The purpose of this report is to imagine the unthinkable - to push the boundaries of current research on climate change to better understand the potential implications on United States national security. The authors interviewed leading climate change scientists, conducted additional research, and reviewed several iterations of the scenario with experts. The scientists support this project, but caution that the scenario depicted is extreme in two fundamental ways. First, they suggest the occurrences outlined in this report would most likely happen in a few regions, rather than on globally. Second, they say the magnitude of the event may be considerably smaller. The authors have created a climate change scenario that although not the most likely, is plausible, and would challenge United States national security in ways that should be considered immediately.