Book excerpt: Most debates and studies addressing potential climate change have focused on the buildup of industrial greenhouse gases in the atmosphere and a gradual increase in global temperatures. But this "slow ramp" climate change scenario ignores recent and rapidly advancing evidence that Earth's climate repeatedly has become much colder, warmer, wetter, or drier-in time spans as short as three to 10 years. Earth's climate system appears to have sensitive thresholds, the crossing of which shifts the system into different modes of operation and triggers rapid, non-linear, and not necessarily global changes. This new paradigm of abrupt climate change does not appear to be on the radar screens of military planners, who treat climate change as a long term, low-level threat, with mostly sociological, not national security, implications. But intense and abrupt climate changes could escalate environmental issues into unanticipated security threats, and could compromise an unprepared military. The global ocean circulation system, often called the Ocean Conveyor, can change rapidly and shift the distribution patterns of heat and rainfall over large areas of the globe. The North Atlantic region is particularly vulnerable to abrupt regional coolings linked to ocean circulation changes. Global warming and ocean circulation changes also threaten the Arctic Ocean's sea ice cover. Beyond the abrupt climatic impacts, fundamental changes in ocean circulation also have immediate naval implications. Recent evidence suggests that the oceans already may be experiencing large-scale changes that could affect Earth's climate. Military planners should begin to consider potential abrupt climate change scenarios and their impacts on national defense.