Just to be clear-- The radiative forcing due to CO2 increases is logarithmic--that is, the radiative forcing going from 300 to 600 ppm is the same as going from 600 to 1200 ppm. Thus, the forcing due to the rising CO2 concentration does decrease on a per ppm basis.
However, forcing is not sensitivity, and like Tom Wigley, I recall papers that have done a good bit of testing of plausible changes in concentration and the sensitivity (that is, the temperature change for a doubling of the CO2 concentration) is, near as it can be estimated, pretty linear. At lower temperatures one may have more snow/ice albedo feedback, but at higher temperatures one has more water vapor and, very likely, carbon cycle feedback (carbon cycle feedback including thawing permafrost and releasing CO2/CH4, out-gassing of CO2 from warmer ocean, higher airborne fraction as ocean overturning slows, etc.). Given the warmth of the Cretaceous, it is hard to be sanguine about adding more and more CO2 to the atmosphere. And given the heat of Venus, which absorbs less solar per square meter than the Earth even though closer to the Sun, it seems really difficult to argue that adding greenhouse gases to an atmosphere leads to a plateau in the response. Mike MacCracken On 2/20/12 7:38 AM, "Tom Wigley" <wig...@ucar.edu> wrote: > Sensitivity is the equilibrium change in global-mean temperature per > unit of radiative forcing. Linearity has been demonstrated up to much > higher forcings than will ever be reached by even the most pessimistic > scenarios. > > Early IPCC reports might cover this. I recall work by Kiehl on this back > in the mid 1980s -- too far back to recall the reference. > > Tom. > > +++++++++++++++++ > > On 2/20/2012 5:28 AM, Robert Chris wrote: >> I am engaged in discussion with a modestly prominent climate skeptic >> who argues that global warming isn't a problem because as CO2 >> concentrations rise climate sensitivity reduces. I recall coming >> across this notion before but I don't know how much peer-reviewed work >> has been done on it. I'd appreciate some help with references to peer- >> reviewed papers that address the idea that climate sensitivity may be >> logarithmic rather than linear so that as atmospheric CO2 >> concentrations rise the effective climate sensitivity reduces and >> discuss the likely levels at which this reduction becomes significant >> in terms of reducing the GWP of CO2. >> >> >> ----------------------------- >> Robert Chris >> The Open University >> r.g.ch...@open.ac.uk >> > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
