Wednesday, August 26, 2009

There are two meanings of the term "greenhouse effect". There is a "natural" greenhouse effect that keeps the Earth's climate warm and habitable. There is also the "man-made" greenhouse effect, which is the enhancement of Earth's natural greenhouse effect by the addition of greenhouse gases from the burning of fossil fuels (mainly petroleum, coal, and natural gas). In order to understand how the greenhouse effect operates, we need to first understand "infrared radiation". Greenhouse gases trap some of the infrared radiation that escapes from the Earth, making the Earth warmer that it would otherwise be. You can think of greenhouse gases as sort of a "blanket" for infrared radiation-- it keeps the lower layers of the atmosphere warmer, and the upper layers colder, than if the greenhouse gases were not there.

How Do Humans Contribute to the Greenhouse Effect
While the greenhouse effect is an essential environmental prerequisite for life on Earth, there really can be too much of a good thing.
The problems begin when human activities distort and accelerate the natural process by creating more greenhouse gases in the atmosphere than are necessary to warm the planet to an ideal temperature.

Burning natural gas, coal and oil —including gasoline for automobile engines—raises the level of carbon dioxide in the atmosphere.
Some farming practices and land-use changes increase the levels of methane and nitrous oxide.
Many factories produce long-lasting industrial gases that do not occur naturally, yet contribute significantly to the enhanced greenhouse effect and “global warming” that is currently under way.
Deforestation also contributes to global warming. Trees use carbon dioxide and give off oxygen in its place, which helps to create the optimal balance of gases in the atmosphere. As more forests are logged for timber or cut down to make way for farming, however, there are fewer trees to perform this critical function.
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.
Types of Greenhouse gases

The major greenhouse gases in the atmosphere are carbon dioxide (CO2), methane, (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone (O3). Atmospheric water vapour (H2O) also makes a large contribution to the natural greenhouse effect but it is thought that its presence is not directly affected by human activity.

Trends in greenhouse gas concentrations

a) Carbon Dioxide (CO2)

High-quality observations of the concentration of CO2 began in 1958, with flask measurements at the Mauna Loa Observatory in Hawaii. Fig. 2 shows that the average annual concentration of CO2 in the atmosphere has risen from about 315 ppmv (part per million by volume) in 1958 to around 363 ppmv in 1997. There is a clear annual cycle in the Mauna Loa data that corresponds to the annual cycle of plant respiration in the Northern Hemisphere : CO2 concentration increase during the Fall and Winter and decline during Spring and Summer. This cycle, follows the growth and die back of vegetation, is reversed and of smaller amplitude in the Southern Hemisphere, and disappears almost entirely in the data measured near the Equator.

b) Methane (CH4)

The rate of increase of the atmospheric abundance of methane has declined over the last decade, slowing dramatically in 1991 to 1992, though with an apparent increase in the growth rate in late 1993 . The average trend over 1980 to 1990 is about 13 ppbv/year (part per billion by volume/year).

c) Nitrous Oxide (N2O)

Over the last four decades, the average growth rate of N2O is about 0.25%/year (Fig. 4). Current tropospheric concentration of N2O is around 312 to 314 ppbv.

d) Chlorofluorocarbons (CFCs)

Among the family compounds of chlorocarbons, CFCl3 (CFC-11) and CF2Cl2 (CFC-12) are receiving more attention because of their larger concentrations and potentially significant effects on stratospheric ozone. CFC-11 and CFC-12 have the highest concentrations of the man-made chlorocarbons, around 0.27 and 0.55 ppbv, respectively (measured at Mauna Loa in 1997, ). As indicated in their GWP values, these two gases are strong infrared absorbers. It is thought that CFC-11 and CFC-12 have contributed about one-third of the radiative forcing of gases other than CO2 during the 1980s.

Consequences of Enhanced Greenhouse Effect

i) Global Warming
Increase of greenhouse gases concentration causes a reduction in outgoing infrared radiation, thus the Earth's climate must change somehow to restore the balance between incoming and outgoing radiation. This ¡§climatic change¡¨ will include a ¡§global warming¡¨ of the Earth's surface and the lower atmosphere as warming up is the simplest way for the climate to get rid of the extra energy. However, a small rise in temperature will induce many other changes, for example, cloud cover and wind patterns. Some of these changes may act to enhance the warming (positive feedbacks), others to counteract it (negative feedbacks).
Using complex climate models, the "Intergovernmental Panel on Climate Change" in their third assessment report has forecast that global mean surface temperature will rise by 1.4¢J to 5.8¢J by the end of 2100. This projection takes into account the effects of aerosols which tend to cool the climate as well as the delaying effects of the oceans which have a large thermal capacity. However, there are many uncertainties associated with this projection such as future emission rates of greenhouse gases, climate feedbacks, and the size of the ocean delay ...etc.

ii) Sea Level Rise
If global warming takes place, sea level will rise due to two different processes. Firstly, warmer temperature cause sea level to rise due to the thermal expansion of seawater. Secondly, water from melting glaciers and the ice sheets of Greenland and the Antarctica would also add water to the ocean. It is predicted that the Earth's average sea level will rise by 0.09 to 0.88 m between 1990 and 2100.

Potential Impact on human life

a) Economic Impact
Over half of the human population lives within 100 kilometres of the sea. Most of this population lives in urban areas that serve as seaports. A measurable rise in sea level will have a severe economic impact on low-lying coastal areas and islands, for examples, increasing the beach erosion rates along coastlines, rising sea level displacing fresh groundwater for a substantial distance inland.

b) Agricultural Impact
Experiments have shown that with higher concentrations of CO2, plants can grow bigger and faster. However, the effect of global warming may affect the atmospheric general circulation and thus altering the global precipitation pattern as well as changing the soil moisture contents over various continents. Since it is unclear how global warming will affect climate on a regional or local scale, the probable effects on the biosphere remains uncertain.

c) Effects on Aquatic systems
The loss of coastal wetlands could certainly reduce fish populations, especially shellfish. Increased salinity in estuaries could reduce the abundance of freshwater species but could increase the presence of marine species. However, the full impact on marine species is not known.

d) Effects on Hydrological Cycle
Global precipitation is likely to increase. However, it is not known how regional rainfall patterns will change. Some regions may have more rainfall, while others may have less. Furthermore, higher temperatures would probably increase evaporation. These changes would probably create new stresses for many water management systems.

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