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Tuesday, July 1, 2008

Combined Solar and Volcanic Aerosol Effect – Part 2


Solar irradiance has been found to have, over the course of its cycle, no more than a 0.1 to 0.2 deg C influence on global temperature. And the overall rise in total solar irradiance is not sufficient to cause the increase in temperature we’ve experienced over the past century. There has to be another cause or group of causes. Maybe it’s not a rise in temperature after all; maybe it’s a drop in temperature at the beginning of period of the instrument temperature record that gives the appearance of an unusual rise.

Within the Smith and Reynolds SST data set, there are a significant number of ocean areas that exhibit major decreases in temperature from the later 19th to the early 20th centuries. What natural occurrence can generate significant drops in temperature? Explosive volcanic eruptions. They launch aerosols into the stratosphere that cool the planet. The oceans also have a memory. Climatologists repeatedly warn us that there’s more heat in the pipeline. How does this oceanic heat reveal itself? Thermohaline circulation (THC), Meridional Overturning Circulation (MOC), whatever, transport surface waters into subsurface layers and return it decades later. The upwelled waters bring with it signatures of past climate events.


Figure 1 is an illustration of the H.H. Lamb volcanic Dust Veil Index, from 1800 to 2000. It is a data base created to indicate the effects of explosive volcanic eruptions on climate. The 1883 eruption of Krakatao is the reference volcano, given a value of 1000, against which all others are calibrated. The study was started in 1970 and completed in 1983. The data for the 1991 eruption of Mount Pinatubo is the value assigned by Robock.
Figure 1

Figure 2: To distribute the effects of the volcanic eruption over the years that follow, I used the method discussed by Bradley and Jones in "Climate Science Since A.D. 1500" (page 608), during their explanation of their Figure 31.1. It states, "Cumulative DVI for the northern hemisphere, assuming the dust from an individual eruption is apportioned over four years with 40% of each DVI assigned to year 1, 30% to year 2, 20% to year 3, and 10% to year 4… …(DVI values from Lamb 1970, 1977, 1983)."


Figure 2

Studies of the Mount Pinatubo eruption provide a wide span for the resulting drops in annual global temperature, from 0.2 to 0.5 deg C. Figure 3 illustrates the Global Temperature Anomaly from January 1990 to December 1994. The vertical red line highlights the time of the Mount Pinatubo eruption. At first glance, a 0.5 deg C drop in global temperature appears extreme. However, there was a moderate El Nino in 91/92. Its maximum ONI value was 1.8 deg C. Assuming a 0.09 coefficient for converting El Nino anomalies to global temperature, the El Nino would have raised global temperature almost 0.16 deg C. Given that, a 0.35 deg C drop appears too little, while 0.5 deg C seems possible. Keep in mind, though, that this graph is of monthly temperatures and that the estimates of temperature drops are for annual temperature. For this post, 0.5 deg C will initially be used as a reference temperature change for the Mount Pinatubo eruption. It will later be changed to 0.2 deg C to illustrate the difference.
Figure 3

Inverting the Cumulative DVI Index and multiplying the DVI values by a coefficient determined by the 0.5 deg C drop in temperature and the Cumulative DVI value of 400 for the Mount Pinatubo eruption creates the graph of the impact on global temperature of volcanic aerosols. Refer to Figure 4.
Figure 4

Figure 5 illustrates Total Solar Irradiance (TSI) Anomaly from 1800 to 2000, using the Lean et al (2000) annual values WITHOUT the background component. The base value for the anomaly is the average of the annual TSI values from 1800 to 2000.
Figure 5

For determining the global temperature response to changes in Total Solar Irradiance (TSI), the average global temperature of 288 deg K will be divided by the Solar Constant of 1366 Watts/Meter^2. This creates a coefficient of 0.21 Deg/ (Watts/Meter^2). Figure 6 illustrates the change in global temperature anomaly attributable to changes in solar irradiance.
Figure 6

In Figure 7, the Solar and Volcanic Aerosol effects on Global Temperature are combined. The magnitudes of the impacts of the early 19th Century volcanic eruptions illustrate the relative insignificance of changes in solar irradiance.
Figure 7

In Figure 8 I’ve reduced the climate sensitivities to changes in solar irradiance and to volcanic eruptions. The solar component was reduced to the commonly proposed value of approximately 0.1 deg C for the last two solar cycles, minimum to maximum, and the volcanic coefficient was changed so that the Mount Pinatubo eruption only caused a 0.2 deg C drop in global temperature. Again, the solar variations are small compared to those of the volcanic eruptions.
Figure 8

In the next post, the data of the Combined Solar and Volcanic Aerosol Effects will be compared to sample SST data that have a significant drop in the late 19th through the early 20th centuries.


Anonymous said...

Bob-simple coefficients aren't the best approach to use here-you need an energy balance model, because it is quite obvious that the responce to early volcanoes in the record which you get here is erroneous-temperatures simply did not drop that much.

I recommend following the instructions given by pliny here:
and use the inputs for sensitivity and delay suggested by Douglass and Knox for volcano forcing:

EBMs can be fun to play around with. you may also want to try out how this works out with other forcings.

Bob Tisdale said...

Andrew: Before you jump to conclusions about how far temperature dropped after the two earlier major eruptions, visit here and check out the discussions of Tambora and Coseguina.


But, yes, I agree that eventually I will have to start looking at EBMs. Right now, I'm using spreadsheets so that anyone familiar with Excel can duplicate what I’ve done.

Just now, I found a recent, very detailed South Pacific SST reconstruction that adds another 100+ years to the timeline and captures the time of those two big volcanoes. It should prove enlightening, one way or the other.



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