I don’t do any climate sensitivity calculations in this post. I’ve cited Hegerl 2000 which uses proxy data (not instrumental data) over the last 750 years to calculate a best guess climate sensitivity of around 3 degrees C.

Fair enough – let’s test that theory out on Cook’s graph :

First step, we will remove the instrumental data (red) since it isn’t being used.

Now, let’s do the math.

One thousand years ago, CO2 was about 260 ppm. By 1960, it was 317 ppm – a gain of 57 ppm. From Cook’s paleoclimate graph above, temperatures declined about 0.2C over that same period. The climate sensitivity for a doubling of  pre-industrial values (280 ppm) is therefore :

(-0.2 degrees C / 57 ppm CO2) * 280 ppm = -1 C