Book 2.indb - US Climate Change Science Program

Abrupt Climate Change7. What Factors ThatInfluence the OverturningCirculation Are Likely ToChange in the Future, andWhat is the ProbabilityThat the OverturningCirculation Will Change?As noted in the Intergovernmental Panel forClimate Change (IPCC) Fourth AssessmentReport (AR4), all climate model projectionsunder increasing greenhouse gases lead toan increase in high-latitude temperature aswell as an increase in high-latitude precipitation(Meehl et al., 2007). Both warming andfreshening tend to make the high-latitudesurface waters less dense, thereby increasingtheir stability and inhibiting convection.In the IPCC AR4, 19 coupled atmosphere-oceanmodels contributed projections of future climatechange under the SRES A1B scenario (Meehlet al., 2007). Of these, 16 models did not useflux adjustments (all except CGCM3.1, INM-CM3.0, and MRI-CGCM2.3.2). In makingtheir assessment, Meehl et al. (2007) notedthat several of the models simulated a late 20thcentury AMOC strength that was inconsistentwith present-day estimates: 14–18 Sv at 24°N.(Ganachaud and Wunsch, 2000; Lumpkin andSpeer, 2003); 13–19 Sv at 48°N. (Ganachaud,2003a); maximum values of 17.2 Sv (Smethieand Fine, 2001) and 18 Sv (Talley et al., 2003)with an error of ± 3–5 Sv. As a consequenceof their poor 20th century simulations, thesemodels were not used in their assessment.The full range of late 20th century estimatesof the Atlantic MOC strength (12–23 Sv) isspanned by the model simulations (Fig. 4.17;Schmittner et al., 2005; Meehl et al., 2007).The models further project a decrease in theAMOC strength of between 0% and 50%,with a multimodel average of 25%, over thecourse of the 21st century. None of the modelssimulated an abrupt shutdown of the AMOCduring the 21st century.Schneider et al. (2007) extended the analysisof Meehl et al. (2007) by developing a multimodelaverage in which the individual modelsimulations were weighted a number of ways.The various weighting estimates were basedon an individual model’s simulation of thecontemporary ocean climate, and in particularits simulated fields of temperature, salinity,pycnocline depth, as well as its simulated AtlanticMOC strength. Their resulting best estimate21st century AMOC weakening of 25–30% wasinvariant to the weighting scheme used and isconsistent with the simple multimodel mean of25% obtained in the IPCC AR4.In early versions of some coupled atmosphereoceanmodels, (e.g., Dixon et al., 1999), increasedhigh-latitude precipitation dominatedover increased high-latitude warming in causingthe projected weakening of the AMOCunder increasing greenhouse gases, while inothers (e.g., Mikolajewicz and Voss, 2000), theopposite was found. However, Gregory et al.(2005) undertook a recent model intercomparisonproject in which, in all 11 models analyzed,the AMOC reduction was caused more bychanges in surface heat flux than changes insurface freshwater flux. Weaver et al. (2007)extended this analysis by showing that, in onemodel, this conclusion was independent of theinitial mean climate state.A number of stabilization scenarios have beenexamined using both coupled Atmosphere-Ocean General Circulation Models (AOGCMs)(Stouffer and Manabe, 1999; Voss and Mikolajewicz,2001; Stouffer and Manabe, 2003;Wood et al., 2003; Yoshida et al., 2005; Bryanet al., 2006) as well as Earth System Models ofIntermediate Complexity (EMICs) (Meehl etal., 2007). Typically the atmospheric CO 2 concentrationin these models is increased at a rateof 1%/year to either two times or four times thepreindustrial level of atmospheric CO 2 , and heldfixed thereafter. In virtually every simulation,the AMOC reduces but recovers to its initialstrength when the radiative forcing is stabilizedat two times or four times the preindustriallevels of CO 2 . Only one early flux-adjustedmodel simulated a complete shutdown, and eventhis was not permanent (Manabe and Stouffer,1994; Stouffer and Manabe, 2003). The onlymodel to exhibit a permanent cessation of theAMOC in response to increasing greenhousegases was an intermediate complexity modelwhich incorporates a zonally averaged oceancomponent (Meehl et al., 2007).157