Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeinto the Northeastern United States and easternCanada, most of the multiproxy evidence formiddle Holocene dryness is focused on themidcontinent, in particular the Great Plainsand Midwest, where the evidence for aridityis particularly clear. There, the expression ofmiddle Holocene dry conditions in paleoenvironmentalrecords has long been known, aswas the case for the “Prairie Period” evident infossil-pollen data (see Webb et al., 1983), andthe recognition of significant aeolian activity(dune formation) on the Great Plains (Formanet al., 2001; Harrison et al., 2003) that wouldbe favored by a decrease in vegetation cover.Temporal variations in the large-scale controlsof North American regional climates as well assome of the paleoenvironmental indicators ofthe moisture changes are shown in Figure 3.13.In addition to insolation forcing (Fig. 3.13A,B),the size of the Laurentide Ice Sheet was amajor control of regional climates, and whilediminished in size from its full extent at theLast Glacial Maximum (21 ka), the residualice sheets at 11 ka and 9 ka (Fig. 3.13C) stillinfluence atmospheric circulation over easternand central North America in climate simulationsfor those times (Bartlein et al., 1998;Webb et al., 1998). In addition to depressingtemperatures generally around the NorthernHemisphere, the ice sheets also directly influencedadjacent regions. In those simulations, thedevelopment of a “glacial anticyclone” over theice sheet (while not as pronounced as earlier),acted to diminish the flow of moisture from theGulf of Mexico into the interior, thus keepingthe midcontinent cooler and drier than it wouldhave been in the absence of an ice sheet.Superimposed on these “orbital time scale”variations in controls and regional responsesare millennial-scale variations in atmosphericcirculation related to changes in the AtlanticMeridional Overturning Circulation (AMOC)and to other ocean-atmosphere variability(Shuman et al., 2005, 2007; Viau et al., 2006).Of these millennial-scale variations, the “8.2 kaevent” (Fig. 3.13D) is of interest, inasmuch asthe climate changes associated with the “collapse”of the Laurentide Ice Sheet (Barber etal., 1999) have the potential to influence themidcontinent region directly, through regionalatmospheric circulation changes (Dean et al.,2002; Shuman et al., 2002), as well as indirectly,through its influence on AMOC, and relatedhemispheric atmospheric circulation changes.The record of aridity indicators for the midcontinentreveals a more complicated history ofmoisture variations than does the African case,with some locations remaining dry until the lateHolocene, and others reaching maximum aridityduring the interval between 8 ka and 4 ka,but in general showing relatively dry conditionsbetween 8 ka and 4 ka. Lake-status records(Fig. 3.13E, Shuman et al., 2009) show the highestfrequency of lakes at relatively low levelsduring the interval between 8 ka and 4 ka, anda higher frequency of lakes at relatively highlevels before and after that interval. Records ofwidespread and persistent aeolian activity andloess deposition (dust transport) increase infrequency from 10 ka to 8 ka, and then graduallyfall to lower frequency in the late Holocene,with a noticeable decline between 5 ka and 4 ka.Pollen records of the vegetation changes thatreflect dry conditions (Fig. 3.13G; Williams,2002; Williams et al., 2004) show a somewhatearlier onset of dryness than do the aeolian orlake indicators, reaching maximum frequencyaround 9 ka. Increased aeolian activity can alsobe noted during the last 2000 years (Fig. 3.13F,Forman et al., 2001; Miao et al., 2007), but wasless pronounced than during the mid-Holocene.The pollen record from Steel Lake, MN, expressedin terms of tree-cover percentages (seeWilliams, 2002, for methods) provides an exampleto illustrate a pattern of moisture-relatedvegetation change that is typical at many sitesin the Midwest, with an abrupt decline in treecover at this site around 8 ka, and over an intervalequal to or less than the sampling resolutionof the record (about 200 years, Fig. 3.13H). Thisdecrease in tree cover and inferred moisturelevels is followed by relatively low but slightlyincreasing inferred moisture levels for about4,000 years, with higher inferred moisturelevels in the last 4,000 years. The magnitudeof this moisture anomaly can be statisticallyinferred from the fossil-pollen data using modernrelationships between pollen abundance andclimate, as was done for the pollen record at ElkLake, MN, which is near Steel Lake (Fig. 3.13I;Bartlein and Whitlock, 1993; see also Webb etal., 1998). Expressed in terms of precipitation,101