Stacking Up Volcanoes

Guest Post by Willis Eschenbach [See Update at end]

As readers of my posts know, I’ve held for many years that there are a variety of emergent phenomena that regulate the earth’s temperature. See my posts The Thermostat Hypothesis and Emergent Climate Phenomena for an overview of my hypothesis.

One of the predictions derivable from my hypothesis is that the earth should be relatively insensitive to small changes in forcing. According to my hypothesis, if the total energy entering the system changes in such a manner that the global temperatures start to drop, inter alia the system responds through changes in the time and strength of the daily emergence of the tropical cumulus field and the associated thunderstorms. This allows more sunlight to enter the system and decreases the thunderstorm-caused surface heat losses, balancing out the energy lost elsewhere and maintaining the temperature.

In this regard, I got to thinking about the Berkeley Earth Land Temperature dataset. It is the longest observational global temperature dataset, stretching back to 1750. This gives me the opportunity to test my hypothesis against the volcanic eruptions that have occurred since 1750.

Now, the general belief is that volcanic eruptions cool the globe … but me, I don’t believe much of anything until I run the numbers myself. And I’ve run the numbers on the eruptions a number of times. I list my previous posts on the subject in the endnotes. But I’ve never looked at the Berkeley Earth record, so let me do that. Remember, if my hypothesis is correct, the volcanic eruptions should not cause a noticeable drop in the temperature.

Let me start with the Berkeley Earth land temperatures. Here is that dataset, along with a Gaussian average to show the underlying variations.

Figure 1. Berkeley Earth global average land temperature (gray lines) and 10-month full width at half maximum (FWHM) Gaussian average (black line). Vertical lines show the 24 largest eruptions, those with a Volcanic Eruptive Index (VEI) of 5 or above. Heavy dark lines show the six largest eruptions, those with a VEI of 6 or 7.

Now, to get the average response to an eruption, what we need to do is to “stack” the eruptions. This means that for each eruption, we look at say the three years before and the three years after the eruption. We stack up this data and average it to find out the average change from before the eruptions to after the eruptions. Figure 2 shows that result for the 24 largest eruptions since 1750. I’ve used the Gaussian average shown in Figure 1 to reduce the amount of noise in the data.

Figure 2. Stacked 24 largest eruptions since 1750. The black and yellow line shows the average of all 24 eruptions. All values are 10-month FWHM Gaussian averages.

As you can see, the prediction from my hypothesis is completely verified. On average there is no global temperature response to the 24 largest eruptions.

But wait, I hear you thinking, maybe it’s only the biggest ones that cause such a response. So I looked at the six largest eruptions shown in Figure 1 above, those with a volcanic explosivity index (VEI) of 6 or larger. Figure 3 shows that result.

Figure 3. Stacked 6 largest eruptions since 1750. The black and yellow line shows the average of all 6 eruptions. The blue line shows the only VEI 7 eruption in the group, Tambora in 1815.

Once again, the prediction from my hypothesis is confirmed. Even the largest six eruptions since 1750 do not result in any measurable global cooling.

The largest eruption, the VEI 7 eruption of Tambora in 1815 (blue line) is widely believed to be responsible for something called a “year without a summer” … but there is little sign of that in the Berkeley Earth dataset. It was just as cold thirty months or so before the Tambora eruption as it was after the eruption. I discussed this in a post called Missing the Missing Summer.

Conclusions?

Well, I’d say that this is very strong evidence that the global temperature is not at the mercy of changes in forcing as is generally believed. Volcanic eruptions clearly and measurably reduce the incoming sunlight due to volcanic aerosols both reflecting and absorbing solar energy.

However, this does not cause a corresponding reduction in global average temperature. Instead, the climate system responds to reductions in forcing from eruptions by increasing the amount of energy entering the system, as well as by reducing the heat loss from the surface, in order to stabilize and maintain the surface temperature within a fairly narrow range (e.g. ± 0.3°C over the 20th century).

UPDATE: People in the comments said no, you’re not looking at the right volcanoes. You need to look at only volcanoes that put significant amounts of sulfates into the atmosphere … well, we are nothing if not a full-service website, so here’s that graph. I got ice core SO2 data from a paper called Variability of sulfate signal in ice core records based on five replicate cores. Here are all of the peaks that they identified since 1750 …

As you can see, even the high-sulfate eruptions don’t affect the global land temperature.

It’s a foggy morning here on the California coast. Two days ago it was blazing hot both here and in the California Central Valley, 108°F (42°C) in Sacramento. When that happens, the hot air rises, drawing the “marine layer” of cool air in off of the Pacific and causing today’s cool foggy weather at our house … the system responds to moderate the changes.

Best of sunshine, fog, weather, and climate to everyone,

w.

NOTE 1: When you comment quote the exact words you are referring to, so we can all understand your precise subject.

NOTE 2: Data Sources. The Berkeley Earth data is here. The volcanic eruption data is from the Smithsonian. Enjoy.

NOTE 3: Some of my previous posts on volcanoes:

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