Book 2.indb - US Climate Change Science Program

Abrupt Climate Changerelated to the same large-scale control—greaterthan-presentsummer insolation in the NorthernHemisphere. In North America, however, theclimate changes were also strongly influencedby the shrinking (but still important regionally)Laurentide Ice Sheet. In contrast to the situationin Africa, and likely related to the existence ofadditional large-scale controls (e.g., the remnantice sheet, and Pacific ocean-atmosphereinteractions), the onset and end of the middleHolocene moisture anomaly was more spatiallyvariable in its expression, but like the Africanhumid period, it included large-scale changesin land cover in addition to effective-moisturevariations. Also in contrast to the African situation,the vegetation changes featured changesin the type of vegetation or biomes (e.g., shiftsbetween grassland and forest, Williams et al.,2004), as opposed to fluctuations between vegetatedand nonvegetated or sparsely vegetatedstates. There are also indications that, as inAfrica and Asia, the North American monsoonwas amplified in the early and middle Holocene(Thompson et al., 1993; Mock and Brunelle-Daines, 1999; Poore et al., 2005), although asin the case of the dry conditions, there probablywas significant temporal and spatial variation inthe strength of the enhanced monsoon (Barronet al., 2005). The modern association of dryconditions across central North America andsomewhat wetter conditions in North Africaduring a La Niña phase (Palmer and Brankovic,1989) led Forman et al. (2001) to hypothesizethat changes in tropical sea-surface variability,in particular the persistence of La Niña-typeconditions (generally colder and warmer thanthose at present in the eastern and western partsof the basin, respectively), might have playedan important role in modulating the regionalimpacts of mid-Holocene climate.A variety of paleoenvironmental indicatorsreflect the spatial extent and timing of thesemoisture variations (Figs. 3.12 and 3.13), andin general suggest that the dry conditionsincreased in their intensity during the intervalfrom 11 ka to 8 ka, and then gave way toincreased moisture after 4 ka, and during themiddle of this interval (around 6 ka) werewidespread. Lake-status indicators at 6 kaindicate lower-than-present levels (and hencedrier-than-present conditions) across muchof the continent (Shuman et al., 2009), andquantitative interpretation of the pollen data inWilliams et al. (2004) shows a similar pattern ofoverall aridity, but again with some regional andlocal variability, such as moister-than-presentconditions in the Southwestern United States(see also Thompson et al., 1993). Although theregion of drier-than-present conditions extendsFigure 3.12. North American lake status (left) and moisture-index (AE/PE) anomalies (right) for 6 ka. Lake (level)status can be inferred from a variety of sedimentological and limnological indicators (triangles and squares), and fromthe absence of deposition (hiatuses, circles) (Shuman and Finney, 2007). The inferred moisture-index values are basedon modern analog techniques applied to a network of fossil-pollen data. Figure adapted from Shuman et al. (2009).99