Feedback between temperature and global carbon storage not only illuminates Earth's history but also informs climate change predictions. In their 2000 paper, Sigman and Boyle seek to account for the roughly 100 ppmv observed decrease in atmospheric carbon dioxide during glacial periods. We will review their arguments that (1) the land provided a source, not sink, of carbon; and (2) changes in the oceanic biological pump are most likely responsible for glacial-age carbon sequestration.