Guest Post by Willis Eschenbach [also, see update at the end of the post]
Anthony recently highlighted a couple of new papers claiming to explain the current plateau in global warming. This time, it’s volcanoes, but the claim this time is that it’s not the big volcanoes. It’s the small volcanoes. The studies both seem to follow what I call “Willis’s Rule of Author Count”. The first study is Total volcanic stratospheric aerosol optical depths and implications for global climate change, by D. A. Ridley, S. Solomon, J. E. Barnes, V. D. Burlakov, T. Deshler, S. I. Dolgii, A. B. Herber, T. Nagai, R. R. Neely III, A. V. Nevzorov, C. Ritter, T. Sakai, B. D. Santer, M. Sato, A. Schmidt, O. Uchino andJ. P. Vernier. The second study is Observed multi-variable signals of late 20th and early 21st century volcanic activity, by Benjamin D. Santer, Susan Solomon, Céline Bonfils, Mark D. Zelinka, Jeffrey F. Painter, Francisco Beltran, John C. Fyfe, Gardar Johannesson, Carl Mears, David A. Ridley, Jean-Paul Vernier, Frank J. Wentz.
Now, Willis’s Rule of Author Count says that the quality of any study is inversely proportional to the square of the number of listed authors. And these two studies have seventeen and twelve authors respectively … not a good sign.
The abstract of the first paper says:
Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, Aerosol Robotic Network, and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at middle to high latitudes and therefore underestimate total radiative forcing resulting from the recent eruptions.
The abstract of the second paper, in turn, says:
The relatively muted warming of the surface and lower troposphere since 1998 has attracted considerable attention. One contributory factor to this “warming hiatus” is an increase in volcanically-induced cooling over the early 21st century. Here, we identify the signals of late 20th and early 21st century volcanic activity in multiple observed climate variables. Volcanic signals are statistically discernible in spatial averages of tropical and near-global SST, tropospheric temperature, net clear-sky short-wave radiation, and atmospheric water vapor.
Now, it is certainly possible that “small-magnitude volcanic eruptions substantially perturb incoming solar radiation”. There are lots of things that perturb incoming solar radiation, with clouds heading the list. Whether small volcano emissions in turn perturb the global surface temperature is a separate question.
But for eruptions to be an explanation for the current plateau in global warming, the authors would have to show a significant increase in volcanic eruptions in the 21st century. And unfortunately (but predictably) I see no sign in either paper that they have even tried to do that.
So let’s do their job for them by taking a look at the actual records of eruptions, both large and small. The data on all known eruptions is available from the Smithsonian Volcanism Project.
Now, we have some choices in how to display this data. Let me show three of these different ways.
First, we can show the total numbers of eruptions by year, without regard to the size of the eruption. Figure 1 shows that information:
As you can see, there is very little difference between the post-2000 (or post 1998, depending on the study) eruption count and the number of eruptions during the end of the 20th century. After 2000 (or 1998), it went up a bit, then it went down a bit … overall, little change.
But wait, I can hear you saying, the eruptions are not all of the same strength … what is the average strength of the eruptions? And reasonably so, since strong eruptions would have a bigger effect than small eruptions. So let’s look at that data.
The strength of an eruption is measured by the volcanic explosivity index, or VEI. This is a logarithmic scale. This means that an eruption with a VEI of 5 is ten times stronger than an eruption with a VEI of 4, and so on.
In order to properly average these, it’s necessary to use a “logarithmic mean” To do this, you first convert the VEIs to actual values (by taking ten to the power of the VEI). Then you average the actual values, and then take the logarithm of the resulting average to convert it back into the logarithmic VEI scale. Figure 2 shows that result:
In 1991, there were two strong eruptions, Pinatubo (VEI of 6) and Cerro Hudson (VEI of 5). Other than that, there’s not a lot of variation.
Once again, you can see that the post 2000 (or post 1998) average strength of the volcanic eruptions are little different from the strength of the eruptions prior to the turn of the century. So that cannot be the cause of 21st century plateau in global surface temperatures.
Finally, we could read the implicit claim as being that there is some kind of increase in the number of small volcanic eruptions. After all, the authors say that these are the overlooked eruptions. So let’s take a look at the small pre- and post-2000 eruptions.
Once again, we see little change in the number of small volcanoes. After 2000, it goes above the average, and then it goes about the same amount below the average.
Conclusions? Well, the papers may be correct in their claim that the effect of eruptions on the clarity of the atmosphere may have been underestimated.
But they are absolutely not correct in the claim that this underestimation reveals the cause of the recent 18+ year plateau in temperatures as being eruptions. There is almost no post-2000 change in either the number of eruptions, the strength of eruptions, or the number of small eruptions.
Overall? I’d say that Willis’s Rule of Author Count, that the quality of any study goes down inversely proportional to the square of the number of listed authors, is validated once again …
[UPDATE: The underlying claim of these two papers is that although there have been no large eruptions in the 21st century, it is the weaker eruptions that are causing the plateau in temperature. These are eruptions with a volcanic explosivity index (VEI) of four. Some commenters below still think that the eruptions of VEI four are significant. The Santer document shows the effect of some of the VEI 4 eruptions on the stratospheric aerosol optical depth (SAOD), which is their main indication of volcanic change. The study says:
We use stratospheric aerosol optical depth (SAOD) data from Vernier et al.  to study changes in stratospheric loadings of volcanic aerosol.
The eruptions make a change of about .002 in the SAOD, viz:
Now, that looks kind of impressive … until you compare it to the larger volcanoes (source):
You can see the sizes of Krakatoa in the 1880s, Pinatubo in 1991, El Chichon in 1982, and Mt. Agung in 1963, along with some smaller volcanoes … and then you can see the part that Santer et al. are discussing. This is the almost-flat line in the post-2000 era. Sorry, but given the short-term, weak, local effects of even the largest volcanoes, I’m not buying the idea that those tiny post-2000 wiggles have any discernible effect at all.
My best regards to you all,
ONCE AGAIN: If you disagree with someone, please do everyone the favor of QUOTING THE EXACT WORDS THAT YOU DISAGREE WITH. This prevents all kinds of misunderstandings and misrepresentations.
FURTHER READING: I note that this is not the first time that Susan Solomon has made the claim that volcanoes are the cause of the current pause in temperatures. In addition, she was the main mover behind one of the IPCC reports, from memory the Fourth, and is fully and completely invested in the meme of “CO2 Roolz Everything, OK” … whenever I see her name on a study, I’m sad to report that I just wince. See here for my discussion of her previous work.