The world’s icy and snowy regions—the cryosphere—are where the most profound changes will occur as the globe continues warming. In many areas, the levels of cryospheric change today are surpassing any seen in the past hundreds to thousands of years. This amplified response has a simple explanation: Most of the cryosphere is, on average, near the freezing point. Small shifts in temperature push large regions to a different physical state. However, while the processes leading to the loss of ice are quickly started, they do not quickly stop. We are on the verge of committing ourselves to sizable increases in sea level. The 2007 Intergovernmental Panel on Climate Change (IPCC) report estimated sea level rise in this century at just 20 to 60 centimeters, but that total did not include contributions from the break-up and flow of ice sheets. The melting of mountain glaciers and ice in Greenland and Antarctica could add an additional meter of sea level rise. An equally important effect may be the feedback that changes in ice—especially the ice-covered ocean—have on climate in both the polar and the temperate regions of the world. The author describes the processes that are rapidly eroding polar ice.