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 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.
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
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,
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.
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.
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
Moore. T. G., 1995. "Global Warming: A Boon for Humans and Other
Animals." Hoover Institution: Public Policy Essay.
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