I do occasionally receive comments to my posts. Some agree with the post, some point me in a different direction, and some disagree. A blogger going by the initials JC replied to my recent post Dip and Rebound and advised:
“There’s been several studies examining why climate changed so dramatically in the early 20th century when forcing from greenhouse gases was low. Eg – Tett 2002 and Reid 1997. The dip and rebound was largely due to volcanic and solar forcing. Eg – strong volcanic activity in the late 19th century had a strong cooling effect coupled with a drop in solar activity. In the early 20th Century, volcanic activity dropped and solar activity rose.”
“Thanks for the links, but a decline in solar forcing can’t account for the decrease in SST anomalies–not with the present opinions about climate sensitivities to changes in TSI. For SST’s to drop more than 0.3 deg C over 40 years, TSI would have had to have dropped more than 3 times the average of the trough-to-peak range of the last 3 solar cycles. That didn’t happen.“The effects of volcanic aerosols last only for a few years and would not be able to cause the decrease in SST over the 40 years illustrated in the ERSST.v2 version of Global SST anomalies. Note the timing of the decreases in global mean forcings illustrated in Figure 1 of the Hadley Centre paper you linked. They do not coincide with the dip and rebound. Therefore, volcanic aerosols do not work. Combining solar and volcanic aerosols won’t work either, because most of the change occurs in the Northern Hempisphere, confirming that it is not related to solar or volcanic aerosol forcings. “Again, the dip and rebound remains unaccounted for by the IPCC and CCSP. Now I can add the Hadley Centre to the list. Thanks.”
Much of the dip and rebound (Figure 1) can be explained with Thermohaline Circulation/Meridional Overturning Circulation, as I noted in a latter comment, but regardless of whether or not THC/MOC can or can’t be used, the fact remains that the IPCC and CCSP are using a natural variation (the dip) to amplify their claims of anthropogenic global warming.
The intent of this post is to further illustrate how volcanic aerosol and solar variations cannot account for the dip and rebound.
Figure 2 is a comparative graph of long-term global SST anomaly data (ERSST.v2) and the Sato Index of Mean Optical Thickness, which represent the effects of stratospheric aerosols from explosive volcanic eruptions. The Sato Index data has been scaled to produce a 0.35 deg C response to the Mount Pinatubo eruption, which is the midpoint of the range of 0.2 to 0.5 deg C of proposed values. As noted earlier in this thread and again in the illustration, the effects of volcanic aerosols last for only a few years after the eruption. This is reflected in the decay rates of the Sato Index. Note also that the start (~1870) of the decrease in SST anomaly is well before the Krakatau eruption in 1883 and that the decrease in SST changes direction (~1910) during the period of volcanic activity, which subsides ~1914.
The thought that volcanic eruptions are responsible for the decrease in SST anomalies from ~1870 to ~1910 doesn’t work.
Figure 3 illustrates monthly Sunspot Number from 1854 to 2008. The data is raw and smoothed with a 133-month filter. The period in question is marked. Since I have yet to find monthly TSI data over the term of the SST data, I’ll use sunspot number as a proxy for TSI.
In Figure 4, I’ve simply scaled the Sunspot data to the current consensus of a 0.1 deg C response in global SST to the trough to peak rise in solar irradiance for Solar Cycles 21 and 22. Even if the actual response is twice that number, it makes little difference as will be illustrated in the following. I’ll key on the smoothed data to make my point, since SST response to changes in solar irradiance is so long. The response time varies depending on the researcher and there’s considerable debate about it, so I won’t list a specific response time.
The raw sunspot cycle data has been removed in Figure 5, to eliminate the distraction and skewing of the scale. There is in fact a decrease in the smoothed solar impact data from ~1870 to ~1900, which might be the basis for the thoughts that the reduction in TSI is responsible for the decline in SST anomaly. BUT the scale is wrong. The reaction of SST, based on the current understanding of climate sensitivities to solar variations, is off by an order of magnitude, at least.
If the global SST anomaly data is compared to the scaled data of what is supposed to be the SST response to changes in Solar Irradiance, Figure 6, the failings of this line of though becomes obvious.
I wanted to determine the scaling factor required for the change in TSI to cause the drop in SST anomalies, so I took the comparison one step farther in Figure 7. To adjust the curve of the Sunspot Numbers, which have been scaled to SST impact, I needed to multiply the data by 22. Also, the period of 1950 to 1990 becomes difficult to explain.
The though that solar irradiance is the cause of the dip and rebound is fatally flawed.
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).
The Sato Index data is available from GISS.
The monthly Sunspot data from January 1749 to October 2008 is available through NASA’s Marshall Space Flight Center.