Subtitle: Some Physical Thoughts on the notion of “Climate Sensitivity”.
It is generally agreed that the central issue in the climate debate is the magnitude of warming one would expect to occur as a result of a doubling of CO2, which results in a “forcing” of ~3.7 watts per meter squared. However, in a moment of sudden insight, I realized that, from a physics perspective, there are several problems with the way this is talked about. But let’s start with a definition. The “equilibrium climate sensitivity”-what most people in this debate think they want to know-is equal to the eventual change in temperature of the Earth that would occur per unit change in the radiative “forcing” of the system. Think about that for a few seconds.
If you have any knowledge of physics, you have probably already realized how this notion can get you into serious trouble. Consider the following case: We shut off the sun. The Earth will begin to cool, and it will do so for a very long time, until it reaches ~0 degrees Kelvin, or a cooling of ~288.15 K. Again, this is eventually, not on a timescale which is really relevant to policy, say. But never mind that. Incoming solar radiation in this circumstance has been reduced ~342 w/m^2. Thus the “equilibrium climate sensitivity” MUST be ~0.84 K/wm^-2, or a little over three degrees for a doubling of CO2. That is, of course, assuming that the figures I’m using for incoming solar radiation and mean surface temperature are correct. Of course, this does not correspond to the sensitivity to a doubling CO2 necessarily. For one thing, the physics governing the feedback mechanisms will be very different at temperatures that low, since the atmospheric gases would have long since ceased to be gases. However, I can only think of things which might plausibly mean that the sensitivity thus derived is too high, not too low. So one might say that very large negative forcing is a “special case” because the response is sure to be very different when the greenhouse effect can’t apply because water vapor will have frozen out, along with CO2 and everything else.
Timescale is also a serious issue. If it continues to warm indefinitely, never reaching “equilibrium” after doubling CO2, just very slowly, well, who cares if in a billion years the warming would reach 5 degrees if in the next thousand it was only 2? I know I wouldn’t. And yet it is technically the long, irrelevant period which is the “equilibrium” sensitivity. Indeed, it stands to reason that the Earth never gets to realize true equilibrium response to external perturbation, if it operates on timescales that long, the forcing is constantly changing on timescales far too short to fully realize the response.
Now I don’t mean to say that the whole concept is bunk, but there seem to be details which people overlook about this.