Ice cores, bored through thousands of feet of stable glacial ice in Greenland, have proved to be our best record of global climate over more than a hundred thousand years. Elizabeth Kolbert describes how mass spectrometric measurements of isotope ratios on the cores have changed fundamentally how climatologists look at long-term trends in global temperatures, and she does a great job of describing how the samples are obtained and the people who get them. Unfortunately, she doesn't do nearly as well describing why the ratios of H218O to H216O in snow are different when the earth is warm than when it is cold. If you want to read more about that, look atThe GISP2 ice core record--paleoclimate highlights by Paul A. Mayewski and Michael Bender, in Reviews of Geophysics vol.33 Supplement, which you can read online.