>>235
Core samples of quickly forming ice (in greenland, for example) reveal the history of (local/global) temperature and atmospheric composition, for one.
There are a lot of ways these things are measured, and the amount of factors going into this kind of thing make it far more complex than some would have you believe (CO2 is far from being the only thing that affects this).
Seeing as it is unlikely there are any respected climatologists, geologists or atmospheric scientists frequenting this board, you'd best learn from a google search for "climate change"