Re: What methods do scientists use to determine the climate of a specific era

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."