Isotope Ratios

An isotope problim is an ate not of a particular element, with he different number of neutrons in it. And the balance of the two of them tells you what temperature of the ice or snow was when it first formed. So this air was first trapped in the ice before humans were even invented. You can go all that way back and for 800 thousand years, layer after layer. We have an accurate record, or a reasonably acr record, of climate stretching back hundreds of thousands of years.