How to compare today to the past
3 hours ago
By Andy May
In the last post, I discussed the problems comparing modern instrumental global or hemispheric average temperatures to the past. Ocean temperature coverage was sparse and of poor quality prior to 2005. Prior to 1950, land (29% of the surface) measurements were also sparse and of poor quality. Only proxy temperatures are available before thermometers were invented, but, again, these are sparse and poorly calibrated prior to 1600. So how can we compare modern temperatures to the distant past? We can’t do it globally or hemispherically, the past data are too poor or too sparse or both. Why not pick the best proxies and compute comparable modern temperatures to compare to the proxies at the specific proxy locations? It is easier to lessen resolution than to increase it.
Rosenthal, et al.’s temperature reconstruction, plotted in Figure 1, shows ~500-meter temperatures from the “Indonesian Throughflow” (Rosenthal, Linsley, & Oppo, 2013). Their data are from sediment cores taken in the Makassar Strait east of Borneo Island. This strait is a portion of the main connection between the Indian, Southern and Pacific Oceans. Its temperature is reflective of the temperature of significant portions of these three large water masses at 500 meters.
Rosenthal, et al. used an Mg/Ca (Magnesium/Calcium ratio) proxy from benthic foraminifers to estimate the 500-meter temperatures and they claim a temperature accuracy of ±0.7°C. Dating was done using radiocarbon (14C) and is probably no better than ±50 years. The radiocarbon dating was checked using distinctive volcanic ash layers and lead isotope dates (210Pb).
https://wattsupwiththat.com/2021/06/24/how-to-compare-today-to-the-past/
