Global Climate
Change
Thomas W. Blaine
The study of global climate
incorporates a number of variables which include temperature and precipitation,
as well as the distribution of these over space and time. In recent years
concern has emerged over the possibility that human activities may change global
climate. Most climatologists agree that if there is any global climate change
over the next 50-100 years, it will be a direct result of the more specific
problem of global warming, the causes and implications of which are discussed in
this fact sheet.
The Greenhouse Effect
The earth orbits the sun at
an average distance of nearly 93 million miles. When the radiation from the
sun reaches the earth, a portion is absorbed as heat on the surface, but most
is radiated back toward space. Due to the nature of the atmosphere, however,
much of the escaping heat is trapped, where it contributes to the warming of
the planet. This phenomenon is known as the greenhouse effect. It is an
extremely important and natural phenomenon, and if it did not exist, the earth
would be far too cold to support life as we know it. The specific gases that
occur naturally in the atmosphere to sustain the greenhouse effect include
water vapor, carbon dioxide (CO2), methane(CH4), ozone
(O3), and nitrous oxide (N2O). These are often referred
to as "greenhouse gases."
It is widely agreed that
concentrations of greenhouse gases present in the atmosphere (particularly
carbon dioxide and methane) have varied tremendously over the period of the
earth's existence, and there is considerable agreement that these changes
correlate with temperature change. However, until recently in the earth's
history, human activities have not been significant enough to influence the
concentrations of these gases. That began to change about two hundred years
ago.
Since the time of the
Industrial Revolution in the 1700's humans have released nearly 1 trillion
tons of carbon dioxide into the atmosphere by burning fossil fuels. A
significant portion of this has been taken up by absorption in the oceans and
by the terrestrial ecosystem. The net effect, however, has been to increase
atmospheric concentrations of carbon dioxide from roughly 280 parts per
million(ppm) to the current level of about 360 ppm. Moreover, annual emissions
are expected, at a minimum, to double in the next 50-100 years.
Over the same period, the
production and use of coal, oil, and natural gas, along with expanded
agricultural enterprises (particularly cattle and wetland rice production)
have led to greatly increased methane emissions. Atmospheric concentrations of
methane have increased from 0.80 ppm in 1800 to 1.75 ppm in 1995.
In addition to naturally
occurring gases such as carbon dioxide and methane, whose levels in the
atmosphere are enhanced by human activities, chloroflourocarbons (CFC's --
primarily used as refrigerants), which are uniquely manmade, also contribute
to the greenhouse effect. However, the production of CFC's has been restricted
due to other environmental concerns. Emissions of CFC's are expected to
decline substantially in the future.
Temperature Trends
Climate models are
basically computer programs that replicate the earth's climate and/or attempt
to predict climate conditions on the basis of selected variables or factors
which might bring about climate change. According to most current climate
models, the combined amounts of greenhouse gases emitted by activities for
which humans are responsible to date have been sufficient to raise the average
temperature of the earth by 2 degrees Fahrenheit.
Modern methods of measuring
and recording temperatures began around 1860. Most analyses of the data show
that since that time, a statistically significant increase in temperature has
occurred. This trend has accounted for a roughly 1 degree Fahrenheit increase
in global temperature, about half what would be predicted on the basis of the
concentration of greenhouse gases. A closer look at the data reveals that
temperatures were relatively stable between 1860 and 1920, when an upward
trend developed which lasted until the late 1940's. A downward trend prevailed
until the mid 1970's, when a warming trend continued into the early 1990's.
The question arises as to
the place of these trends in historical context. Archeological analyses reveal
that the earth's temperature has varied greatly over time. It was about 18
degrees F warmer than at present about 100 million years ago when the
dinosaurs roamed the earth. It is estimated that at the other extreme, the
earth was more than 10 degrees F cooler than at present during the last ice
age, which ended about 10 thousand years ago.
The specter that alarms
those who fear global warming is that many climate models that assume a
doubling of carbon dioxide emissions over the next 50-100 years forecast a 3-7
degree F increase in global temperature, an increase that is unprecedented in
such a short period of time.
Implications of Warming
Most climate models predict
that if global warming occurs, it will not produce globally uniform effects.
Most places will become warmer, but some will actually cool down. Global
precipitation will increase due to increased evaporation from the oceans, but
some areas will receive substantially less rainfall than today. It is expected
that temperatures will rise more near the poles and less in tropical regions.
Nights and winters are expected to warm more than daytime and summer
temperatures. Thus in general, warming will tend to occur at the lower ends of
current temperature ranges. This has led some to argue that global warming
will be generally beneficial to mankind, potentially opening new areas in the
upper temperate zones to agricultural enterprises that are not practical today
due to the cold climate. Also, increased concentrations of carbon dioxide
would have a fertilizing effect on crops and other vegetation, stimulating
growth.
Current climatological
models indicate that a 3-7 degree F increase in global average temperature
would cause a melting of substantial portions of polar ice, which when
combined with thermal expansion of the oceans would result in a global sea
level rise of between one and three feet. Many low lying coastal areas would
be jeopardized, such as the country of Bangladesh, where many of the lands
adjacent to the ocean are at or below sea level. In the United States numerous
barrier islands, primarily on the Atlantic and Gulf coasts would be adversely
influenced. Some coastal marshes and wetlands would be inundated. This might
have a mitigating effect on warming, since these types of environments are one
of the leading natural sources of methane. However, the loss of these
ecosystems would probably have far-reaching consequences, and in any case,
marsh like environments might possibly "migrate" inland. Several
major urban areas such as New Orleans, Miami, and New York City would also be
threatened. Projects built to keep the waters out are feasible, but would come
at a substantial expense.
Another consequence of
warming would be an increase in the number of tropical cyclones. Currently, an
average of ten named storms (which include tropical storms and hurricanes)
develop in the Atlantic/Gulf/Caribbean waters off the south/east coasts of the
U.S. during hurricane season (June l-November 30). About half of these
actually strike the U.S. coast. Conditions that are required for the
development of such systems include warm ocean water (79 degrees F or
greater). Global warming would increase the length of the season, and also
would expand the area of ocean over which these storms could develop. Since
these storms typically produce very large amounts of precipitation after
making landfall, they would partially offset tendencies for drought which
might otherwise be expected to result from warming in the eastern U.S. While
these types of showers would be hit and miss, and therefore not very reliable
for unirrigated areas, they could certainly play a role in replenishing
aquifers and reservoirs which could be tapped for irrigation. Even with the
most severe types of warming forecast, however, the west coast of the U.S.
would still remain unaffected by tropical cyclones, since the temperatures of
the adjacent Pacific surface would tend to remain well below 79 degrees F.
As for implications for
Ohio, most climate models predict substantially warmer winters and slightly
hotter summers. Some indicate that summers would also be drier than at
present. Thus, agriculture would be faced with a longer and drier growing
season. Adaptations might include the introduction of crops (such as
cotton)that are not possible given the current climate. The reduction of the
number and severity of winter storms would unquestionably benefit farmers, but
the increasing likelihood of summer droughts could present a real challenge
for agriculture. In contrast with rising sea levels, the levels of the Great
Lakes, including Lake Erie, are expected to drop by an average of 7-8 feet,
primarily because of reduced rainfall and increased use of irrigation.
Need for Further Study
Numerous questions remain.
For example, why has there been only a 1 degree F increase in global
temperature, when climate models predict it should have been twice that
amount, given current greenhouse gas emissions? What other factors are
influencing global climate? Have the oceans - been responsible for moderating
warming by absorbing excessive greenhouse gases? What is their capacity to
absorb still further increases? Do current climate models tend to exaggerate
the effects of increased greenhouse gas emissions?
It seems very unlikely at
this point that humans will take the types of action necessary to prevent
carbon dioxide and methane emissions from increasing enough to cause a
predicted increase of global temperatures in the 3-7degree F range
over the next 50-100 years. Even if the high income countries are able,
through environmental regulations, to control emissions of carbon dioxide,
most developing countries, where living standards present a constant daily
struggle, will almost certainly opt to follow paths which will lead to higher
emissions. Under this scenario, we should be taking steps to understand better
the determinants of global climate change and how we can best cope with the
consequences.
Suggested Reading
Ausubel, J. H. 1991. "A
Second Look at the Impacts of Climate
Change." American Scientist, Vol.79: 210-221.
Council for Agricultural
Science and Technology. 1992. "Preparing U.S. Agriculture for Global
Climate Change." Ames IA: Council for Agricultural Science and Technology.
Matthews. S. W. 1990.
"Under the Sun -- Is Our World Warming?"?" National
Geographic. 178(4): 66-99.
Miller, G. T. 1994.
"Global Warming and Ozone Loss: Apocalypse Soon?" Living in the
Environment.Belmont, CA: Wadsworth Publishing Co.
Moore. T. G., 1995.
"Global Warming: A Boon for Humans and Other Animals." Hoover
Institution: Public Policy Essay.
Ohio State University
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