The volcanic aerosol factor of the CSALT model is an example of a perfectly interlocking piece in the larger global surface temperature puzzle.  I thought I would present a more detailed description in response to the absolutely hapless recent volcano posts at the WUWT blog (here and here).  The usual deniers in the WUWTang Clan can’t seem to get much right in their quest to intelligently spell out ABCD (Anything But Carbon Dioxide).

Fig 1 :  CSALT model using the GISS Stratospheric Aerosol forcing model.

The addition of the volcanic aerosol factor is no different than the other components of the CSALT model. Two flavors of volcanic aerosol forcings are provided. The standard forcing table is the GISS stratospheric aerosol optical thickness model maintained by Sato [1] and I use this table as is (see Figure 1).   The more experimental model that I generated is a sparse table that features only the volcanoes of Volcanic Explosivity Index (VEI) of 5 or higher.

The VEI scale is logarithmic so that a VEI of 6 contains 10 times as much ejected particulates by volume than a VEI of 5 (which recursively is 10 times as much as VEI=4, and so on). This means that by modeling VEI of 5 or 6 we should capture most of the particulates generated as discrete events.

Figure 2 shows the residual of a fit given that the volcanic aerosol forcing is excluded from the model. The majority of the sharp negative excursions map to the major eruptions of VEI scale 5 or 6 over the past 130+ years.

Fig 3 :  Identification of eruptions of VEI scale 5 or 6 with major cold spikes identified by the CSALT model residual.

To create a sparse table, we look up the volcanic eruptions from the Smithsonian Institute Global Volcanism Program database and attach an intensity scaled to compensate the negative excursion of the residual.

Some volcanic eruptions of VEI=5  such as Mt. St. Helens in 1980 and Bezymianny in 1955 are not included in the table because they do not appear to have generated  cooling of any magnitude or duration.

A scaled response function is attached to each event so that a sparse forcing profile is generated as in Figure 4.

Fig 4 : Sparse forcing profile for volcanic eruptions of VEI = 5 and VEI=6.

The dates of the eruptions are accurately matched to the Smithsonian records. The table below shows the historic Month start since 1980 and the corresponding Model start.  These are coincident apart from a few cases where Delay = Model – Month is set to a few months after the eruption occurs. The last column shows the Lag defined as the time the Minimum residual cooling dip is observed after the Model start month.  In general, the observed cooling spike minimum is consistently seen about 1 to 1.5 years after the initial eruption is recorded. The CSALT model has resolution in terms of months so that these observed lags have significance.

After this sparse model is included in the CSALT model, the residual error is much reduced as shown in Figure 5.  The sparse model works just as well in improving the model fit as the more sophisticated GISS stratospheric forcing model, indicating how important the major volcanic events are in determining climate response.

Fig 5 :  Fit using sparse volcanic forcing model. The major negative excursions are removed and the correlation is much improved.

The likelihood that the cooling excursions shown in the original residual are volcanic eruption related is extremely high. This kind of agreement in timing and in scale does not happen by accident. By the same token, the planetary cooling of 0.1 C for the largest events matches the historical observations of the transient impact that eruptions have on the climate.  The CSALT model is able to extract the contribution of volcanoes to the natural variability with skill. There are still cooling spikes that occur which do not appear to be associated with major volcanic eruptions (or other obvious natural variability factors captured in CSALT) such as the dip in 1976-1977 or the Cold Sunday of January 1982, but these are the exception.

Compare this work to the posts at WUWT.  In the first post, the author “Wondering” Willis Eschenbach makes the claim that :

“In particular, despite widespread skepticism, I have persisted in saying that volcanoes basically don’t do jack in the way of affecting the global temperature.”

The reason the wondering one had so much difficulty was likely because he was dealing with a temperature profile that had not been sufficiently defluctuated of other natural variability factors, such as SOI and TSI.   Same issue with the second WUWT post, which did have a good start by concentrating on the high VEI eruptions, but likewise was not able to find evidence of volcanic aerosols in the global temperature anomaly.  So a similar conclusion was found:

“The effect of aerosol emissions on global temperatures from volcanic eruptions appears very small and may not be discernable from natural variation.”

This is very representative of the poor quality of research that climate skeptics and deniers consistently demonstrate.  It often appears that they have no interest in pursuing the science but instead make only an effort to create scenarios that supports their underlying agenda.  The only good that comes out of WUWT posts is that their “motivated reasoning” provides me with my own motivation to demonstrate just how wrong they are.  Yes, climate science can be complex, but there are always simplifications that can reveal a more concise path to follow.  Whether the skeptics and deniers even bother or care to learn is another story.

References