John Nielsen-Gammon has an article called your logic escapes me. It’s about Judith Curry’s testimony to the US Senate’s EPW committee (that I discussed here). I recommend reading the article yourself; it’s very good. Basically, Judith was presenting evidence that she thought weakened the IPCC’s attribution statement. John Nielsen-Gammon was arguing that even though the evidence presented was factually correct, using it to conclude that the IPCC statement is weaker than it appears, doesn’t make sense (or, at least, that’s how I interpret his article).
Judith Curry has responded with a post about the logic (?) of the IPCC statement. I, however, really don’t get the logic of what Judith is trying to say. I’ll only consider her conclusions, which start with
N-G’s reasoning about how to think about the relative contributions of natural vs anthropogenic anthropogenically is something like the way I have approached this, but not quite. I look at it the following way. Consider two periods: 1975-1998 (warming), and 1998-2013 (hiatus). Play with the percentages of natural variability (assuming warming in the first period and cooling in the second period) and anthropogenic forcing, accounting for the relative lengths of the two periods, and see what percentage breakdown works for both periods. And what the implications are of the hiatus extending another 10 years. You will not get numbers that exceed 75% for anthropogenic.
If I use HadCRUT4, I get a trend for the period 1975-1998 of 0.171 ± 0.072oC per decade, and for the period 1998-2013 of 0.042 ± 0.125oC per decade. If I assume natural variability provides an equivalent amount of warming during the earlier period as cooling during the latter period, then I get a mean anthropogenic trend of 0.106oC per decade. If the “hiatus” continues for another 10 years, then the total mean warming since 1975 will be 0.171×2.3 + 0.042×2.5 = 0.5oC. If the average of these two trends is anthropogenic, then the total anthropogenic contribution is 0.106×4.7 = 0.498oC. This is virtually 100% of the warming. What am I missing here? Sure, I could take the uncertainty intervals into account and could assume that the natural contribution to warming during 1975-1998 far exceeded the contribution to cooling in the later period. However, I seem to be able to easily get numbers that exceed 75% for anthropogenic.
Judith continues with
If I were given a 44% range to work with, I would put the range at 28-72% anthropogenic. My range overlaps with the IPCC in domain 51-70%, but I also allow for numbers below 50%. The main uncertainties seem all in the direction of increasing the contribution from natural variability.
So, Judith seems to be suggesting that natural variability could contribute 72% of the warming. If I assume she’s still referring to the periods 1975-2013, then we have a total mean warming of 0.46oC. If Judith is correct, then natural variability could have been responsible for 0.33oC of that warming. If natural variability has been providing a cooling during the period 1998-2013, then that suggests that the natural variability trend was at least 0.14oC per decade during the period 1975-1998 (and that would require that it was actually providing no cooling during 1998-2013). That would also imply that the anthropogenic trend during 1975-1998 was lower than during 1998-2013.
Again, we could consider the uncertainty ranges, but if natural variability is to provide some cooling (as Judith suggests) during the 1998-2013 period, then if it is to provide as much as 72% of the warming during the 1975-2013 period, the contribution it makes during the earlier period just gets bigger. Also, according to the IPCC attribution figure, there is less than a 2.5% chance of natural/internal variability providing more than 0.15oC of the 0.65oC warning since 1950. If all if this were to occur after 1975, that would still mean that there was a less than 2.5% chance of natural/internal variability providing more than 33% of the warming since 1975. So, how does Judith get that it could provide as much as 72%? Is it just me, or is everyone else finding it hard to follow Judith’s logic?