Greenhouse Gases

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INTRODUCTION [SLIDE 2]
Many chemical compounds found in the Earth’s atmosphere act as “greenhouse gases.” These gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. Over time, the amount of energy sent from the sun to the Earth’s surface should be about the same as the amount of energy radiated back into space, leaving the temperature of the Earth’s surface roughly constant. Greenhouse gases greatly affect the temperature of the Earth. Without them, Earth's surface would average about 33°C colder than the present average of 14 °C (57 °F). However, due to certain human activities, there is an increase in the amount of greenhouse gases causing the earth to “warm” but we will talk about that later. EXAMPLES [SLIDE 4]

Many gases exhibit these “greenhouse” properties.In order, the most abundant greenhouse gases in Earth's atmosphere are: * water vapour (H2O)
* carbon dioxide (CO2)
* methane (CH4)
* nitrous oxide (N2O)
* ozone (O3)
And some are exclusively human-made like aerosols and halocarbons.

SOURCES [SLIDE 6]
There are three primary anthropogenic (human caused) contributors to rising greenhouse gas concentrations: (1) our excessive burning of fossil fuels for energy,
(2) our accelerating removal of the earth's natural carbon storage through our destruction of forests and our removal of previously vegetative land for habitat, grazing, and agriculture for livestock production and (3) our various manufacturing processes which release near-permanent and extremely potent man-made greenhouse gases.

(4)Population growth is another factor in global warming, because as more people use fossil fuels for heat, transportation and manufacturing the level of greenhouse gases continues to increase. As more farming occurs to feed millions of new people, more greenhouse gases enter the atmosphere.

Let’s look at the sources of the primary greenhouse gases. [SLIDE 8] |
• Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour. Indirectly, humans have the potential to affect water vapour substantially by changing climate. For example, a warmer atmosphere contains more water vapour. Human activities also influence water vapour through CH4 emissions, because CH4 undergoes chemical destruction in the stratosphere, producing a small amount of water vapour. • Carbon dioxide has increased from fossil fuel use in transportation, building heating and cooling and the manufacture of cement and other goods. Deforestation releases CO2and reduces its uptake by plants. Carbon dioxide is also released in natural processes such as the decay of plant matter. • Methane has increased as a result of human activities related to agriculture, natural gas distribution and landfills. Methane is also released from natural processes that occur, for example, in wetlands. Methane concentrations are not currently increasing in the atmosphere because growth rates decreased over the last two decades. • Nitrous oxide is also emitted by human activities such as fertilizer use and fossil fuel burning. Natural processes in soils and the oceans also release N2O. • Ozone is a greenhouse gas that is continually produced and destroyed in the atmosphere by chemical reactions. In the troposphere, human activities have increased ozone through the release of gases such as carbon monoxide, hydrocarbons and nitrogen oxide, which chemically react to produce ozone. As mentioned above, halocarbons released by human activities destroy ozone in the stratosphere and have caused the ozone hole over Antarctica. Human activities have resulted in these gases to accumulate in the atmosphere, causing concentrations to increase with time....
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