by Andy May, Watts Up With That:
I last wrote about Climate Change and Civilization for the past 4,000 Years in 2016. Since then, a lot has changed, and I’ve learned a lot more about the subject. First, we learned that various air and sea temperature proxies, such as ice core δ18O or tree rings, are all different. For a discussion of some temperature proxies used and the problems with them, see here. Proxies have different accuracies, they are often sensitive to the temperature of different seasons, and they have different temporal resolutions. Thus, as pointed out by Soon and Baliunas in 2003, they are all local and “cannot be combined into a hemispheric or global quantitative composite.”
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The global average surface temperature (GAST) reconstruction relied upon in the IPCC AR6 report was by Kaufman, et al. The authors admit that the average spacing of each temperature (the temporal resolution) is 164 years. Thus, to compare the entire global instrumental temperature record to the proxies in a valid way, one must average all the daily readings since 1860 into one point. That is, the rate of warming since 1860 is irrelevant, the proxy record cannot see a 164-year increase. The problem of comparing daily modern instrumental temperature records to proxies is discussed by Renee Hannon here.
Temperature Proxies
Most temperature proxies are sensitive to only one season, yet it is clear that seasonal temperatures vary at different rates and yearly average temperatures vary differently than seasonal temperatures. Mixing them statistically to create an accurate GAST record of the distant past is not possible. This problem is discussed in more detail here and here. Further most proxies are affected by precipitation frequency and/or CO2 levels as well as temperature. We know CO2 is higher today than in the past few thousand years and cannot correct for precipitation frequency or amount.
While combining hundreds of proxies into one composite “global” or “hemispheric” record is a fool’s errand, we can look at high-quality, high-resolution local proxies from multiple places to get a qualitative sense of global or hemispheric climate changes, which is also what Soon and Baliunas did in 2003. Two records are especially helpful, they are the Greenland ice core record by Vinther, et al. and the Indonesian Throughflow 500-meter water depth record from the Makassar Strait by Rosenthal, et al. The Makassar Strait record is representative of northern Pacific sea surface temperatures and the Vinther Greenland record is representative of air temperature in the Greenland-Renland-Agassiz area. The Vinther record is superior to the more commonly used GISP2 record (Alley, 2004) & (Alley, 2000) because it accounts for elevation changes and ice flow. Both corrupt the GISP2 reconstruction.
Both the Vinther and Rosenthal temperature records have a 20-year resolution over our 4,000-year period, which is a good resolution for proxies. Accuracy is good in both records and about ±0.3°C. Both are Northern Hemisphere proxies but are located 9,500 miles apart. They are compared to a comparable (10-year resolution) Antarctic proxy (Jouzel, et al., 2007) in figure 1. As you can see, averaging Northern Hemisphere proxies with Southern Hemisphere proxies is not always a good idea, temperature trends vary with latitude.
As figure 1 suggests the long decline in temperature labeled the “Neoglacial” is mainly a Northern Hemisphere phenomenon, this is more clearly seen in figure 1 here. Figure 2 compares the Vinther and Rosenthal records from figure 1 with selected historical events and Usoskin’s solar grand minima (SGM) record (Usoskin, 2017) shown as orange outlined black dots. Notice the smoothed version of the Vinther record is plotted in figure 1 and the unsmoothed 20-year record is plotted in figure 2. To see figure 2 in full resolution, click on the image or here. The figure prints well on 8.5×11 inch or A4 paper.
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