One of the best sources of historical climate data for the past 100,000 years is the ice covering Greenland. Alley describes the process of digging into the ice and what they have discovered so far. Dramatic climate shifts in the past were driven by much smaller changes in atmospheric energy than we have experienced over the past 60 years. Abrupt world-wide climate change in the past has taken place over three to five years, and is likely to do so again. Fascinating facts from Alley and his team.