Submitted by: Anonymous

Answer:
Perhaps a better way of phrasing what we believe this question asks would be to inquire about the degree of global warming caused by an incremental increase of CO2 at different initial, or starting, concentrations of CO2; for each new increment of a radiatively-active greenhouse gas that is added to a planetary atmosphere produces less warming than the preceding increment.

To provide a numerical illustration of this phenomenon, consider the planet Venus, which has a massive 93-bar atmosphere that is about 96% CO2 (Kasting et al., 1988).  Relative to Earth, we could say that its atmospheric CO2 concentration is 0.96 x 93 bar x 1,0000,000 ppm CO2 per bar = 89,300,000 ppm CO2.  This amount of CO2 produces a greenhouse warming on Venus of approximately 500°C (Kasting et al., 1988).  Hence, the mean global warming produced by a 300 ppm increment of CO2 over this vast concentration range (0 to 89,300,000 ppm) is (500°C / 89,300,000 ppm CO2) x 300 ppm CO2 = 0.0017°C.

Now consider Mars, which has a nearly pure CO2 atmosphere of about 0.0085 bar (McKay, 1983) and a greenhouse warming of approximately 5.5°C (Kasting et al., 1988).  In this case, the mean global warming produced by a 300 ppm increment of CO2 over this much smaller concentration range (0 to 8,500 ppm) is (5.5°C / 8,500 ppm CO2) x 300 ppm CO2 = 0.19°C

In an extension of a very similar type of analysis, Idso (1988) has shown that a 300 ppm increase in the air's CO2 concentration from a base value of 300 ppm (which would correspond to Earth at about the year 1920) has the ability to warm a planet by approximately 0.4°C, while the initial 300 ppm increment is responsible for about 1°C of warming.

Within the context of the current debate over the ongoing rise in the air's CO2 content, it is the 0.4°C warming that is of greatest relevance; for the thermal consequence of a 300 to 600 ppm doubling of the atmospheric CO2 concentration is what people typically attempt to calculate.  We feel, however, that this result is an upper limit to what could actually occur; for as Idso (1998) has argued, there are a number of biological phenomena that would likely be set in motion by such a rise in the air's CO2 content that could well negate this primary impetus for warming.  Consequently, although the CO2 greenhouse effect is definitely real, as demonstrated by measurements of the surface temperatures and atmospheric compositions of Mars and Venus, Earth - the "living planet" - may well depart somewhat from the expectations provided by its lifeless planetary neighbors.

Even if Earth's biosphere were unable to thwart the primary CO2-induced impetus for warming, however, the maximum temperature rise that is predicted by this comparative planetology analysis is still much less than what is currently being predicted by most general circulation models of the atmosphere.  And that is one of the reasons why we do not believe they are providing the correct answer to this problem.  There must be large positive feedbacks involved to get the type of warming they are predicting; but most of the evidence we have seen suggests that negative feedbacks predominate.  Ergo, the debate continues.

References

Idso, S.B.  1998.  CO2-induced global warming: a skeptic's view of potential climate change.  Climate Research 10: 69-82.

Kasting, J.F., Toon, O.B. and Pollack, J.B.  1988.  How climate evolved on the terrestrial planets.  Scientific American 258 (2): 90-97.

McKay, C.  1983.  Section 6. Mars. In: R.E. Smith and G.S. West (Eds.), Space and Planetary Environment Criteria Guidelines for Use in Space Vehicle Development.  1982 Revision (Vol. 1). NASA Tech. Memo. 82478, Marshall Space Flight Center, AL.