The second order draft of the 5th IPCC report have been leaked. Not much of excitement from what little I have seen discussed. There is very little difference in the "forcing" estimates. What I would like to see is a actually estimate of the absolute value of the average surface temperature with legitimate error bars. The starting point for this whole fiasco is 33C, the number of degrees the Earth is warmer due to the "Greenhouse Effect" assumption. As that number changes all the impact estimates would change.
Since the 33C assumption was brought up in a recent comment and I haven't had a good rant in a while, I mentioned some of the uncertainty involved in that assumption. First lets look at what the assumption creates.
With the solar energy absorbed on average being ~240Wm-2 +/- 11 roughly and the "average" surface radiant equivalent energy of 398 +/-5, the impact of the "greenhouse effect" is 398-240=202 Wm-2 plus/minus the range of uncertainty of both measurements. The effective temperature of the effective radiant energy absorbed, 240Wm-2 is ~255.5 K degrees with the range from 258 to 249 K degrees. For the "surface", the one we actually live on, the effective temperature of the 398 Wm-2 is 289.4 K with a range of roughly 287.4 to 291.4 K degrees.
Using the center of the ranges (289.4-255.5)/(398-240) = 34.4/158 = 0.22 K/Wm-2 If we add 4 Wm-2 of "forcing" 4*.22=.88 degrees of warming due to an approximate doubling of CO2.
The original estimate was 33 C degrees with 154 Wm-2 GHE for 33/154 = 0.21 K, however, since that is way too simple, the head scientists in charge of confusion constructed an imaginary "surface" to produce an imaginary energy source at an imaginary temperature. Using the 33C estimate and (333-240)=93Wm-2 33/93= 0.35K/Wm-2 which would yield 4*.35=1.4 degrees per doubling. That energy from that imaginary "surface" has to filter back through the system to produce a "real" impact.
The 0.22 or 0.21 is a change in temperature to a change in energy ratio, the Atmospheric R-value. As a ratio, averaging is complicated and can lead to errors. The confusing value from the imaginary "surface" is also a ratio which is subject to the same potential errors. For a small range of change, both though can be useful.
With a larger change, the ratio based on the imaginary "surface" would be subject to a larger percentage error since it starts with questionable assumptions and is more sensitive to the range of uncertainty.
The oceans as I have mentioned before, would also have a relationship between temperature and energy. 4C is that rough average temperature of the total mass of the oceans and 0C is the freezing point of those oceans, using the effective radiant energy of both temperatures, (4-0)/(334-316)= 4/18 = 0.22, amazing is it not? The freezing point of the oceans can vary down to -1.9C, (4-(1.9)/(334-309) = 5.9/25 = 0.236 T/Wm-2. Since the 0 and the -1.9 are more fixed by the physical properties of water, salt and fresh, for a 4 Wm-2 increase in "forcing" the impact range would be 0.88 to 0.94 degrees per doubling. It is a Tale of Two Greenhouses which allows comparison.
Since the head scientists in charge of confusion have selected the imaginary reference "surfaces", a change in the absolute temperature of the surface we actually live on will produce some interesting press releases. My BEST estimate for an absolute temperature of the surface is 17.5C or 2.5C greater than the initial estimate and 3.5C greater than the revised estimate. Time will tell.