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VCE Environmental Science - Greenhouse Effect
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VCE Environmental Science - Greenhouse Effect


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  • 1. Unit 3 Environmental Science: The greenhouse effect
  • 2. The atmosphere
    • The thin envelope of air that surrounds our planet, the atmosphere , is a mixture of gases. Two elements, nitrogen and oxygen , make up 99% of the volume of air. The other 1% is composed of "trace" gases.
  • 3. The atmosphere
    • The atmosphere is divided vertically into four layers:
      • troposphere
      • stratosphere
      • mesosphere
      • thermosphere .
    • The temperature, density and concentration of different gases varies in each level. Electromagnetic energy from the sun, in the form of solar radiation , is reflected, transmitted and absorbed according to changes in these factors.
    • .
  • 4. The atmosphere
    • The troposphere extends from the earth's surface to an average of 12 km, and contains about 75% of the gases of the atmosphere. All of the earth's weather occurs in the troposphere.
    • The temperature and moisture of the troposphere generally decreases with increasing altitude up to the tropopause. The temperature averages 15°C near the surface and -57°C at the tropopause. Moisture levels are greatest near the Earth’s surface, but vary across this surface, with the greatest moisture in the air over the oceans.
    • Heat and moisture are transported vertically within the troposphere by convection, updrafts and downdrafts.
    • Q1
  • 5. The atmosphere
    • Energy from the sun heats the Earth's surface and is then radiated back to the troposphere, where greenhouse gases absorb this energy, causing the lower atmosphere to heat up. It results from the presence of certain atmospheric gases, including water vapour (H 2 O) carbon dioxide (CO 2 ), methane (CH 4 ) nitrous oxide (NO 2 ), and ozone (O 3 ) which absorb energy. This is the greenhouse effect and is a naturally occurring process.
    • The major components of the atmosphere, nitrogen and oxygen, mainly exist as two-atom molecules (N 2 and O 2 ) which are too tightly bound together to vibrate and thus they do not absorb heat and contribute to the greenhouse effect.
    • Without greenhouse gases in the atmosphere, life on this planet would probably not exist, as the average temperature of the Earth would be -18° C, rather than the present (average) 15°C.
  • 6. Atmospheric processes Figure 2.1: Radiation emitted by the sun Energy from the sun - solar radiation – is a form of electromagnetic energy and contains radiation in various wavelengths of the spectrum. The yellow line in the graph in Figure 2.1 shows the amount of energy in solar radiation contained in the different wavelengths, with the greatest energy in visible light (43%), less in long wave infrared (IR) and less again as shortwave ultraviolet (UV). Wavelengths shorter than visible light account for only 7 - 8% of the total, but are extremely important because of their high energy per photon.
  • 7. Atmospheric processes
    • The amount and nature of solar radiation reaching the Earth’s surface differs from that entering the Earth’s atmosphere , due to processes that occur within the atmosphere. This is because gases and particles in the Earth's atmosphere reflect, scatter and absorb some of this energy,, during its passage through the atmosphere.
    Figure 2.2: Changes in the solar radiation spectrum during its passage through the atmosphere Figure 2.2 shows the total amount of energy is reduced by about half by the time it reaches the Earth’s surface. And while the atmosphere is relatively transparent to visible light, most ultraviolet light is absorbed by ozone (O 3 ) in the stratosphere, and much infrared is absorbed by water vapour (H 2 O), mainly in the troposphere. This results in most of the energy reaching the Earth’s surface in the form of visible light. Q 2.
  • 8. The greenhouse effect Figure 2.3: The destiny of incoming solar radiation. The remaining energy (51%) is absorbed by the Earth’s land, oceans and vegetation. It heats the Earth’s surface and provides energy to melt ice and snow, evaporate water, create wind and air and ocean currents, and provide energy for photosynthesis. As incoming solar radiation passes through the atmosphere, a proportion of the energy (26%) is reflected back to space by clouds and other atmospheric particles, while some of the energy (19%), mostly in the form of infrared radiation, is absorbed by clouds, gases and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth's atmosphere, 4% is reflected from the Earth’s surface back to space.
  • 9. The greenhouse effect
    • The solar radiation (mainly as visible light) is absorbed at the Earth’s surface.
    • Because the Earth’s surface is now warmed, it then re-r adiates/emits heat, in the form of longwave infrared radiation, to the lower atmosphere.
    • Some of this infrared radiation leaves the atmosphere, but the greenhouse gases in the troposphere absorb most (90%) of this heat energy and re-emits it as infrared energy back to Earth and the atmosphere.
    • This is the greenhouse effect , as it produces a warming effect similar to a greenhouse.
    • Q 3
    Figure 2.4: The greenhouse effect
  • 10. The greenhouse effect
    • Carbon dioxide (CO 2 ) consists of one carbon atom with an oxygen atom bonded to each side. This bonding allows the carbon dioxide molecule to absorb infrared radiation and the molecule starts to vibrate. Eventually, the vibrating molecule will emit the radiation again, and it will most likely be absorbed by yet another greenhouse gas molecule. This absorption-emission-absorption cycle serves to keep the heat near the surface, effectively insulating the Earth’s surface from the cold of space. This is how all the greenhouse molecules warm the Earth.
    Figure 2.5: A carbon dioxide molecule Only molecules with two or more bonds joining the atoms can absorb and re-emit energy in this way. Nitrogen and oxygen make up most of the atmosphere but are not greenhouse gases for this reason. Water vapour, carbon dioxide and methane are the most important greenhouse gases, with water vapour having the greatest contribution.
  • 11. Atmospheric processes
    • The oceans cover nearly three-quarters of the earth's surface and play an important role in exchanging heat and moisture between the Earth and its atmosphere. Oceans not only act as an abundant moisture source for the atmosphere but also as a heat source and sink (storage). They heat up less rapidly than the land, but are also slower to cool. Ocean and atmospheric currents play a significant role in moving heat around the world; they move heat away from the equatorial regions, and bring cool water from the poles.
  • 12. Enhanced greenhouse effect
    • The enhanced greenhouse effect is caused by changes generated by human activities , which have increased concentrations of greenhouse gases in the atmosphere due to:
    • increased the sources of greenhouse gases, including the burning of biomass and fossil fuels, agricultural and industrial activities
    • increased deforestation, resulting in less carbon ‘sinks’
  • 13. Enhanced greenhouse effect
    • This has resulted in more infrared radiation being absorbed and re-emitted in the troposphere – global warming . This causes:
    • changes to global climates due to increasing temperatures and changing ocean and atmospheric circulation patterns;
    • changes to precipitation , with some regions experiencing higher precipitation and others less;
    • changes to soil moisture levels due to changing precipitation and evaporation rates;
    • rising sea levels , as more land ice melts and flows into the oceans, as well as due to the thermal expansion of sea water;
    • increasing extreme weather events due to the above changes, including tropical storms, drought, floods and forest fires.
    • These changes to the world’s natural environment will, in turn, cause:
    • migration, population decline and loss of plant and animal species in response to the above changes;
    • impacts on human populations through reduced water availability, loss of coastal regions, increasing diseases, and impacts on industries dependent on climate and natural resources.
  • 14. The enhanced greenhouse effect
    • Human activities have increased the concentration of greenhouse gases such as carbon dioxide, methane, nitrous oxide and ozone, as well as introducing a group of synthetic gases, which also act as greenhouse gases, including CFCs and halons. Increases in carbon dioxide levels have contributed most to the enhanced greenhouse effect, which has increased by approximately 1/3 since pre-industrial times and by 30 ppm in the past 20 years.
  • 15. The enhanced greenhouse effect
    • Scientists have studied the Earth's past climate extending back millions of years. Evidence in the fossil record indicates that the Earth's climate has shown great variability over time, even before humans existed. However, climatologists have detected a steady though small increase in global average temperatures over the last few decades, based on weather data collected from all around the world. Most scientists agree that the recent global warming is due to anthropogenic causes , and whilst it may be within the realm of past variability, is of great concern.
  • 16. Atmospheric Processes
    • While the earth's temperature is dependent upon the greenhouse-like action of the atmosphere, the amount of heating and cooling are strongly influenced by:
      • the type of surface that solar radiation encounters when it hits the Earth. Forests, grasslands, ocean surfaces, ice caps, bare soil, deserts, and cities all absorb, reflect, and radiate heat energy differently. Sunlight falling on a white icy surface is strongly reflected back into space, resulting in minimal heating of the surface and lower atmosphere. Sunlight falling on a dark, bare soil, however, is strongly absorbed, on the other hand, and contributes to significant heating of the surface. Albedo is the percentage of solar energy reflected back by a surface.
      • water vapour and clouds: water vapour in the atmosphere both reduces the amount of solar radiation reaching the earth's surface but also acts as a greenhouse gas. The amount of water vapour in the atmosphere is also highly dependent on air temperature.
      • These factors complicate the extent of the enhanced greenhouse effect.
  • 17.
    • Some animated diagrams of the greenhouse effect
    • Q 4 - 12
  • 18. Sources of greenhouse gases include:
    • Natural:
    • water cycle – evaporation and transpiration
    • respiration
    • decay
    • enteric fermentation
    • fires
    • volcanic eruptions
    • attrition of shells and limestone
    • ozone formation
  • 19. Sources of greenhouse gases include:
    • Anthropogenic:
    • burning and combustion
    • agriculture
    • landfill
    • transport
    • sewerage treatment
    • industrial processes
    • generation of dust and aerosols
    • land clearance
  • 20. Carbon sinks include:
    • Natural:
    • vegetation
    • soils
      • roots of plants
      • organic debris
    • oceans
      • dissolution
      • photosynthesis by phytoplankton and algae
    • Anthropogenic:
    • geosequestration
    • biochar