Greenhouse Gases
Greenhouse gases are those gaseous constituents of the atmosphere,
both natural and human-made,
that absorb and emit infrared radiation emitted by the Earth's
surface, the atmosphere and clouds.
Naturally-occurring greenhouse gases include water vapor, carbon
dioxide (CO2), methane (CH4), nitrous
oxide (N2O), and ozone (O3). Several
classes of halogenated substances that contain fluorine, chlorine
or bromine are also greenhouse gases, but they are, for the most
part, solely a product of industrial activities. Halocarbons,
perfluorocarbons (PFCs), and sulfur
hexafluoride (SF6) are
all potent greenhouse gases.
In June 2003, the Maine State Legislature passed a law requiring the
Department to develop an action plan with the goal of reducing greenhouse
gas emissions from state sources. The Department has initiated a stakeholder
process, seeking input and building consensus on how best to meet the
required emissions reductions. The
Maine Greenhouse Gas Initiative website details the work of the stakeholder
group in developing that action plan.
Carbon dioxide (CO2) is naturally-occuring
gas, part of the global carbon cycle where carbon is cycled between
various atmosphere, oceanic, terrestrial life, marine line and mineral
reserves. The predominant source of human-made carbon dioxide
emissions is the combustion of fossil fuels. Forest clearing,
other biomass burning, and some non-energy production processes (such
as cement production) also emits notable quantities of carbon dioxide. It
is the principle greenhouse gas that affects the Earth's radiative balance. It
is also the reference gas against which all other greenhouse gases are
measured, and therefore, has a Global Warming Potential of 1.
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Methane (CH4) is
greenhouse gas with both natural and human-produced sources. In
biological systems, methane is produced by the anaerobic decomposition
of organic matter. Agricultural processes such as wetland rice
cultivation and the decomposition of animal wastes emit methane, as
does the decomposition of municipal solid waste in landfills. Methane
is also emitted during the production and distribution of natural gas
and petroleum and is released as a by-product of coal mining and incomplete
fossil fuel combustion. Slightly more than half of current methane
emissions are from human-produced sources. Methane has a Global
Warming Potential of 23.
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Nitrous Oxide (N2O) comes
from many sources, including the use of synthetic and manure fertilizers
and manure deposition by livestock; fossil fuel combustion, especially
from mobile sources (i.e., cars, trucks); nylon and nitric acid production;
wastewater treatment; and waste combustion and biomass burning. The
atmospheric concentration of nitrous oxide has steadily increased during
the Industrial Era and is now 16% (46 ppb) larger than in 1750. Relative
to carbon dioxide, nitrous oxide has a Global Warming Potential of 296.
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Ozone (O3) is
present in both the upper stratosphere where it shields the Earth
from harmful levels of ultraviolet radiation and in the lower troposphere
where it is the main component of "smog." Ozone-depleting
chemicals, such as chlorine, fluorine and bromine-containing halocarbons
(CFCs) have depleted stratospheric ozone concentrations, allowing radiative
forcing (warming) of the atmosphere. Tropospheric ozone is produced
from complex chemical reactions of volatile organic compounds (VOCs)
mixing with nitrous oxides (NOx) in the presence of sunlight.
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Halocarbons are
carbon compounds that contain fluorine, chlorine, bromine or iodine. For
most of these compounds, human activities are the sole source. Halocarbons
that contain chlorine (also known as CFCs and HCFCs) and bromine (also
known as halons) cause depletion of the stratosphere ozone layer and
are potent greenhouse gases. For example, methyl chloroform and
carbon tetrachloride have Global Warming Potentials of 140 and 1800,
respectively.
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Perfluorocarbons (known as PFCs)
and sulfur hexafluoride (SF6) are powerful greenhouse
gases. These substances have been been used in recent years
as replacements for ozone-depleting halocarbons. Perfluorocarbons
and sulfur hexafluoride are predominantly emitted from various industrial
processes, including aluminum smelting, semiconductor manufacturing,
electric power transmission and distribution, and magnesium
casting. Currently, the radiative forcing (warming) impact
of perfluorocarbons and sulfur hexafluoride is small, but they have
a significant growth rate, extremely long atmospheric lifetimes and
are strong absorbers of infrared radiation, and therefore, have the
potential to influence climate far into the future. Relative
to carbon dioxide, sulfur hexafluoride has a Global Warming Potential
of 22,200.
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Useful links
- EPA's Global Warming Site - U.S. EPA (http://yosemite.epa.gov/oar/globalwarming.nsf/content/) Information
for both concerned citizens and industry on climate change, greenhouse
gas emissions, and what is being done both in the United States and
around the world.
- Climate Protection Partnerships - U.S. EPA (http://www.epa.gov/cpd.html) EPA
works with businesses, organizations, governments, and consumers
to reduce emissions of the greenhouse gases that contribute to global
climate change by promoting greater use of energy efficient and other
cost-effective technologies.
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