evidence: We rely on proxies. Trees, for example, can be very sensitive to annual changes in temperature and moisture, and the width and density of their annual rings reflect those year-to-year changes. We can then sample hundreds of trees all over the world along with other natural archives to infer what climate was like in the past. For this study in particular we used several different statistical methods that combine all of these proxies to produce global maps of temperature change going back 2,000 years. [...] Yes. Paleoclimate proxies can be used to infer past temperatures, but are not thermometers per se, and so they include non-temperature "noise." We have therefore tried to use as many proxies as feasible for our study, but we are limited by where the data exists and the quality of the data. Uncertainties are usually largest in places without good quality proxy data. But for the particular hypothesis we’re testing, we don’t think these uncertainties significantly impact the results. We find the same results regardless of which proxy networks or which statistical methodologies we use. So we’re pretty confident in the results.