Carbon dioxide is a trace gas in the atmosphere measuring in currently at around 380 parts per million (ppm). Represented instead as a percent, this equates to 0.038%. Another way to look at it this is with colored balls. Suppose you had 100,000 balls of two colors red and blue. If the red balls represented carbon dioxide molecules, then only 38 of the balls would be red. Why is it important to have this perspective? Heat in the form of infrared radiation must "find" one of these red balls as it is escaping from the Earth's surface. As you might guess, the odds are not very favorable for this.
Carbon dioxide is a fairly poor absorber of infrared radiation. The illustration below is the infrared absorbance of both carbon dioxide and water vapor. The peak areas are regions in the infrared spectrum where each molecule absorbs this heat energy. Note that water vapor is a much better absorber (more peak regions) than the carbon dioxide. Also, some of the absorption bands for water vapor and carbon dioxide overlap. This means that they compete for the same heat energy. Since there is usually more water vapor in the atmosphere, the water vapor has a greater chance of absorbing the radiation.
In chemistry, we learn that the absorbance of radiation is logarithmic. For those not familiar with logarithmic functions, a logarithm rises up rapidly in the beginning and then increases at an ever-slowing rate. A graph showing the cumulative temperature increase versus increasing parts per million of carbon dioxide is seen below.
The first 20ppm of carbon dioxide accounts for over 55% of the net warming. When the level of carbon dioxide reaches 100ppm, this accounts for over 81% of the net warming effect. Man has added about 100ppm since 1940, which is represented by the last five data points. This last 100ppm has added roughly 0.11 degrees Celsius or about 3.8% of total warming that can be attributed to man's emission of carbon dioxide.
What if we continue to add even more carbon dioxide? We would see an even smaller and smaller addition to the net warming. Increasing the carbon dioxide from 380ppm to 480ppm would produce only about another 0.05 degrees Celsius increase in temperature.
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