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Observations of anthropogenic global warming

Comments on Schwartz et al. (2010), version 2

Posted by Ari Jokimäki on January 27, 2010

I first wrote about Schwartz et al. (2010) here but as some of it was based on my misunderstandings I wrote it again, resulting text below. I mistakenly thought that ocean thermal lag would cause 0.5 K cooling to the observed temperature at all climate sensitivity values and then interpreted their results based on that mistake, which lead me to believe that they had forgot to include the ocean thermal lag to their final analysis. Also, my calculated value of 0.5 K was wrong (0.3 K would have been correct), see the comment section of the first version. So here’s the second, and hopefully more correct version:

Newly published paper by Schwartz et al. (2010) (abstract, full text) 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.

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.

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. 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.

Countervailing forcings over the industrial period also have an effect to the global temperature. They are discussing aerosol forcing here. In their Figure 2 they present also how aerosol forcing would effect the situation (however, see below for a minor error in the Figure 2 relating to this). The red lines in Figure 2 are with aerosol forcing; three lines for different amounts of assumed aerosol forcing. They say that with the IPCC best estimate aerosol forcing (1.2 W/m2) the warming “would be compatible with the lower end of the IPCC “likely” range of climate sensitivity”, but actually if we would consider the possible natural variability, we can see from their Figure 2 that resulting climate sensitivity would approach the nominal 3 K value, the climate sensitivity might in that case be about 2.8 K (but of course, as natural variation might work to other direction too, the resulting sensitivity might also be only about 1.6 K).

They then enter to a discussion about the methods of determining the climate sensitivity and possible actions for improving the aerosol forcing uncertainty.

Some notes

Their Figure 2 is not very clear, so it might be good to go explain some of the things in it. The yellow line there represents the GHG-forcing only. The black line represents the greenhouse forcing minus the ocean thermal lag of 0.37 W/m2. This is the expected forcing from the GHG’s without taking other forcings to consideration. The effect of aerosol forcing (and tropospheric ozone forcing) is then presented in three red lines. The highest line has aerosol forcing of 0.6 W/m2, the middle red line has aerosol forcing of 1.2 W/m2, and the lowest red line has aerosol forcing of 2.4 W/m2. The values of 0.6 and 2.4 are from the IPCC best estimate 5 % and 95 % range.

However, here is apparently a minor mistake (thanks to AJ for catching that, see the comment section of the first version), the highest and lowest red lines are drawn too high so that at the equilibrium climate sensitivity of 1 K/(W/m2) they are 0.15 K too high. The reason for this is unknown.

The equilibrium climate sensitivity of 1 K/(W/m2) (corresponding to CO2 doubling temperature of 3.7 K) is a good point to see how the numbers add because at that equilibrium sensitivity the forcing and temperature have numerically the same value (i.e. the ocean thermal lag forcing of 0.37 W/m2 is 0.37 K at that point). Let us check if the numbers add up. Here are the numbers as I have “measured” (based on the pixel amounts in the image) them from the Figure 2 (the difference from the yellow line is given in parentheses):

Yellow line:  2.63 K (0.00 K)
Black line:   2.26 K (0.38 K)
Red 0.6 line: 2.13 K (0.50 K)
Red 1.2 line: 1.36 K (1.28 K)
Red 2.4 line: 0.33 K (2.31 K)

As described above, the yellow line presents the GHG forcing which is said to be 2.6 W/m2, so at this point it would be expected to be 2.6 K. My measured value agrees very well with that.

The black line is expected to be 0.37 K below the yellow line at that point, and the measured difference from the figure is 0.38, so that agrees well.

The highest red line has the forcings of GHG’s, tropospheric ozone, ocean thermal lag, and aerosol forcing of 0.6 W/m2, so they would add up to (2.6 + 0.35 – 0.37 – 0.6) W/m2 = 1.98 W/m2. Here my measured value is 2.13 K, which is 0.15 K higher than it should be (this was discussed briefly above).

The middle red line has the same forcings as the highest red line, but now the aerosol forcing is 1.2 W/m2, so now all the forcings would add up to (2.6 + 0.35 – 0.37 – 1.2) W/m2 = 1.38 W/m2. Here my measured value is 1.36 K, a good agreement.

The lowest red line has the same forcings as the highest red line, but now the aerosol forcing is 2.4 W/m2, so now all the forcings would add up to (2.6 + 0.35 – 0.37 – 2.4) W/m2 = 0.18 W/m2. Here my measured value is 0.33 K, so here again is the 0.15 K discrepancy discussed above.

Note that solar and volcanic forcing have not been mentioned. They should be included in the natural variability, so there is no need to handle them separately here.

I’m little disappointed of the lack of references to the preceeding studies on the subject. For example, Lean & Rind (2008) determined the relative sizes on forcings, finding no such problems as Schwartz et al. are suggesting.

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.

Note that James Annan has also made some comments on this (thanks to Paul Middents for pointing it out in the comment section of the first version).


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.

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.


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