I believe one of the tools scientists use is isotope ratios to research ancient climates. I was reading yesterday about how this technique came about. A paper called "Pleistocene Temperatures" was published in the Journal of Geology in 1955 written by Cesare Emiliani at the University of Chicago. He had pioneered the modern use of sediment cores by crushing up the shells of a once living, group of singled celled marine animals called a foraminiferas. "...Washed them in distilled water, pulverised them in a mortar and baked them at 482 degrees C in a stream of helium gas. From that perfectly clean powder of calcium carbonate, Emiliani extracted the oxygen the forams themselves had built into their shells thousands of years ago when they were alive. With a mass spectrometer he counted how many of those oxygen atoms were the light isotope, oxygen 16 which makes up more than 99 percent of all the oxygen on Earth, and how many were the heavier and much rarer isotope, oxygen 18. Normally the heavy oxygen prefers to be in calcium carbonate over water, because that reduces the overall vibrational energy of the molecular system. But as the temperature of the seawater goes up, that slight preference goes down, and with it the ratio of oxygen 18 to oxygen 16 in the foram shells. That ratio was the thermometer Emiliani used to take the temperature of the Pleistocene..." Thats an except from a book called, Fixing Climate, The story of climate science - and how to stop global warming by Robert Kunzig and Wallace Broecker. It's a pretty interesting book which might answer some of your questions.