Jai,
from the paper, Schwartz stated your point succinctly. . .
The change in forcing is due largely to a reduction in the magnitude of the negative
aerosol forcing between AR4 and AR5, -1.2 W m-2 in AR4 vs. -0.9 W m-2 in AR5; there is as
well an increase in forcing by greenhouse gases (GHGs), from 1.66 to 1.82 W m-2,
This is an error in the Schwartz et al. 2014 paper. The boldface numbers in the above quote are
not the total greenhouse gas forcing, but only the
CO2 forcing. E.g., see
Figure 8.16 of the AR5 report, which I already posted in my last post. That figure shows that the best estimate for total greenhouse gas forcing in AR4 is about
2.9 W m-2, and in AR5 it's about
3.2 W m-2.
See also Figure 8.15 in AR5:
[...] what "expert judgments" were used [...] ?
In my last post, I provided references to the relevant parts of
Chapter 7 of the IPCC report: {Sections 7.4, 7.5.3, 7.5.4, Figure 7.19}. There's more information there.
Regarding the Schwartz et al. 2014 paper to which you linked above: from my reading of that paper, I don't think that Figure 1 of that paper is an apples-to-apples comparison. The pink circles in that Figure correspond to the CMIP5 models. However the equilibrium climate sensitivity (ECS) and forcing (F) associated to these models, as used by Schwartz et al. in the Figure, turn out to be different from the "usual" definitions. I would take that Schwartz et al. Figure with a grain of salt...
hmmm, I disagree. Do you have proof? have you reviewed Taylor et al (2012) and Forster et al (2013) to compare inputs? I find your assertion highly suspect (sorry) compared to the documentation in Schwartz et al (2014)
Take a look at the
supplementary information of the Schwartz et al. 2014 paper, more precisely
Table S2.
According to that table, the two CMIP5 models with the highest ECS values are the
CSIRO-Mk3-6-0 model (ECS:
5.87°C)
HadGEM2-ES model (ECS:
5.78°C)
However, I looked it up in Table 1 of the
Forster et al. 2013 paper, which is the source of the Schwartz et al. numbers. It turns out that the "true" ECS associated to the latter 2 models is
4.08°C and
4.59°C respectively. What Schwartz et al. call "ECS" is based on an alternative, simplified formula which also uses some model output from the Forster et al. paper. So Schwartz et al. are
not using the ECS as used in the Forster et al. paper, or as used in the AR5 report for that matter, e.g. see Fig. 9.42 of the AR5 report (horizontal axis):
Moreover, the forcing (F) associated to the CMIP5 models, as used by Schwartz et al., turns out to be "adjusted forcing", as discussed in the Forster et al. 2013 paper. This cannot be directly compared to the assessment of anthropogenic forcing as used in the AR5 report.
Moreover, for the estimates of forcing, Schwartz et al. use model output for the year 2003, whereas the AR5 report assesses the forcing for the year 2011. Throughout the paper, Schwartz et al. are comparing data from different years or periods. It's not apples-to-apples.
In my opinion, the Schwartz et al. 2014 paper should be taken with a grain of salt. I don't think this is very surprising, given Schwartz' earlier work on this subject; upthread I've already mentioned the absurdly low estimate for ECS in the
Schwartz 2007 paper.