Daniel Sigman is Dusenbury Professor of Geological and Geophysical Sciences at Princeton University. He studies the global cycles of biologically active elements, in particular, nitrogen and carbon, and he is active in the development of analytical techniques for studying nitrogen in the environment. He also investigates the history of these cycles in order to understand the causes of past changes in the atmospheric concentration of carbon dioxide, the role of this greenhouse gas in the waxing and waning of ice ages, and the ocean’s response to climate change.