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Anthropogenic Greenhouse Gases

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  • Mike Neuman
    Once, all climate changes occurred naturally. However, during the Industrial Revolution, we began altering our climate and environment through changing
    Message 1 of 1 , Feb 1, 2005
      Once, all climate changes occurred naturally. However, during the
      Industrial Revolution, we began altering our climate and environment
      through changing agricultural and industrial practices. Before the
      Industrial Revolution, human activity released very few gases into
      the atmosphere, but now through population growth, fossil fuel
      burning, and deforestation, we are affecting the mixture of gases in
      the atmosphere.

      What are the Greenhouse Gases?

      Some greenhouse gases occur naturally in the atmosphere, while others
      result from human activities. Naturally occuring greenhouse gases
      include water vapor, carbon dioxide, methane, nitrous oxide, and
      ozone. Certain human activities, however, add to the levels of most
      of these naturally occurring gases:

      Carbon dioxide is released to the atmosphere when solid waste, fossil
      fuels (oil, natural gas, and coal), and wood and wood products are

      Methane is emitted during the production and transport of coal,
      natural gas, and oil. Methane emissions also result from the
      decomposition of organic wastes in municipal solid waste landfills,
      and the raising of livestock. Methane traps over 21 times more heat
      per molecule than carbon dioxide.

      Nitrous oxide is emitted during agricultural and industrial
      activities, as well as during combustion of solid waste and fossil
      fuels. Nitrous oxide absorbs 270 times more heat per molecule than
      carbon dioxide.

      Very powerful greenhouse gases that are not naturally occurring
      include hydrofluorocarbons, perfluorocarbons, and sulfur
      hexafluoride, which are generated in a variety of industrial

      Hydrofluorocarbons (HFCs) - HFCs are man-made chemicals, many of
      which have been developed as alternatives to ozone-depleting
      substances (ODS) for industrial, commercial, and consumer products.
      The global warming potentials of HFCs range from 140 (HFC-152a) to
      11,700 (HFC-23). The atmospheric lifetime for HFCs varies from just
      over a year for HFC-152a to 260 years for HFC-23. Most of the
      commercially used HFCs have atmospheric lifetimes less than 15 years;
      e.g., HFC-134a, which i sused in automobile air conditioning and
      refrigeration, has an atmospheric life of 14 years.

      The HFCs with the largest measured atmospheric abundances are (in
      order), HFC-23 (CHF3), HFC-134a (CF3CH2F), and HFC-152a (CH3CHF2).
      The only significant emissions of HFCs before 1990 were of the
      chemical HFC-23, which is generated as a byproduct of the production
      of HCFC-22. HFCs are primarily used as a substitute for ozone-
      depleting chemicals. Between 1978 and 1995, HFC-23 concentrations
      have increased from 3 to 10 parts per trillion (ppt), and continue to
      rise. Since 1990, when it was almost undetectable, global average
      concentrations of HFC-134a have risen significantly to almost 10 ppt
      (parts per trillion). HFC-134a has an atmospheric lifetime of about
      14 years and its abundance is expected to continue to rise in line
      with its increasing use as a refrigerant around the world. HFC-152a
      has increased steadily to about 0.3 ppt in 2000, however its
      relatively short life time (1.4 years) has kept its atmospheric
      concentration below 1 ppt (IPCC, 2001 ).

      Perfluorocarbons (PFCs) - Primary aluminum production and
      semiconductor manufacture are the largest known man-made sources of
      two perfluorocarbons – CF4 (tetrafluoromethane) and C2F6
      (hexafluoroethane). The GWP of CF4 and C2F6 emissions is equivalent
      to approximately 6,500 and 9,200 tonnes, respectively. PFCs are also
      relatively minor substitutes for ozone-depleting substances (ODSs).

      PFCs have extremely stable molecular structures and are largely
      immune to the chemical processes in the lower atmosphere that break
      down most atmospheric pollutants. Not until the PFCs reach the
      mesosphere, about 60 kilometers above Earth, do very high-energy
      ultraviolet rays from the sun destroy them. This removal mechanism is
      extremely slow and as a result PFCs accumulate in the atmosphere and
      remain there for several thousand years. The estimated atmospheric
      lifetimes for CF4 and C2F6 are 50,000 and 10,000 years respectively.
      Measurements in 2000 estimate CF4 global concentrations in the
      stratosphere at over 70 parts per trillion (ppt). Recent relative
      rates of increase in concentrations for two of the most important
      PFCs are 1.3% per year for CF4 and 3.2% per year for C2F6 (IPCC,
      2001 ).

      Sulfur Hexafluoride (SF6) - The global warming potential of SF6 is
      23,900, making it the most potent greenhouse gas the IPCC has
      evaluated. SF6 is a colorless, odorless, nontoxic, nonflammable gas
      with excellent dielectric properties. SF6 is used for insulation and
      current interruption in electric power transmission and distribution
      equipment, in the magnesium industry to protext molten magnesium from
      oxidation and potentially violent burning, in semiconductor
      manufacturing to create circuitry patterns on silicon wafers, and as
      a tracer gas for leak detection.

      Like the other high GWP gases, there are very few sinks for SF6, so
      all man-made sources contribute directly to its accumulation in the
      atmosphere. Measurements of SF6 show that its global average
      concentration has increased by about 7% per year during the 1980s and
      1990s, from less 1 ppt in 1980 to almost 4 ppt in the late 1990's
      (IPCC, 2001).

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