Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Programstate. This latter transition provides an exampleof a climate change that would have significantsocietal impact if it were to occur today in anyregion, and provides an example of an abrupttransition to drought driven by gradual changesin large-scale external controls.In North America, drier conditions than thoseat present commenced in the mid-continentbetween 10 and 8 ka (Thompson et al., 1993;Webb et al., 1993a; Forman et al., 2001), andended after 4 ka. This “North American midcontinentalHolocene drought” was coeval withdry conditions in the Pacific Northwest, andwet conditions in the south and southwest, in amanner consistent (in a dynamic atmosphericcirculation sense) with the amplification ofthe monsoon then (Harrison et al., 2003). Themid-Holocene drought in mid-continentalNorth America gave way to wetter conditionsafter 4 ka, and like the African humid period,provides an example of major, and sometimesabrupt hydrological changes that occurred inresponse to large and gradual changes in thecontrols of regional climates.These continental-scale hydrologic changesobviously differ in the sign of the change (wetto dry from the middle Holocene to present inAfrica and dry to wet from the middle Holoceneto present in North America), and in the specifictiming and spatial coherence of the hydrologicchanges, but they have several features in common,including:• the initiation of the African humid period andthe North American Holocene drought wereboth related to regional climate changes thatoccurred in response to general deglaciationand to variations in insolation;• the end of the African humid period and theNorth American Holocene drought were bothultimately related to the gradual decrease inNorthern Hemisphere summer insolationduring the Holocene, and to the response ofthe global monsoon;• paleoclimatic simulations suggest thatocean-atmosphere coupling played a rolein determining the moisture status of theseregions, as it has during the 20th centuryand the past millennium;• feedback from local land-surface (vegetation)responses to remote (sea-surfacetemperature, ocean-atmosphere interaction)and global (insolation, global ice volume,atmospheric composition) forcing may haveplayed a role in the magnitude and rapidityof the hydrological changes.Our understanding of the scope of the hydrologicchanges and their potential explanationsfor both of these regions has been informedby interactions between paleoclimatic datasyntheses and climate-model simulations (e.g.,Wright et al., 1993; Harrison et al., 2003; Liu etal., 2007; see Box 3.3). In this interaction, thedata syntheses have driven the elaboration ofboth models and experimental designs, whichin turn have led to better explanations of thepatterns observed in the data (see Bartlein andHostetler, 2004).4.2 The African Humid PeriodOne of the major environmental variations overthe past 10,000 years, measured in terms ofthe area affected, the magnitude of the overallclimatic changes, and their rapidity, was thereduction in magnitude around 5,000 years agoof the African-Asian monsoon from its early tomiddle Holocene maximum, and the consequentreduction in vegetation cover and expansionof deserts, particularly in Africa south of theSahara. The broad regional extent of enhancedearly Holocene monsoons is revealed by thestatus of lake levels across Africa and Asia(Fig. 3.10), and the relative wetness of the intervalis further attested to by similarly broad-scalevegetation changes (Jolly et al., 1998; Kohfeldand Harrison, 2000). Elsewhere in the regioninfluenced by the African-Asian monsoon, theinterval of enhanced monsoonal circulation andprecipitation also ended abruptly, in the intervalbetween 5.0 and 4.5 ka across south and eastAsia (Morrill et al., 2003), demonstrating thatthe African humid period was embedded inChapter 394