Leads and Lags at the End of the Last Ice Age

Over the course of Earth history it is generally believed that atmospheric carbon dioxide (CO2) and climate are closely coupled ( 1 ). The most direct evidence comes from polar ice cores. Snow falling in Antarctica and Greenland gradually compacts to form solid ice and trap air. Polar ice also records past temperatures in the ratio of heavy to light isotopes in the water molecule. Ice core analyses have shown that Antarctic temperature and atmospheric CO2 concentrations are highly correlated over the large-scale climate cycles of the past 800,000 years ( 2 ). But which came first? Does CO2 drive climate cycles or is it a feedback in the system that contributes to warming? On page 1060 of this issue, Parrenin et al. ( 3 ) address this question in a study of CO2 concentrations and Antarctic temperatures during the last deglaciation.