuries rather than millennia. The increase in
carbon dioxide and in global temperature since about 1850
have been the most rapid changes known to have occurred
in the history of the Earth, with the exception of the events
following the asteroid that caused the Cretaceous extinction.
Even the global warming that occurred at the end of
each of the recent ice ages was less rapid than the one that is
now occurring as a result of human activity. Human activity
is causing the extinctions of many species, which will only
increase more as a result of continued global warming. Many
scientists estimate that the Earth is entering the sixth of its
mass extinctions. The greenhouse effect is highly unlikely
to cause massive disasters by itself, but it certainly can alter
the climatic patterns of the Earth to an extent that the economic
systems of modern human civilization would be unable
to cope with them. The greenhouse effect, then, keeps all
humans alive, but human activity may be creating too much
of a good thing.
Further Reading
Alley, Richard B. “Abrupt climate change.” Scientific American,
November 2004, 62–69.
Barnola, J. M., et al. “Historical carbon dioxide record from the
Vostok ice core.” In Trends: A Compendium of Data on Global
Change. Carbon Dioxide Information Analysis Center, Oak Ridge
National Laboratory, U.S. Department of Energy, Oak Ridge,
Tenn., 2000. Available online. URL: http://cdiac.ornl.gov/trends/
co2/vostok.htm. Accessed April 21, 2006.
Bradshaw, William E. and Christina M. Holzapfel. “Evolutionary
response to rapid climate change.” Science 312 (2006): 1477–
1478.
Pagani, Mark, et al. “Marked decline in atmospheric carbon dioxide
concentrations during the Paleogene.” Science 309 (2005):
600–603.
Petit, J. R., et al. “Historical isotopic temperature record from the
Vostok ice core.” In Trends: A Compendium of Data on Global
Change. Carbon Dioxide Information Analysis Center, Oak
Ridge National Laboratory, U.S. Department of Energy, Oak
Ridge, Tenn., 2000. Available online. URL: http://cdiac.ornl.gov/
trends/temp/vostok/jouz_tem.htm. Accessed April 21, 2006.
Weart, Spencer R. The Discovery of Global Warming. Cambridge,
Mass.: Harvard University Press, 2003.
group selection The process of natural selection favors
characteristics that maximize the fitness of individuals within
populations. Most scientists maintain that evolution operates
by this process of individual selection. Any characteristics,