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					 Global Warming - Earth's Greenhouse Effect,
 Atmospheric Concentrations Of Greenhouse Gases,
 Predictions And Evidence Of Global Warming
Global warming refers to a long-term increase in the Earth's surface temperature
that results in large-scale changes in global climate, namely redistribution of
climatic zones defined by temperature, precipitation, and associated adapted
ecosystems. Global climate changes, and episodes of global warming, have
occurred throughout geologic history as a result of natural variations in incoming
solar radiation, atmospheric chemistry, and oceanic and atmospheric
circulation. Anthropogenic, or human-caused, global warming and climate
change are a potential outcome of human activities during the last 150 years.
Scientific data show that atmospheric concentrations of carbon dioxide,
methane, nitrous oxide, and man-made chemicals called halocarbons are
increasing as a result of emissions associated with human activities, and models
predict that this environmental change may lead to global warming.

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 Global Warming - Earth's Greenhouse Effect
Solar radiation is the major source of energy to Earth's surface. Much of that
incoming short-wave-length energy is absorbed by the surface where it drives
atmospheric and oceanic circulation, and fuels biological processes like
photosynthesis. The land and sea surfaces then reradiate extra longer-
wavelength heat, or infrared, energy. If Earth's atmosphere were transparent to
the emitted infrared radiation, the planet would cool relatively efficiently and
would have an average surface temperature of about 0°F (-18°C). However, the
Earth's naturally occurring "greenhouse effect" maintains the planet's average
temperature at a more livable 59°F (15°C) by trapping some of the escaping heat
within the atmosphere. Small concentrations of so-called "green-house gases,"
also known as radiatively active gases, absorb some of the infrared energy and
thereby delay its passage to space. Water vapor (H2O), carbon dioxide (CO2),
methane (CH4), nitrous oxide (N2O), and ozone (O3) are the most concentrated
and effective greenhouse gases. The greenhouse effect has been extremely
important to the evolution and survival of life on Earth. A surface temperature of
59°F is sufficient to maintain the Earth's reservoirs of life-sustaining liquid water,
and to impel climatic processes, whereas 0°F is too cold for most organisms to
live or for ecological processes to function well.

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 Global Warming - Atmospheric Concentrations Of
 Greenhouse Gases
Prior to the modern influence of human activities on atmospheric chemistry, the
naturally occurring greenhouse gases had fairly stable atmospheric
concentrations: carbon dioxide about 280 ppm (or parts per million by volume),
methane 0.7 ppm, and nitrous oxide 0.285 ppm. (Human activities do not appear
to affect the concentration of water vapor, which varies naturally over time.)
Today, the atmospheric concentration of CO2 has increased to about 364 ppm,
while that of CH4 is 1.7 ppm, and N2O is 0.304 ppm. The concentrations of
chlorofluorocarbons (CFCs), and other completely man-made, or synthetic,
greenhouse gases, have increased from essentially zero to about 0.7 ppb (parts
per billion by volume).

Atmospheric concentrations of the greenhouse gases have increased particularly
quickly since the middle of the twentieth century, coinciding with rapid human
population growth and intensive global industrialization. The combined effects of
fossil fuel use and deforestation have increased the atmospheric concentration
of CO2. Fossil fuels, like oil, natural gas, and coal contain carbon in their
chemical structure that, when liberated by combustion, combines with oxygen
to create CO2. Trees, like all plants, take in CO2, incorporate carbon in their
structure, and emit O2 back into the atmosphere; deforestation destroys carbon
"sinks" that lower the atmospheric concentration of CO2. Fossil-fuel mining,
decomposition of organic materials in human and livestock waste treatment
facilities, and flooding in rice agriculture have led to increased emissions of CH4.
Agricultural fertilizers, and combustion of fossil fuels and solid wastes account
for increased N2O emissions. Industrial processes emit a variety of powerful
synthetic greenhouse gases like CFCs, hydrofluorcarbons (HFCs),
perfluorocarbons (PFCs) and sulfur hexafluoride (SF6).

The greenhouse gases vary greatly in their ability to absorb infrared radiation. On
a per-molecule basis, methane is about 25–40 times more absorptive than carbon
dioxide, nitrous oxide is 200–270 times stronger, and CFCs are 3–15 thousand
times more effective. CO2, however, has by far the largest atmospheric
concentration, and has experienced the greatest increases; CO2 is responsible for
about 60% of the human contribution to increased atmospheric heat retention.

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 Global Warming - Predictions And Evidence Of
 Global Warming
Most atmospheric scientists assume that the well-documented increase in
greenhouse gases will result in an intensification of Earth's naturally occurring
greenhouse effect, and to global warming. The exact climatic response to
increased concentrations of radiatively active gases, and its potential effects on
humans are, however, difficult to measure or predict. However, if global warming
were to occur as most scientific studies predict, it would have substantial climatic,
ecological, and sociopolitical consequences.

The Earth's surface is surface temperature is extremely variable from place to
place, and over time. Furthermore, the systems that interact to maintain the
planet's temperature and climate are extremely complex; cause-and-effect
relationships between changes in one system, the atmosphere in this case, and
results in another, global climate, are very difficult to predict, observe, and
"prove." In spite of these scientific challenges, there is significant evidence that
the Earth has warmed significantly during the past 150 years or so, and that
global climate has responded to the temperature increase. Climate records show
a 1°F increase in the average temperature of the Earth's oceans, atmosphere, and
solid surface since the late 1900s. Geologic and historical studies document
dramatic thinning and shrinkage of the polar ice caps, and retreat of Earth's
alpine glaciers. Less conclusive, but still suggestive, data supporting
anthropogenic global warming include a several centimeter increase in global
sea-level since 1900, and alterations in large-scale weather phenomena like the
southeast Indian monsoon, Atlantic hurricane season, El Niño Southern
Oscillation, and North African drought cycle.

The empirical, or observed, data listed above generally agree with predictions
computed by mathematical models of global climate processes. These "virtual
experiments," called three-dimensional general circulation models (GCMs),
simulate the complex movements of energy and mass involved in the global
circulation of the atmosphere and oceans. Scientists use GCMs to predict the
effects of a change in a specific variable, like the concentration of atmospheric
CO2, on the rest of the global climate system. Because of the complexity of the
computational problem, GCMs that attempt to predict global climate change
have had somewhat variable results. However, most experiments do suggest that
the increased concentration of atmospheric greenhouse gases has resulted, and
will continue to result, in global warming. For example, one GCM that doubles the
present CO2concentration to about 700 ppm predicts a 2°-6°F rise in global
temperature, and suggests that the warming would be 2–3 times more intense at
high latitudes than in the tropics.

Other predicted consequences of warming include large-scale shifts in
atmospheric and oceanographic circulation patterns, melting of the polar ice
caps, global sea-level rise, reorganization of the Earth's climatic zones, and
establishment of new large-scale weather patterns. Such changes in the
distribution of heat, precipitation, and weather phenomena like storms and
floods would affect the productivity and distribution of natural and managed
vegetation. Animals and microorganisms would experience dramatic changes in
their habitats, and perhaps face much higher rates of species extinction. Most
ecologists consider that global warming, if were it to occur as predicted, would
represent a serious threat to biodiversity and to the health of ecosystems
The predicted climatic and biological changes associated with anthropogenic
global warming could have potentially disastrous outcomes for the Earth's human
population. In 1998, more than half of the world's population, some 3.2 billion
people, lived with in 120 miles of the ocean. Even small increases in global sea
level, and in the intensity of coastal storms and floods, would threaten the lives
and property of large numbers of people. Changes in regional temperature,
precipitation, and weather, as well as biological health, would affect the managed
agriculture, fishing, and forestry that provide food and shelter for the Earth's
burgeoning human population.

Most scientists, and many international policy-makers, now consider global
warming to be a credible threat to the Earth's natural environment and human
population. However, because the specific consequences of global warming are
difficult to predict, and in some cases unknown, the scientific community remains
divided about the potential effects of the phenomenon. Attempts to prevent
anthropogenic global warming, especially measures that require socioeconomic
sacrifice, have therefore been extremely controversial. The 1992 United Nations
Framework Convention on Climate Change (UNCCC), also called the Kyoto
Protocol, acknowledges that human activities can alter global climate, and
requires signatory nations to reduce greenhouse gas emissions. As of November
2002, 181 nations had signed, ratified, or acceded to the conditions of the Kyoto
protocol. However, the United States, by far the world's largest per-capita
producer of greenhouse gases, did not sign the treaty on the grounds that the
science of global warming remains inconclusive, and that the economic
consequences of action would be too great.

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