Stable Isotopes in Ice: Tracers of the Global Environment

Significant advances in geophysical sciences most often follow from development of new abilities to measure Earth's properties. One major development of the past half century has been the measurement of stable isotopic composition of precipitation and its variations on vast spatial and temporal scales, the latter especially in Arctic and Antarctic glacial ice. The venerable tradition of research in this subject emanates directly from work of Dansgaard, Craig, and Epstein. Here I discuss how isotopic variations induced by atmospheric distillation offer a compelling example of a geophysical phenomenon arising from microphysical properties, but one that is dependent on the global-scale environment. I discuss how the geography of precipitation isotopes is explicable by treating the problem as an advective diffusive reaction system. Three of the most important results of environmental geophysics have emerged from analyses exploiting (in part) the record of this system in polar ice: the strong but quixotic coupling of climate and biogeochemistry on multi-millennial time scales; the high but plausible (and contentious) values for global climate sensitivity to radiative forcings; and the documentation of past very rapid climate changes. Looking forward, I also discuss the major unresolved issues lurking behind this facade of success, including poor understanding of the controls on deuterium excess at low temperatures, and inability to quantify many non-temperature effects on isotope time series (many of which were clearly discussed by Dansgaard nearly forty years ago).