Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 1A large-scale change in the climatesystem that takes place over a fewdecades or less, persists (or is anticipatedto persist) for at least a fewdecades, and causes substantialdisruptions in human and naturalsystems.3. ORGANIZATION OF REPORTSynthesis and Assessment Product 3.4 considersfour types of change documented in thepaleoclimate record that stand out as beingso rapid and large in their impact that theypose clear risks to the ability of society andecosystems to adapt. These changes are (i) rapiddecrease in ice sheet mass with resulting globalsea level rise; (ii) widespread and sustainedchanges to the hydrologic cycle that inducesdrought; (iii) changes in the Atlantic MeridionalOverturning Circulation (AMOC); and(iv) rapid release to the atmosphere of the potentgreenhouse gas methane, which is trapped inpermafrost and on continental slopes. Based onthe published scientific literature, each chapterexamines one of these types of change (sealevel, drought, AMOC, and methane), providinga detailed assessment of the likelihood of futureabrupt change as derived from reconstructionsof past changes, observations and modeling ofthe present physical systems that are subjectto abrupt change, and where possible, climatemodel simulations of future behavior of changesin response to increased GHG concentrations.In providing this assessment, we adopt theIPCC AR4 standard terms used to define thelikelihood of an outcome or result where thiscan be determined probabilistically (Box 1.1).4. ABRUPT CHANGE INSEA LEVELPopulation densities in coastal regions and onislands are about three times higher than theglobal average, with approximately 23% of theworld’s population living within 100 kilometers(km) distance of the coast and 99% probability>95% probability>90% probability>66% probability>50% probability33 to 66% probability