The parliamentary briefing linked in the previous posting is very interesting. There are several bits and pieces worthy of comment. In this post I'm going to pick up on something Keith Shine FRS told the MPs about climate sensitivity.
- The presentation gave an overview of the fundamentals of the climate system and discussed how sensitive the climate system is to increases in CO2 concentrations. AR5 stated that it was extremely likely that the temperature increase will be between 1.5°C‐4.5°C, although extremely unlikely that it will be less than 1°C and very unlikely of being greater than 6°C.
- This is a slightly lower sensitivity limit than AR4 as the models become more refined, with representation of uncertainties in temperature rise in understandable semi‐quantitative confidence bands in AR5.
Now as readers at BH know, the reason for the change in the lower limit is nothing to do with improving climate models at all. As noted in the previous post it is a minor concession to the existence of a considerable body of observationally based studies that show that climate sensitivity is much lower than suggested by the GCMs. In fact the lower end of the GCM estimates given by the IPCC in AR5 is precisely where it was for AR4, at 2°C.
Why, we wonder, do climate scientists do not point the existence of the observational studies to parliamentarians?
Bishop Hill