We do not inherit the earth from our ancestors - we borrow it from our children
What degree of proof about the human catastrophe from global climate change do we need
before we are motivated to act to prevent it? - Eric Chivian, M
Greenhouse Gases
What is a Greenhouse Gas?
Greenhouse gases are simply gases in the atmosphere that serve to warm the earth by trapping and re-emitting longwave (infrared) radition that is emmitted by the earth. Some of these gases are natural, others are anthropogenic (human) in origin.
Is it Really a "Greenhouse" Effect?
No. The word "greenhouse" is used because of the supposed analogy to the way the glass or plastic of the greenhouse keeps the temperature inside above what it would be on the outside, without the 'greenhouse effect'. In greenhouse however the increase in temperature is mostly due to solar energy from the sun heating the air inside which then remains trapped by the walls. Unlike a real greenhouse the atmospheric greenhouse effect is due to the trapping of infrared radiation that would otherwise escape into space, cooling the earth. So it is not due to the warming and trapping of air but rather due to the trapping of radiation which then prevents cooling, so that it is not really a "greenhouse effect". However this is the term that most people are familiar with so it is the one we will use as well.
The Natural Greenhouse Effect
Without the greenhouse effect the earth would be about 33 degrees Celsius cooler than it is at present. This obviously would have catastrophic consequences for life on earth as we know it. That mid summer day when it may often reach 30 degrees or more in BC, the temperature instead would be hovering at zero.
The reason for the natural greenhouse effect is the effect of clouds and infrared -absorbing (ie: greenhouse) gases. Clouds prevent the escape of radiation at night when the surface cools; during the day they reflect the sun's radiation back into space. The amount of radiation they trap or reflect is highly dependent upon cloud type, thickness (opaqueness), height, coverage etc. Natural infrared absorbing gases include carbon dioxide (CO2) and water vapour (H2Ov), plus a small amount of methane (CH4) and other trace gases. The man-made greenhouse gases include CO2, CH4 and chloro - flouro - carbons or CFC's. Because these gases do not absorb visible radiation they let solar radiation in but serve to trap (absorb) some of the outgoing infrared radiation emmitted by the earth, preventing cooling.
Anthropogenic Climate Change
Anthropogenic climate change is the climatic shifts that are caused by the release of man made greenhouse gases into the atmosphere. The major one of concern is CO2. For almost 10,000 years, the CO2 concentration was approximately constant at 280ppm. Since the industrial revolution of the 1800's when we began burning mass quantities of fossil fuels including coal, gas, oil, etc, the concentration has increased 30% and is rising steadily, with current concentrations over 365 ppm (2000 measurement) (see table below). If no efforts are made to mitigate this increase concentrations could easily reach 500ppm by 2100.
However, CO2 is not the only man made gas of concern. Others include methane (industry, garbage, cattle raising etc), nitrous oxides ( a byproduct of combustion - comes primarily from automobiles), and of course CFC's. Shown in the table below is the concentrations of anthropogenic greenhouse gases in 1800 before the industrial revolution compared to the values measured in 1999. Also shown is the rate of change and the average lifetime of one of these gases in the atmosphere. So while CFC's have a small concetration they are relatively inert in the atmosphere, lasting 50 to more than 100 years.
Gas 1800 1999 Change/yearAtms. Lifetime
1800 1999 Change/yearAtms. Lifetime Water Vapour 3000 ppm3000 ppm ? ppm -
3000 ppm ? ppm -Carbon Dioxide 280 ppmv353 ppmv1.8 ppm (.5%) 40 years
353 ppmv1.8 ppm (.5%) 40 yearsMethane .8 ppmv 1.75 ppmv0.015ppm(.9%) 11 years
.8 ppmv 1.75 ppmv0.015ppm(.9%) 11 yearsNitrous Oxide288 ppbv 310 ppbv 0.8 ppb(.25%) 130 years
288 ppbv 310 ppbv 0.8 ppb(.25%) 130 yearsCFC-11 0 280 pptv 9.5 ppt (4%) 55 years
0 280 pptv 9.5 ppt (4%) 55 yearsCFC-12 0 484 pptv 17 ppt (4%) 116 years
0 484 pptv 17 ppt (4%) 116 yearsWhat are the Most Important Greenhouse Gases?
By far the most abundant greenhouse gas is water vapour (0-4% of the atmospheric gases); H2O traps one third of the heat trapped by all the gases. However, it should be kept in mind that when the effect of clouds is included the effect may be postive or negative since while clouds trap infrared radiation they also reflect solar raditiation back into space before it has a chance to reach, and warm, the earth's surface. Clouds furthermore absorb the most outgoing radiation since they contain liquid water which, while it has a very high heat capacity, it is an excellen absorber. The next most important greenhouse gas is carbon dioxide (0.03% of the atmospheric gases) which traps about 12% of the total radiation directly trapped by all the gases. In terms of human activities this is the gas of creates the greatest concern due to the large quantities in which they are released (more than 30% increase in atmospheric concentrations since a little over 100 years ago). The concentration of H2O vapour is essentially unaffected by human activities because it is already so high. Furthermore the concentraion is determined also by the saturation vapor pressure of the air which is determined by temperature and will cause the vapour to condense into a liquid which will result in its removal due to gravity (rain).
What is the Atmospheric Window?
Atmospheric CO2 strongly absorbs infrared radiation at 15µm band; there is so much CO2 (and H2O) that all the radiation in this band is completely absorbed by the present atmosphere. For one thing, there is a "window" between the wavelengths of 8-11µm (microns) where infrared raditiaton is allowed to escape as CO2 does not absorb much in this window. Gases such as CFC's that can absorb in these windows are of greater concern because they are able to trap radiation that would have otherwise been able to escape, even with higher CO2 levels.
Other Greenhouse Gases
Other contributors are ozone (which traps ~3% of the total), methane, nitrous oxide, and the CFC's. Estimates place the relative contributions of (CO2 : CH4 : CFC's : N2O) at (1 : 0.4 : 0.4 : 0.1). It is worth noting, however, that the gases that have the greatest warming potential are the fully fluorinated compounds (FFCs), because there is no significant mechanism for their destruction on this planet as a result their atmospheric lifetimes are estimated to be up to 50,000 years. SF6 is the most potent greenhouse gas of all. The concentration of SF6 is rising at 8% per year, it is used in agricultural production, and, perhaps ironically, its use has also increased rapidly due to it's use as replacements for the CFC's which are damaging the ozone layer. However SF6 is a much more dangerous greenhouse gas than CFC's. .
For much more information on CO2 go the Carbon Cycle
