There is a strong possibility of confusing two subtly different concepts here.
Firstly, with respect to the linear response to small changes in GHG (especially CO2) forcing: it must be borne in mind that the forcing of 4W/m^2 is really very small compared to the range of changes that the climate system already sees locally, which may be over a thousand W/m^2 on a daily basis and at least 100 W/m^2 on a seasonal basis (which is long enough for the large land masses and atmosphere to reach local equilibrium given the sea surface temperature). To first order, all responses to small perturbations are linear. This is also supported by basically every model - we can occasionally create threshold effects, especially on a regional basis, but it is hard to create something that has a broad effect. One phenomenon I am aware about in some models is a runaway warming that may be due to the mis-specification of ozone in the stratosphere - briefly, the ozone distribution is generally held fixed in space and if the tropopause rises, this may result in "stratospheric" ozone appearing in the troposphere where it buggers up the radiation code. Similarly, very high GHG (10x) concentrations may invalidate the approximations in the code. But we have good reasons to believe that the equilibrium response to small increases in CO2 is near-linear, and the question is merely regarding the slope of that line. Now, when we are talking about substantially different forcing mechanisms, especially large ice sheets, and also rather different climate states - involving things like continental rearrangement and appearance of the major mountain ranges, which may result in large changes to the global climate state including things like dominant weather patterns and the mean humidity of the atmosphere, the same argument as above would very likely apply to modest CO2 perturbations on top of that state (though the modelling support will be far thinner simply because people aren't doing so many experiments), but the slope of the linear response is likely to be different. In fact, our modelling work (and that of other people) strongly supports the belief that the background climate state does affect the response to co2 changes. So while there may be some nonlinearity, ie a change in response as the forcing changes, there is no reason to either think this is likely to be a significant threshold, or that we are close to such a threshold. People are looking at this (especially wrt N Atlantic overturning circulation) but they seem to find it hard to get anything interesting - even on this regional basis, let alone global. James 2009/12/10 [email protected] <[email protected]>: > The Early faint Sun paradox is interesting. Having done a bit of > reading about it, it seems that one plausible explanation is that the > steady increase in the Sun's solar output may have been compensated by > a (less steady) decline in greenhouse gas concentrations, notably the > very powerful, but low concentration ones like methane. > > Having said that, I still have trouble with the notion that forcings > always have the same level of feedbacks. I find it quite plausible on > the face of it that clouds, ice sheets, greenhouse gases etc. > sometimes could act as a near perfect thermostat, and sometimes they > might act to amplify even the tiniest of external perturbations. So, > in the ice ages, a solar related external perturbation of virtually > nothing got amplified to 6C, and over a longer time scale, a massive > solar perturbation didn't get amplified, but rather regulated away by > the Earth system response. > > What do I conclude from that about how the system might respond to a > perturbation now? Well, I clearly don't buy Lindzen's line that it's a > near certain fact that a few W/m2 will now give virtually no response. > But I also fail to see, where the certainty comes from that as James > Annan put it, climate sensitivity is 3C. By analogy with past climate > alone it's certainly plausible to expect both the possibility of > massive amplification and a virtually perfect thermostat. > > So, maybe in the ice ages, cloud feedbacks due to ice sheets were very > different to how cloud feedbacks might operate due to a forced > increase in CO2 concentrations together with massive aerosol > injections plus quite a few land use changes. And, while those > feedbacks might be thermostat like now, why shouldn't there be a > threshold, where clouds suddenly become strongly amplifying instead? > > -- > You received this message because you are subscribed to the Google Groups > Global Change ("globalchange") newsgroup. 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