My opinion is that those who would propose that the logarithmic curve of CO2 in any way suggests no additional warming if pre-industrial levels simply don’t know very much about radiative transfer and how saturation of CO2 in the atmosphere actually works. I don’t blame them, as it is actually pretty complicated stuff, but I do blame them if they simply believe what is posted on a skeptical blog rather than do some deeper reading on the subject from a wide variety or sources. I would highly suggest that anyone serious about actually learning about the science behind AGW read “The Warming Papers” (available on Amazon) cover to cover. If you don’t have the time and money for that, but are serious about understanding the true science and physics behind CO2 and the saturation effect in the atmosphere, one of the very best postings on this can be found here:
http://scienceofdoom.com/2010/05/12/co2-an-insignificant-trace-gas-part-eight-saturation/
The above post is just part 8 of an 8 part series about this, but really, anyone who is serious about this subject and who really wants to try to get a handle on the actual science behind what CO2 really does as it relates to radiative transfer in the atmosphere and doesn’t want to read The Warming Papers, ought to at least read the complete series from the above link starting at the very beginning here:
http://scienceofdoom.com/2009/11/28/co2-an-insignificant-trace-gas-part-one/
Here’s the thing to keep in mind. We’ve only seen the transient climate response (TCR) to our current 392 ppm of CO2, and have not yet seen the equilibrium climate response (ECR) as it takes many decades to unfold…i.e. the cryosphere is responding to levels of CO2 we had met years ago, and will be responding many years from now to our current level, even if they somehow froze at 392 ppm) but we never will know what the ECR would be from 1990′s, 2002′s, or today’s level because the atmosphere continues to accumulate more and more greenhouse gases every year, far faster than the longer-term equilibrium response will take.
Here’s my current view on the issue of TCR/ECR and what final average global temperature increase of the lower troposphere we might end up with if we are so fortunate as to halt the rise in CO2 at 560 ppm.
The TCR for the lower troposphere might be as low as 1.5C per doubling of CO2 from per-industrial levels, but all that is fairly unimportant, as it is the ECR that really matters in the long term as it is what effects ecosystems, weather patterns, etc.. Where will the temperatures settle once the system has seen all negative and positive slower Earth System feedbacks play out? 3C as an ECR is a pretty reasonable estimate, given that numerous studies give a range for the TCR/ECR ratio from about 0.4 to 0.8. Also, paleoclimate data from the Pliocene and Miocene (the last time CO2 levels were this high or higher) indicate temperatures 3C or more higher. So both the climate models and the paleoclimate data would suggest that a 3C increase for a lower tropospheric ECR is a pretty reasonable, in the range estimate for an Earth that goes from 280 to 560 ppm of CO2. Of course, given the rapidity (from a geologic perspective) with which CO2 (and other GH gases) is increasing, it is entirely possible that various components of the slower earth-system response could lag others, meaning that, even halting the concentration of CO2 at 560 ppm could lead to an over-shoot situation whereby temperatures increase by 4.5C or even higher before settling back at the 3C higher level for the equilibrium response.