Re: A Climate Science Headline You Won't See: Climate sensitivity to CO2 = 1.16 degre
No, you are. The paper even flat out says that it's the second figure in that sentence that is comparable to the IPCC. (equilibrium climate sensitivity rather than transient)
The actual increase in temperature you get when you double CO2 in the atmosphere is the transient climate sensitivity. This, according to the authors, is what people commonly mean by climate sensitivity (line 407-408). What you refer to is the equilibrium climate sensitivity, which is the increase in temperature you'd get if everything was allowed to come to equilibrium after CO2 doubling, after the climate reaches a steady state. There is, of course, no such thing as a steady state in real world climate, it's a figure you get from running climate models.
These guys did a straightforward determination of the transient climate sensitivity from GISS temperature (!) and various tables of forcing data. They call it a lower bound estimate because they think there is uncertainty about forcings that would lead to higher sensitivity, especially aerosols. In other words, if aerosols caused the total forcing to be lower than GISS estimates then the climate sensitivity would actually be higher than the estimates they calculate. But there is very little real world data on the aerosols over time, so the possibility of higher sensitivity than these estimates, taken from real world data, is speculative.
The use of GISS data is an interesting choice since it runs hotter than other data sets, going up ~0.6 degrees since 1979 while satellite data has the temperature going up ~0.3. (The Met office has it going up ~0.45.) You could argue that the lower bound these guys calculate should be divided in half; i.e., 0.7 for the transient climate sensitivity, which would mean that net climate feedbacks are negative. There are a lot of problems with the station data the GISS temperature record is based on. Interesting that they call these estimates "lower bound" when they used the hottest data set out there.
Then, using a bunch of assumptions, they calculate the equilibrium climate sensitivity, which is what the IPCC reports out. They came up with a figure of about 2, but there was uncertainty in this estimate, and the lower bound of the equilibrium climate sensitivity 95% confidence interval was 1.16 meaning that they could say that the chances were >95% that the actual equilibrium climate sensitivity was greater than that number. This puts the actual
lower bound they calculate well below the IPCC's number.
GISS claims that since 1750 total forcing from greenhouse gasses and other factors like aerosols has increased by about 1.7 W/m2. From this we got a temperature increase of about 0.8 degree. Doubling the 1750 level of CO2 would give us an additional 1.6 W/m2. So, all else remaining equal, transient climate sensitivity to doubling of CO2 ought to be about 1.55 degress going by GISS's forcings. And this is transient climate sensitivity. Using satellite data for temperatures would give you a lower figure as it indicates an increase of only 0.3 since 1979, taking that much off of the total, for an increase of 0.5 degrees since 1750. From this the sensitivity is ~0.98.
Of course, CO2 isn't the only greenhouse gas. The next most important one is methane, which tends to increase and decrease along with CO2. So the increase in forcing from greenhouse gasses will be greater than that of CO2 alone. Temperature increases according to the theory would increase by an additional 30% or so with CO2 doubling.
Regardless, this is what the data shows. Again, the possibility of higher sensitivity is speculative. If this and the paper's estimates match the IPCC then I don't know what all the alarm over AGW is for.
