CRYSTALS OF ICE in a sample taken from the Greenland ice sheet and photographed in polarized light are about 1,000 years old. The very small air bubbles that are visible hold clues to the ancient concentrations of trace gases, including the greenhouse gases carbon dioxide and methane. Studies of ice cores from Greenland and Antarctica indicate that the average global concentration of methane held steady near 700 ppb between 10,000 and 300 years ago and then began a dramatic climb about 100 years ago (bottom). The red dots represent data from the ice; the asterisk represents the average global value for the late 1970's; about 1,500 ppb, The micrograph was made by Chester C. Lang way, Jr. of the State University of New York at Buffalo,