Book 2.indb - US Climate Change Science Program

Abrupt Climate Changecorrectly represent the response of the tropicalclimate system to radiative forcing andwhether greenhouse forcing will actuallyinduce El Niño/Southern Oscillation-likepatterns of tropical SST change that willcreate impacts on global hydroclimate inaddition to those caused by overall warming.3. Do We Expect an Abrupt Change inthe Atlantic Meridional OverturningCirculation?This question is addressed in Chapter 4 of thisreport. The Atlantic Meridional OverturningCirculation (AMOC) is an important componentof the Earth’s climate system, characterized bya northward flow of warm, salty water in theupper layers of the Atlantic, and a southwardflow of colder water in the deep Atlantic. Thisocean current system transports a substantialamount of heat from the Tropics and SouthernHemisphere toward the North Atlantic, wherethe heat is transferred to the atmosphere.Changes in this ocean circulation could have aprofound impact on many aspects of the globalclimate system.There is growing evidence that fluctuations inAtlantic sea surface temperatures, hypothesizedto be related to fluctuations in the AMOC, haveplayed a prominent role in significant climatefluctuations around the globe on a variety oftime scales. Evidence from the instrumentalrecord shows pronounced, multidecadal swingsin widespread Atlantic temperature that may beat least partly due to fluctuations in the AMOC.Evidence from paleorecords suggests that therehave been large, decadal-scale changes in theAMOC, particularly during glacial times. Theseabrupt changes have had a profound impacton climate, both locally in the Atlantic and inremote locations around the globe.At its northern boundary, the AMOC interactswith the circulation of the Arctic Ocean. Thesummer arctic sea ice cover has undergonedramatic retreat since satellite records began in1979, amounting to a loss of almost 30% of theSeptember ice cover in 29 years. The late summerice extent in 2007 was particularly startlingand broke the previous record minimum with anextent that was three standard deviations belowthe linear trend. Conditions over the 2007–2008winter promoted further loss of multiyear icedue to anomalous transport through Fram Strait,raising the possibility that rapid and sustainedice loss could result. Climate model simulationssuggest that rapid and sustained SeptemberArctic ice loss is likely in future 21st centuryclimate projections.In response to the question of an abrupt changein the AMOC, Chapter 4 notes:• It is very likely that the strength of theAMOC will decrease over the course ofthe 21st century in response to increasinggreenhouse gases, with a best estimatedecrease of 25–30%.• Even with the projected moderate AMOCweakening, it is still very likely that onmultidecadal to century time scales awarming trend will occur over most of theEuropean region downstream of the NorthAtlantic Current in response to increasinggreenhouse gases, as well as over NorthAmerica.• It is very unlikely that the AMOC will undergoa collapse or an abrupt transition toa weakened state during the 21st century.• It is also unlikely that the AMOC will collapsebeyond the end of the 21st centurybecause of global warming, although thepossibility cannot be entirely excluded.• Although it is very unlikely that theAMOC will collapse in the 21st century,the potential consequences of this eventcould be severe. These might include asouthward shift of the tropical rainfallbelts, additional sea level rise around theNorth Atlantic, and disruptions to marineecosystems.5