Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeentire period of 1320 to 1462 A.D. and forcedan AGCM with this record. They found that theoverall colder tropical Pacific implied by thecoral records forced drying over North Americawith a pattern and amplitude comparable tothat inferred from tree ring records, includingfor two megadroughts (1360–1400 A.D. and1430–1460 A.D.). Discrepancies between modeland observations can be explained throughthe combined effect of potential errors in thetropical Pacific SST reconstruction, a role forSST anomalies from other oceans, other unaccountedexternal forcings, and climate modeldeficiencies.The modeling work suggests that the Medievalmegadroughts were driven, at least in part, bytropical Pacific SST patterns in a way that isfamiliar from studies of the modern droughts.Analyses of the global pattern of Medievalhydroclimate also suggest that it was associatedwith a La Niña-like state in combinationwith a warm subtropical North Atlantic and apositive North Atlantic Oscillation (Seager etal., 2007a; Herweijer et al., 2007). For example,Haug et al. (2001) used the sedimentary recordfrom the Cariaco basin in the Caribbean Sea toargue that northern South America experiencedseveral wet centuries during the Medievalperiod, which is consistent with a La Niña-likePacific Ocean. As another example, Sinha et al.(2007) used a speleothem (a secondary mineraldeposit formed in a cave) record from India toshow that at the same time the Indian monsoonwas generally strong, especially compared tothe subsequent Little Ice Age.It has been suggested that the tropical Pacificadopted a more La Niña-like mean state duringthe Medieval period, relative to subsequentcenturies, as a response to a relatively strongSun and weaker volcanic activity (Mann et al.,2005; Emile-Geay et al., 2007; see also Adamset al., 2003). This follows because a positiveradiative forcing warms the western equatorialPacific by more than the east because in thelatter region strong upwelling and ocean heatdivergence transports a portion of the absorbedheat toward the subtropics. The stronger eastwestgradient then strengthens the WalkerCirculation, increasing the thermocline tilt andupwelling in the east such that actual coolingcan be induced.Further support for positive radiative forcingover the tropical Pacific Ocean inducing LaNiña-like SSTs and drought over the Southwestcomes from analyses of the entire Holocenerecorded in a New Mexico speleothem, whichshows a clear association between increasedsolar irradiance (as deduced from the atmospheric14 C content recorded in ice cores)and dry conditions (Asmerom et al., 2007).However, the theory for the positive radiativeforcing-La Niña link rests on experimentswith intermediate complexity models (Clementet al., 1996, 2000; Cane et al., 1997). Incontrast, the coupled GCMs used in the IPCCprocess do not, however, respond in this wayto rising greenhouse gases and may actuallyslow the Walker Circulation (Vecchi et al.,2006). This apparent discrepancy could arisebecause the tropical response to changes insolar irradiance is different from the responseto rising greenhouse gases or it could be thatthe coupled GCMs respond incorrectly due tothe many errors in simulations of the tropicalPacific mean climate, not the least of which isthe notorious double-intertropical convergencezone (ITCZ) problem.3.5 Megadroughts in the Great Plainsand U.S. “Breadbasket”The emphasis up to now has been on the semiaridto arid Western United States because thatis where the late-20th century drought beganand has largely persisted up to the presenttime. The present drought has therefore largelymissed the important crop-producing States inthe Midwest and Great Plains. Yet, previousstudies (Laird et al., 1996; Woodhouse andOverpeck, 1998; Stahle et al., 2000, 2007)indicate that megadroughts have also occurredin those regions as well. To illustrate this, wehave used the updated NADA to produce an averagePDSI series for the Great Plains rectangleindicated in Figure 3.7. That series is shown inFigure 3.9C and it is far more provocative thaneven the Southwest series. The MCA periodshows even more persistent drought, now onthe centennial time scale, and the 15th centurymegadrought stands out more strongly as well.The duration of the MCA megadrought in ourrecord is highly consistent with the salinityrecord from Moon Lake in North Dakota thatlikewise shows centennial time scale droughtaround that time. More ominously, in com-The present droughthas largely missedthe important cropproducingStatesin the Midwest andGreat Plains.91