Good question. I know of one way, which is to look at sedement and other physical evidence that is laid down gradually over time. For example, tree rings (of tr

Good question. I know of one way, which is to look at sedement and other physical evidence that is laid down gradually over time. For example, tree rings (of trees hundreds or thousands of years old, or maybe even petrified trees) and ice cores (for example, from Antartica; the chemical composition of the ice and the air trapped in it depends on the prevailing environment). As I understand it, your question has received much attention over the years and these techniques are mature and well-developed. To understand these issues and much more, I highly recommend spend a little time on the IPCC reports: https://news.ycombinator.com/item?id=9772353 https://news.ycombinator.com/item?id=9772353