Page:Implications for US National Security of Anticipated Climate Change.pdf/11



Potential Climate Discontinuities and Secondary Surprises
While current climate models project long-term increases in global average surface temperatures, climate scientists warn that more sudden, dramatic shifts could be possible, given the complexity of the system and analogs in the climate record. Looking back over the past 100,000 years, the earth’s climate periodically has undergone extreme shifts, sometimes in as little as a decade. Such abrupt swings can occur in complex systems when seemingly small shifts in the forces in play suddenly trigger dramatic nonlinear change, such as when a slight drop in temperature can suddenly turn water into ice. A body of scientific research indicates that the current rate of increase of carbon dioxide in the atmosphere is the highest in perhaps as long as 66 million years, sea levels are rising faster than in the past 2,700 years, and the oceans are acidifying more rapidly than in the past 56 million years.

Many climate scientists warn that the risk of abrupt climate change—currently low—will increase over the next several decades and beyond. We judge that the possibility of abrupt climate change cannot be discounted over any timeframe because research has not identified indicators to forecast potential tipping points and other thresholds. Even if sudden shifts in the climate do not materialize, gradual shifts in climate could nonetheless spark surprising secondary effects—such as a massive release of gases from melting permafrost, persistent megadroughts, extreme shifts in critical ecosystems, emerging reservoirs of new pathogens, or the sudden breakup of immense ice sheets. The national security implications of such changes could be severe.