Recently, John O'Sullivan and Claes Johnson have discussed the apparent division of the climate debate participants into three categories: The alarmists, the lukewarmers and the deniers. I myself find the distinction between lukewarmers and deniers very interesting for several reasons though, as pointed out, for an outsider it might be difficult to really grasp what the fuss is all about. The reason for this is probably because the discussion has largely been focused on climate politics rather than the politics of science, a distinction which I will try to clarify in the following. Suppose you are interested in climate politics, then the relevant question to ask would be something like "What will be the temperature change from a doubling of CO2". If asked to a lukewarmer the answer would probably be something between 0.5 and 1 degrees warming whereas a denier might end up in a small interval around 0 degrees. Ok, so why all this rancour? The answer, I think, is spelt "the greenhouse effect". The discussion becomes difficult since the very definition of this effect is evasive, but let's try to list some candidates:
A1. The Tyndall Effect. An experiment reproducible in the lab where, by using electrically driven equipment, you selectively excite the degrees of freedom of GHGs causing heating.
A2. The thermodynamic effect of GHGs in the atmosphere, whatever it might be.
A3. The thermodynamic effect of backradiation on the surface temperature, whatever it might be.
A4. The difference between the measured surface temperature and a hypothetical "black-body temperature", whatever the cause.
A5. The alteration of the atmospheric temperature profile as outlined in climatology textbooks and quantified by the use of software like MODTRAN.
Lukewarmers tend to get both irate and personally insulted any time some denier has the audacity to question the well-posedness of this effect. This attitude of course has a very detrimental effect on climate science since any one who proposes some new theory about the atmosphere must relate that theory to some mysterious concept called "the greenhouse effect" that is very ill-defined but nevertheless cannot be questioned. Even the demarkation of denial is not very clear. Anthony Watts seems to be open to discussing the "magnitude" of this effect whereas Fred Singer takes a more orthodox stance: Those who question the ability of these gases to elevate the surface temperature by 33 degrees are also to be considered deniers. (As a side remark, the lukewarers seem to be very certain of what is going to happen if we lower the concentration of CO2 though the effect of an increase is, for some reasons, very uncertain). This attitude is so puzzling that it calls for an explanation. So let's try that too, why do people become lukewarmers? We may speculate in the following reasons:
B1. Credibility reasons
The issue of climate politics is so important that you, for credibility, cannot afford to associate yourself with people whom you percieve as cranks or whom someone else might percieve as cranks.
B2. Social reasons
In order to be able to hang around with the "big" guys, like Lindzen, Singer and Spencer, you have to play the game and not offend their basic beliefs.
B3. Scientific prestige
You don't want to admit that what you once believed was fundamentally misguided.
B4. Political reasons
This could be viewed as a kind of "damage controle". You realise that the scientific establishment has made a huge blunder but want to save them from complete humiliation. Hence, it is better to blame everything on computer models and some convenient patsy like Michael Mann.
B5. Religious reasons
The greenhouse effect has become like an icon for you and if it disappeared it would feel like a deep personal loss and shatter your entire belief system. The greenhouse effect has made the atmosphere "non-trivial" for you.
The reason the lukewarmers would give themselves is probably something similar to the first one, since it would be impossible to admit any of the other. But is this credible? I don't think so. First of all, if you really believe in your arguments the mature way to deal with the situation is simply to, in a calm and pedagogical way, explain any misconceptions the deniers might have. On the contrary, it seems as if they desperately want to avoid the entire discussion, ignore it to death, which makes some of the other reasons more plausible. Moreover, they themselves constantly make attempts to make the deniers look like cranks. Here is where the proliferation of definitions of the GHE becomes handy, they may claim that:
C1. The greenhouse effect can be experimentally verified (i.e. the Tyndall effect can be experimentally verified).
C2. Deniers claim that GHGs have no thermodynamic effect at all.
C3. Deniers claim that backradiation does not exist or does not have any thermodynamic effect.
This is, of course, an active and deliberate way of discrediting the deniers. Constantly they evade the difficult position, namely that of defending the actual greenhouse gas hypothesis (A5). I will try to argue that all of the three smear tactics above (C1-C3) can be dealt with in a more or less straighforward way.
D1. The Tyndall gas effect is real. But what is the significance of this?. If you use precision instruments driven by electricity you can do a lot of things, for example, run computers and refrigerators. The real earth atmosphere system is not electrically driven and there are many other processes to consider.
D2. We do not claim that GHGs have no thermodynamic effect. In fact, many deniers believe that the atmospheric mass causes an incresed surface temperature, and since GHGs are massive particles they contribute to this effect.
D3. This is perhaps the most delicate point. Many deniers have been tempted to cook up very complicated arguments to explain why radiation from a colder atmosphere on a warmer surface has no effect at all. If you support the idea that the atmosphere does indeed have some effect on the temperature this position becomes problematic. Some people have speculated in some kind of pressure induced effect and the obvious question then becomes: Cannot the radiation pressure of CO2 contribute to this effect? If, like me, you instead envision some combined diffusion-convection process where the atmospheric mass acts as a blanket, then backradiation also have a perfectly natural role to play. Thus, I can't see any alternative than to say that backradiation does exist and that it has a thermodynamic effect.
This might come as a surprise to some people. Have I just acknowledged the greenhouse effect?Certainly not. The greenhouse effect is a ghost that can neither be verfied nor denied. As concerns backradiation, on the other hand, after many months of brooding I have come to the following tentative conclusion:
Backradiation is trivial
As simple as that. What the lukewarmers want you to believe is that it is not. Triviality made non-trivial.