Interview with Nathan Urban on his new paper “Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum” | What's New In Science

We look for periods in the Earth’s history when CO₂ levels were higher or lower than today, and see how much warmer or colder the climate was.  From this we can estimate ECS by applying a physical theory relating carbon dioxide to planetary temperatures.

[Why not determine how long women's skirts were during ancient times, then figure out the relationship between skirt length and planetary temperature?]

...the last century hasn’t seen as much climate change as was present during the Last Glacial Maximum; with the latter, there is a stronger climate signal to analyze.