Comments on S c h w a r t z et al. (2010)
Posted by Ari Jokimäki on January 24, 2010
UPDATE: I misunderstood some points about this paper and made some false comments about it here. Therefore I wrote a new version. Use the new version because all the information in this version is not correct.
Newly published paper by Schwartz et al. (2010) (abstract) has been claimed to show that theory of AGW is false or that “global warming has been cancelled”, etc. The claims are based on this statement in the paper’s introduction:
However, the observed increase of GMST over the industrial period is less than 40% of what would be expected from present best estimates of Earth’s climate sensitivity and the forcing (imposed change in energy balance, W m-2) by the observed increases in GHGs.
(GMST = global mean surface temperature, GHG = greenhouse gases).
In other words, they determine expected temperature rise from greenhouse gas forcing and the climate sensitivity, then look at the observed temperature rise and compare the two. Not surprisingly, they found that the two are different. I said “not surprisingly” because they only looked at greenhouse gas forcing and I know that there are other forcings at play and I’m also quite sure that some of those forcings work in different direction than greenhouse gases, aerosols for example. Just a simple example of the situation would be that if GHG’s would cause a warming of 2K and aerosols would cause cooling of 1K, then the resulting warming from these two would be 1K meaning that the observed warming would be only 50 % of the expected warming from GHG’s.
So, the 40 % number they give doesn’t represent the total overall situation, but it only represents the situation if only greenhouse gases are considered and the rest forcings are ignored. Now, Schwartz et al. know this because it is the subject of their paper to study what causes the difference, so Schwartz et al. are not claiming that observed temperature is less than 40 % of the expected all-forcing-temperature. Yet, it is the 40 % number that is the one they are now parroting all over the Internet as if it would represent the total overall situation.
It would be the same as if I would calculate that aerosols in the air would cause cooling of certain amount and then I would note that global temperature has been rising instead of expected cooling from aerosols. I would then say that I will now consider why there is such a difference but somebody else would just quote me on the observed temperatures not showing the expected cooling and would then spread that word as a proof that the theory of aerosols has been now shown wrong.
Well, at this point we are only in the introduction section of the Schwartz et al. and we already have handled most of the false claims circulating in the Internet about this. But Schwartz et al. do have things to say even beyond the introduction.
Rest of the paper
Schwartz et al. are studying if the difference between the observed and expected greenhouse gas warming is due four main things:
– Natural variation in global temperature.
– Lack of attainment of equilibrium.
– Overestimate of climate sensitivity.
– Countervailing forcings over the industrial period.
They calculated that the expected warming from GHG’s would have been 2.1 K. They said that the observed temperature increase had been 0.8 K. That means that they are looking to find 2.1 K – 0.8 K = 1.3 K of cooling from the above mentioned four things.
Natural variation in global temperature can cause up to 0.2 K of cooling according to them. This is how they found it out:
We use variation in preindustrial global temperature as inferred from proxy records, mainly tree rings, ice cores, corals, and varved sediments to estimate the likely magnitude of any natural cooling over the 150-year interval of the instrumental record.
Proxies? Tree-rings??? Surely any self-respecting climate denier at last now will dump this paper as a heretic production. Well, seriously, I think that’s reasonable approach to get a rough idea. However, it’s also bad news for those who think that the global warming is from natural variability. According to Schwartz et al. observed = 0.8 K and natural variability = 0.2 K. That means the observed warming is 400 % of the expected maximum warming from natural variability – a worse result than the observed versus expected from GHG’s.
Note that natural variability can work for both directions, it can cause cooling or warming.
Lack of attainment of equilibrium is a fancy way of saying that there might be delays in the climate system so that not all the warming from GHG’s has yet been realised in surface temperature but is instead hiding somewhere. Ocean is the most obvious and important place to hide the warming from GHG’s. They determine that 0.37 W/(m2) of the forcing could be hiding in the ocean, and they say that it corresponds to 22 % of the warming discrepancy, which would give about 0.5 K of cooling (I might have misinterpreted that though, they don’t express it very clearly).
Note that this effect works only to one direction, it has a cooling effect on global surface temperature.
Overestimate of climate sensitivity suggests that the climate sensitivity would be lower than the expected range. That would explain the discrepancy. They note that IPCC limit for very “unlikely” climate sensitivity is 1.5 K and they say that the observed warming would require the climate sensitivity to be even lower than that. That, however ignores the other factors causing the cooling mentioned above. The situation is presented in their Figure 2. There they present the observed warming as a horizontal line and they have added the natural variability as a horizontal band around the observed line. The expected warming from GHG’s is presented as an increasing line. One can see that when accounting for natural variability, the expected warming goes out of the band at climate sensitivity of about 1.7 K. That already is within IPCC very unlikely limits, and approaching the “likely” limit of 2.0 K.
However, they haven’t included the “Lack of attainment of equilibrium” value of 0.5 K discussed above. If we would include that, we would get a possible climate sensitivity of 2.2 K, well within the IPCC “likely” limits. This wouldn’t even include the aerosol forcing, which is likely to be substantially negative. With aerosol forcing of the size IPCC has determined to be the best estimate we would get even higher possible climate sensitivity (one that would agree quite well with IPCC limits), approaching 4 K. I have reconstructed some relevant parts of their Figure 2 and I have added the 0.5 K lines there as well. See the Figure 1 below.
Figure 1. Reconstruction of the relevant parts of Schwartz et al. Figure 2 – the warming of Earth’s surface (X-axis) as a function of climate sensitivity (Y-axis). Expected increase of global mean surface temperature for GHG’s only (black), expected increase of global mean surface temperature for GHG’s and aerosol’s based on IPCC’s best estimate (green), observed increase of global mean surface temperature (blue thick line) and the possible effect of natural variability to that (blue thin lines), and observed increase of global mean surface temperature when ocean thermal sink has been accounted for (red thick line) and the possible effect of natural variability to that (red thin lines).
Countervailing forcings over the industrial period also have an effect to the global temperature. Aerosol forcing we already discussed briefly above and it is the only forcing they are discussing here. Here too they discuss Figure 2 in a manner that is ignoring other factors. They say that with the IPCC best estimate aerosol forcing the warming “would be compatible with the lower end of the IPCC “likely” range of climate sensitivity”, but actually if we consider the natural variability and the warming wasted to the ocean, we can see from their Figure 2 that resulting climate sensitivity could easily be 4 K. Here are my estimates for the climate sensitivity (values in Kelvins) based on their reconstructed Figure 2 presented above as Figure 1:
GHG GHG + aero Observed 1.3 2.2 Obs & natural 1.0 - 1.6 1.7 - 2.7 Obs + ocean 2.1 3.5 Obs + ocean & nat 1.8 - 2.4 3.0 - 4.0
Schwartz et al. do make an important point about aerosol forcing, the fact that it has large uncertainty. But I’m not quite sure that’s exactly a new finding.
So, at this point it seems I’m disagreeing with them a little. In my opinion they are stressing the low end of their results and not considering the high end much. In fact the warming that goes to the ocean is quite certain component, so they definitely should have considered that in their Figure 2.
They then enter to a discussion about the methods of determining the climate sensitivity and possible actions for improving the aerosol forcing uncertainty.
Claims in the Internet about Schwartz et al. are largely based on misunderstanding and not reading the paper beyond the abstract and/or introduction chapter. However, there is an apparent actual point in Schwartz et al. that other factors contributing to the difference of observed and expected warming are not enough suggesting that we have some forcings wrong or that climate sensitivity is somewhat smaller than we have thought.
To me it seems that Schwartz et al. are mistaken and their point seems to rise from the fact that they didn’t consider ocean thermal lag when they determined the whole situation. They considered the ocean thermal lag separately but did not include it to their discussion of Figure 2 describing the overall situation. When the ocean thermal lag is included, the results seem to agree well with the IPCC values and the best estimate of the climate sensitivity would be 3-4 K.
I’m also little disappointed of the lack of references to the preceeding studies on the matter. For example, Lean & Rind (2008) determined the relative sizes on forcings, finding no such problems as Schwartz et al. are suggesting.
UPDATE (January 25, 2010):
I’ll add one note. As I have been making my paperlists, I have read a lot of introduction sections of papers because there the existing research on the subject in question is given and also the references to the key papers on the subject. I was quite amazed when I had read the introduction section of this Schwartz et al. paper. There isn’t a single reference to peer-reviewed papers, but they only reference IPCC 4th assessment report once. I don’t recall seeing any other papers with so poor introduction section. Also note that Schwartz had problems before with the ocean’s role in his 2007 paper, see the comment section below (thanks to Paul Middents for pointing that out).