Climate Change over the past 4000 Years
18 hours ago Andy May 78 Comments
By Andy May
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.”
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.
https://wattsupwiththat.com/2024/12/03/climate-change-over-the-past-4000-years/
