Scientists are supposed to use a wide range of evidence. Things such as tree growth rings from long dead and/or fossilized trees and other fossil plant records, the ice cores mentioned in other comments, probably geologic evidence for atmospheric gases, and I'm sure a lot more, like old coral etc. They are then supposed to extrapolate a range of possibilities. From this they are to find the overlaps in the data and give a more detailed range. Lather, rinse, repeat, until you have an exact answer or the smallest range feasible. This is still a vast oversimplification. Instead we have "scientists" that do the least work possible then cherry pick the data out of it that fits what will get them the money.