Guest Post by Willis Eschenbach
Anthony put up a post titled “Why the new Otto et al climate sensitivity paper is important – it’s a sea change for some IPCC authors” The paper in question is “Energy budget constraints on climate response” (free registration required), supplementary online information (SOI) here, by Otto et alia, sixteen other alia to be precise. I agree that it’s an interesting and important paper, mostly because of who the authors are. However, I have to say there were some parts that I couldn’t fit together. Here’s their figure S2, showing the radiative forcing since 1850:
Figure 1. Forcings used in Otto 2013. Grey lines represent possible variations based on a monte carlo analysis of the errors in the parts that make up the total radiative forcing. They divide the total forcings into greenhouse gases (GHGs), volcanoes and solar, and a “residual” forcing which is assumed to be mostly aerosols. The estimated errors in these are used to generate a host of possible realizations, shown in gray above.
Now, when I saw that, I just rolled my eyes. Here we go again with the volcanoes, I thought.
See those big dips in the black line above? Those are reckoned to be the change in forcing due to volcanic eruptions. What happens is that the volcanoes spew light-colored aerosols into the stratosphere. This reflects more sunlight, and thus reduces the forcing in a measurable manner. As you can see, the larger volcanoes make a very significant change in the forcing. So I set out once again to see if the claimed temperature change due to the volcanic forcing held up in the real world.
But as often happens, before getting to the volcanoes I got sidetractored, this time by discovering that Otto et al. to the sixteenth power are not discussing eruptions from ordinary active volcanoes . Oh, no indeed.
Otto and his hexadecagonic cohort are discussing retroactive volcanoes.
Here’s why I say that. I digitized the Otto data. Figure 2 shows the results, including the dates of the larger eruptions which are responsible for the large dips in the amount of sunlight reaching the earth:
Figure 2. Forcings from the Otto 2013 paper, along with the volcanoes associated with each of the large dips in forcing. The earliest eruption is assumed to be Cotopaxi because of the very large dust veil index associated with that eruption.
Here’s the oddity I noticed. As you can see, not only are the volcanoes associated with the large drops in forcing. They also apparently are able to cause the temperatures to drop during the year before they occur … in other words, they cool retroactively. In each case, there’s a drop in forcing, not only in the year of the eruption, but in the previous year as well …
Here’s the likely reason why the retroactive drop in forcing occurs. It’s not a timing error in the data. Figure 3 shows the Otto forcing data compared to the GISS forcing data.
Note that in both the GISS and the Otto forcings, the lowest points after the eruptions coincide perfectly, so it’s not from timing … but the years immediately before the eruptions are quite different. Rather than the forcing falling in the year prior to the eruption as the Otto data does, the GISS forcing remains high until the actual year of the eruption. Additionally, the size of the GISS volcanic forcings is about twice the size of the Otto volcanic forcings.
Now, the paper says that they are using the average model forcings from the Forster paper that I discussed recently. However, the Forster forcings look much like the GISS forcing shown above, with large volcanic excursions. In addition, the Forster results are not retroactive—as with the GISS data, the Forster forcing drops in the year of the volcano, and not the previous year as in the Otto forcings.
I suspect (and let me emphasize suspect) that what has happened is that either rashly or quite possibly inadvertently, someone has slightly smoothed the Forster forcing data. The difference is subtle, I didn’t notice myself until I looked at Figure 2 and went whaa?
I say it’s smoothed because if I use a simple 3-year centered moving average on the Forster data, I get something very near to the Otto forcing data. Such a smoothing makes the volcanic drops “retroactive”, since the centered moving average includes the following year’s data. Figure 4 shows those results.
Figure 4. The Forster (blue) and Otto (red) forcing data, along with a smoothed version of the Forster data. The smoothing is done using a simple centered 3-year moving average. The years prior to the eruptions of Krakatoa (orange diamond) and Pinatubo (red squares) are highlighted in both datasets to show the “retroactive” effect of the smoothing on the previous year’s data. The Forster data has a 0.3 W/m2 trend added to match the Otto data, as described in the Otto paper SOI.
Note how similar the simple smooth of the Forster data is to the Otto data. This is the only explanation I can think of for the retroactive nature of the Otto volcanoes.
If the Otto data is indeed incorrect, that could make a large difference in their results. It’s difficult to say, but since the size of the volcanic excursions in forcing have been cut in half in the Otto data, it seems like it might be important. In addition, when you are using a forcing time series dataset for predicting (hind casting or forecasting) results, it’s a lot easier to forecast next years temperature data when your forcing data for this year contains some information about next year … using a smoothed dataset as input to another calculation is almost always a huge mistake.
For me, the best thing about the Otto paper was that via the Forster paper, it provided the data and the impetus to write my last post on climate sensitivity. It also provided an insight into how to analyze the effects of the volcanoes on the historical data versus the claims of the models regarding the volcanic effects … stay tuned, my new findings in that regard are the subject of my next post, interesting stuff.
Best wishes to all,