i mean.....the worlds navys have been keeping track since the 1600s secondly ice cores in antartica, literal miles of ice drilled up you can roughly tell the temperature by ice composition (is it super dense? it was warmer. is it light and fragile? it was probably fair weather world wide then.) secondly they check the ice for minute radiation that will tell you roughly what year it was because we used to be able to tell how much green house gasses human put off via radiation in the air. but after the 70s nuclear tests, that method because shitty and they had to come up with a new way to measure it (which is flawed) either way dont be worrying about the climate