Meanwhile, while we're on the subject, a couple of folks have raised McKitrick's plan, which bases a carbon tax on the instantaneous global mean surface temperature (or, according to one correspondent, the equatorial mean).
The equatorial mean, of course, is plainly silly: why pick the least temperature-sensitive spot to measure the anthropogenic damage, unless you were looking for a way to pretend it away.
There is some appeal to the idea, though; the idea is to find a measure of global change that is independent of the scientific theory, such that people who don't have any faith in the science could agree to it, and to use that measure to calibrate the response.
In practice McKitrick's idea seems either ignorant or disingenuous. I cannot take anybody who proposes it seriously, because they still remain firmly in the class of people who "don't get it".
Let's leave aside the usual confusion about "global warming", which is a symptom of anthropogenic climate change, not the disease. We can reduce global warming in a literal sense to zero easily enough with additional aerosol releases, but this will not avoid massive climate change.
Even if global temperature were the a complete measure of anthropogenic climate change, the problem is that it's a delayed measure. The ocean and sea ice take a while to respond; the land ice even longer; and the clathrates (we hope) still longer than that. (*) What this means is that the temperature we see now is the temperature we bought in the past; some of the response is delayed, and some of it is greatly delayed. This is part of what I would call a basic policy-level understanding of the science. If you miss that point, you don't know enough about what is going on in the climate system to venture a serious policy.
I like the idea that the policy should adjust to the evidence. But the idea that the instantaneous temperature is a measure of the sensitivity is a mark of denialist-influenced confusion. Unfortunately there simply is not a simple metric of how deeply in trouble we are. I think the basic idea is not unreasonable on its face, but the measure we use needs to account for the delays in the system.
We could apply something like this to the trillionth ton constraint. If the system is less sensitive than we think, we can loosen the constraint. The trouble is that is the system is more sensitive than we think, there is very little time to tighten the constraint. There is realistically no more time for dawdling, since even the trillion ton limit carries plenty of climate risk on present evidence. We should shoot for that now; shooting for two trillion and realizing the right goal is one trillion is no good if you find yourself already committed to a trillion and a half.
(*) These can be regarded as exacerbating feedbacks from the point of view of the atmosphere system, or slow modes from the point of view of the whole coupled climate system.