This value has held for the past 30 years with
narrowing error bars.
Narrowing the error bars is actually going to be quite, quite difficult.
What does it tell you about a field that is unwilling or unable to
narrow the error bars on the key quantity driving policy decisions in over 4 decades of extremely well - funded research?
Where I have some difficulty is reconciling
the narrow error bars with Stephens et al. (2012) who state, «The current uncertainty in this net surface energy balance is large, and amounts to approximately 17 Wm — 2.
We can probably estimate its magnitude with pretty
narrow error bars, given that it is a straight chemical / physical process dependent in a simple way on substrate, temperature and so on.
Not exact matches
So there the
error bars are -1 K and +1.5 K. Annan & Hargreaves give a means to
narrow this range somewhat and I think they give 2.8 K as most likely.