|MadSci Network: Earth Sciences|
Hello. Karl! Temperature in the recent past is largely determined by what are called proxy variables, i.e. things that correlate well with temperature and can be measured accurately after the events. Some examples of these are variations in isotope ratios of oxygen and hydrogen in ice cores, types of foraminifera fossils found in undisturbed sediments, types of pollen found in sediments, etc. The subject is quite interesting and more than a little complex. I'd recommend you get and read two books: "Climate Change in Prehistory" by Burroughs "Paleoclimatology" by Bradley The first of these is "easier", and presents data and results that are very interesting. The latter digs deeper into the peculiarities, limitations, and foibles of each method, and is thus more complete, more complex, and "harder."
Try the links in the MadSci Library for more information on Earth Sciences.