Thanks for the response. I just found a site explaining that most of the earth's CO2-related heat comes from the FIRST 20 ppm, so it looks like you're right.
Are you aware of the following characteristic of Carbon Dioxide?
One of the more interesting issues that is often ignored in most discussions is the saturation issue of carbon dioxide. The first 20 ppm in our atmosphere make up most of the induced warming from carbon dioxide. After the first 150 ppm or so, the ability of carbon dioxide to increase temperature by itself become almost nill. (see Lindzen-Choi graph in related link). The doubling of CO2 from the 190 to 380 ppm has followed this trend and seen less then a degree (C) of overall warmth. Doubling again, using this graph, will see almost no additional heating of the planet according to this concept.
http://wiki.answers.com/Q/How_does_carbon_dioxide_contribute_to_global_warming
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