how do people predict an ancient climate?

Answer 1

This is actually a very interesting and excellent question, and I hope somebody with a better understanding than I have of it comes along to answer it... but I'll do my best.

A great way of looking at the ancient climate is by drilling ice cores into the ice caps at the poles and on top of Greenland. This ice has been building up layer after layer each year - since it never thaws out the ice traps gases that give scientists an excellent idea of what the climate was like at that time. There are obvious layers in the ice from each year's deposit - this will give a good idea about precipitation levels and how warm it was at that time.

Studying geologic features can also give you excellent clues about ancient climate, and is probably the best way of finding out about ancient climate. The kinds of sedimentary rocks that build up in a given area can tell a lot about what the climate of that area was (limestone would indicate the area was under water, and if that area is far from the edge of the continent that would mean that the earth must have been very warm during that time period). Areas that have been glaciated in the past (like much of North America) have obvious signs - and for glaciers to advance down an entire continent the earth would have to be very cold.

Fossils are another excellent indicator of climate - especially on a local level. If you find a wooly mammoth fossil in a warm area it would make sense to assume that the area was once much colder. If there are fossils of dinosaurs and other cold blooded creatures all over then it's a good bet that the area was once much warmer.

I wish I could explain these ideas better, but unfortunately I can't. Hopefully this was helpful though.

Answer 2

The science of geology (rocks) helps tell what ancient climates were.

EDIT 02/03/2007 : "To figure out what climates used to be like, scientists work like detectives and look for clues around the planet to solve the mystery. They find clues inside SEDIMENTARY ROCKS, GLACIERS, FOSSILS, TREES, AND CORALS." (emphasis that of responder)
http://www.windows.ucar.edu/tour/link=/earth/climate/cli_paleo.html&edu=elem

I felt I was missing something, and did more research. Check the source for more indepth knowlege from a beginner to an advanced learner.