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The Sun

VIEWS: 16 PAGES: 10

									STS

                   Greenhouse gases
205



      • Greenhouse gases are the gases present in the atmosphere which
      reduce the loss of heat into space and therefore contribute to global
      temperatures.
      • Greenhouse gases are essential to maintaining the temperature of the
      earth; without them the planet would be so cold as to be uninhabitable.
      • Likewise, an excess of greenhouse gases can raise the temperature of
      the planet to unlivable levels.
      • The term greenhouse gas is applied to, in order of relative abundance:
      water vapour, carbon dioxide, methane, nitrous oxide, ozone and CFCs.
      • Greenhouse gases are produced by many natural and industrial
      processes.
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                    Carbon dioxide
205

      Carbon dioxide is a gas at standard temperature and pressure and
      exists in Earth's atmosphere in this state. It is currently at a
      globally averaged concentration of approximately 383 ppm by volume
      in the Earth's atmosphere, although this varies both by location and
      time.
      Carbon dioxide is an important greenhouse gas because it transmits
      visible light but absorbs strongly in the infrared.
      Carbon dioxide is produced by all animals, plants, fungi and
      microorganisms during respiration and is used by plants during
      photosynthesis.
      Carbon dioxide is generated as a byproduct of the combustion of
      fossil fuels or vegetable matter, among other chemical processes.
      And also as a byproduct in lime production.
      Inorganic carbon dioxide is output by volcanoes and other geothermal
      processes such as hot springs.
STS

                             Methane
205


      • Methane is the principal component of natural gas.
      • Methane is a relatively potent greenhouse gas with a high global
      warming potential.
      • Methane in the atmosphere is eventually oxidized, producing carbon
      dioxide and water. As a result, methane in the atmosphere has a half life
      of seven years (every seven years, the amount of methane halves).
      • The abundance of methane in the Earth's atmosphere in 1998 was 1745
      parts per billion, up from 700 ppb in 1750.
      • In addition, there is a large, but unknown, amount of methane in
      methane clathrates in the ocean floors. Global warming could release this
      methane, which could cause a further sharp rise in global temperatures.
      Such releases of methane may have been a major factor in previous
      major extinction events.
      • The Earth's crust also contains huge amounts of methane. Large
      amounts of methane are produced in swamps. Other sources include mud
      volcanoes which are connected with deep geological faults.
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                    Nitrous Oxide
205


      • Nitrous oxide (also known as laughing gas) is a chemical compound
      with the chemical formula N2O.
      • It is used in motor racing as an oxidizer to increase the power output
      of engines.
      • Despite its relatively small concentration in the atmosphere, nitrous
      oxide is the fourth largest greenhouse gas contributor to overall
      global warming, behind carbon dioxide, methane and water vapour.
      • Nitrous oxide is emitted by bacteria in soils and oceans, and thus has
      been a part of Earth's atmosphere for aeons. Agriculture is the main
      source of human-produced nitrous oxide: cultivating soil, the use of
      nitrogen fertilizers, and animal waste handling can all stimulate
      naturally occurring bacteria to produce more nitrous oxide.
      • The livestock sector (primarily cows, chickens, and pigs) produces
      65% of human-related nitrous oxide.
      • Industrial sources make up only about 20% of all anthropogenic
      sources, and include the production of nylon and nitric acid, and the
      burning of fossil fuel in internal combustion engines.
STS

                               Ozone
205



      • Ozone (O3) is a triatomic molecule, consisting of three oxygen
      atoms.
      • Ground-level ozone is an air pollutant with harmful effects on the
      respiratory systems of animals. Ozone in the upper atmosphere
      filters potentially damaging ultraviolet light from reaching the
      Earth's surface.
      • It is not emitted directly by car engines or by industrial
      operations. It is formed by the reaction of sunlight on air
      containing hydrocarbons and nitrogen oxides that react to form
      ozone directly at the source of the pollution or many kilometers
      down wind.
      • Although ozone was present at ground level before the industrial
      revolution, peak concentrations are now far higher than the pre-
      industrial levels and even background concentrations well away from
      sources of pollution are substantially higher. This increase in ozone
      is of further concern as ozone present in the upper troposphere
      acts as a greenhouse gas.
STS
205
       Chlorofluorocarbons (CFCs)

      • Chlorofluorocarbons (CFC) are compounds containing chlorine,
      fluorine and carbon only, that is they contain no hydrogen.
      • They were formerly used widely in industry, for example as
      refrigerants, propellants, and cleaning solvents. Their use has been
      regularly prohibited by the Montreal Protocol, because of effects
      on the ozone layer.
      • They are also powerful greenhouse gases.
      • CFC's have half-lives between 50-100 years, so their presence in
      the atmosphere and reactivity with ozone is long lived. One CFC
      molecule typically degrades around 10,000 ozone molecules before
      its removal, but this number can sometimes be in the millions.
      • Hydrochlorofluorocarbons (HCFCs) are of a class of haloalkanes
      where not all hydrogen has been replaced by chlorine or fluorine.
      They are used primarily as chlorofluorocarbon (CFC) substitutes, as
      the ozone depleting effects are only about 10% of the CFCs.
STS

          The Role of Water Vapor
205


      Water vapor is a naturally occurring greenhouse gas and accounts for
      the largest percentage of the greenhouse effect, between 36% and
      66%. Water vapor concentrations fluctuate regionally, but human
      activity does not directly affect water vapor concentrations except at
      local scales (for example, near irrigated fields).
      Water vapor is special in the sense that the amount of water vapor
      depends directly on the temperature.
      Vapor pressure is the pressure of a vapor in equilibrium with its liquid.
      Water has a tendency to evaporate to a gaseous form, and water vapor
      has a tendency to condense back into liquid form. At any given
      temperature, for a particular substance, there is a pressure at which
      the gas of that substance is in dynamic equilibrium with its liquid or
      solid forms. This is the vapor pressure of that substance at that
      temperature. The air cannot hold more water vapor than is allowed by
      the vapor pressure.
STS

      Clausius-Clapeyron Relation
205


      The Clausius-Clapeyron relation gives a relationship between the
      saturation vapor pressure and the temperature.
                                              17.67T
                         es T   6.112 e   T  243.5



      This equation establishes that warmer air has the potential to hold
      more water vapor per unit volume. As a simple example, the air at
      30 oC can hold about 3.5 times more water vapor than air at 10 oC.
      Current state-of-the-art climate models predict that increasing
      water vapor concentrations in warmer air will amplify the
      greenhouse effect created by anthropogenic greenhouse gases.
      Thus water vapor acts as a strong positive feedback to the forcing
      provided by greenhouse gases such as CO2.
STS
205             Radiative Forcing of
                 Greenhouse Gases
      Most greenhouse gases have both natural and anthropogenic sources.
      During the pre-industrial holocene, levels of the gases were roughly
      constant. Since the industrial revolution, levels of some gases have
      increased due to human actions.

      Gas              1750 Level Current Level   Increase Forcing (Wm2)

      Carbon dioxide   280 ppm       384ppm          104 ppm       1.53
      Methane          700 ppb     1,745 ppb       1,045 ppb       0.48
      Nitrous oxide    270 ppb       314 ppb          44 ppb       0.15
      CFC-12             0           533 ppt         533 ppt       0.17

      As you can see, even though the concentration of CFCs is low, its
      effect is very high, so we need a different measure for the effect of
      these gases.
STS

          Global Warming Potential
205


      Global warming potential (GWP) is a measure of how much a given mass of
      greenhouse gas is estimated to contribute to global warming. It is a relative
      scale which compares the gas in question to that of the same mass of
      carbon dioxide.
      The GWP depends on the following factors:
          • the absorption of infrared radiation by a given species
          • the spectral location of its absorbing wavelengths
          • the atmospheric lifetime of the species

  GWP values and lifetimes from Lifetime               GWP time horizon
  2007 IPCC AR4                  (years)    20 years      100 years       500 years
  Methane                            12        72            25              7.6
  Nitrous oxide                     114       310           298             153
  HFC-23 (hydrofluorocarbon)       270       12000         14800           12200

  HFC-134a (hydrofluorocarbon)      14        3830          1430            435
  Sulfur hexafluoride              3200      16300         22800           32600

								
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