In the last couple years, there have been quite a few papers using the instrumental period to estimate equilibrium*** sensitivity, with most of them finding best estimates heavily concentrated near the lower end of the IPCC 1.5K – 4.5K assessed "likely" range. Examples include Aldrin et al. (2012), Ring et al. (2012), Otto et al. (2013), Lewis (2013), Skeie et al. (2014), Masters (2014), Loehle (2014), and, more recently, Lewis and Curry (2014). I think this general theme stems from a few factors: 1) the lower-than-expected increase in surface temperatures at the turn of the century, 2) a decrease in estimates of the magnitude of the anthropogenic aerosol cooling offset, and 3) more constrained estimates of OHC in recent years.
Many people argue that these studies indicate that sensitivity is less than previously thought, and that policies and risk analysis should reflect the downgraded estimates. Critics, on the other hand, often point to paleo evidence to dismiss the impact of these studies on our current expectations of CO2-induced warming. Therefore, I thought it would be interesting to combine the latest estimates of sensitivity from PALAEOSENS (2012), which I take to represent synthesized evidence from paleo estimates, with my (admittedly ad-hoc) synthesized evidence from a few of these recent instrumental era estimates with published distributions. This Bayesian approach of combining different lines of independent evidence is nothing particularly new, having been done by Annan and Hargreaves (2006), but I was curious to see how it would shake out with these recent estimates. Here are the results:
Numerically, the result of the “combined” evidence is a median of 2.1 K, a “likely” (68%) range of 1.5-2.9 K, and a “very likely” (95%) range of 1.1K-3.9K. There are a few caveats here: first, I used WebPlotDigitizer to quickly digitize the values from the aforementioned studies, which is a nifty tool, but obviously not perfect. Second, your results for this kind of method are always going to be affected by the studies chosen, although I think the “Instrumental Mean” line – with its mode and median between 1.5 and 2.0 K – is probably a fair representation of the evidence from these type of studies. And finally, it could be argued that there are structural uncertainties in our knowledge that permeate both the paleo and instrumental estimates, such that these lines are not truly “independent”, and the resulting uncertainty ranges should be wider.
Script for this post can be found here.
***these methods range from assuming effective=equilibrium sensitivity (e.g. Otto et al 2013) to explicitly considering the modeled differences (e.g. Masters 2014).