Carbon isotope ratios in algae-produced organic compounds provide evidence that 35 million years ago a dramatic decline in atmospheric carbon dioxide levels spurred Antarctic glaciation (Science, DOI: 10.1126/science.1203909). The idea of linking a drop in atmospheric CO2 with glacier growth from the subsequent global cooling has been controversial and difficult to substantiate. Yale University’s Mark Pagani and colleagues extracted alkenones—long-chain unsaturated ethyl and methyl ketones produced by ancient ocean algae—from seafloor sediment cores and analyzed the molecules’ 13C to 12C ratios. These ratios provide a fingerprint of the atmospheric CO2 levels present when the algae produced the alkenones via photosynthesis. The analysis by Pagani’s team suggests that atmospheric CO2 decreased just before Antarctic ice shelves started forming. “Onset of Antarctic glaciation reflects a critical tipping point for Earth’s climate and provides a framework for investigating the role of atmospheric CO2 during major climatic change,” the researchers conclude.