Book 2.indb - US Climate Change Science Program

Abrupt Climate Changeplanetary-scale climatic variations during theHolocene.A general conceptual model has emerged (seeRuddiman, 2006) that relates the intensificationof the monsoons to the differential heating of thecontinents and oceans that occurs in response toorbitally induced amplification of the seasonalcycle of insolation (i.e., increased summer anddecreased winter insolation in the NorthernHemisphere) (Kutzbach and Otto-Bliesner,1982; Kutzbach and Street-Perrott, 1985; Liuet al., 2004). In addition to the first-orderresponse of the monsoons to insolation forcing,other major controls of regional climates, likethe atmospheric circulation variations relatedto the North American ice sheets, to ocean/atmospheric circulation reorganization overthe North Atlantic (Kutzbach and Ruddiman,1993; Weldeab et al., 2007), and to tropicalPacific ocean/atmosphere interactions (Shin etal., 2006; Zhao et al., 2007) likely also playeda role in determining the timing and details ofthe response. In many paleoenvironmental records,the African humid period (12 ka to 5 ka)began rather abruptly (relative to the insolationforcing), but with some spatial variability in itsexpression (Garcin et al., 2007), and similarly,it ended abruptly (deMenocal et al., 2000; andsee the discussion in Liu et al., 2007).The robust expression of the wet conditions(Fig. 3.10), together with the amplitude of the“signal” in the paleoenvironmental data, hasmade the African humid period a prime focusfor synthesis of paleoenvironmental data,climate-model simulations, and the systematiccomparison of the two (COHMAP Members,1988), in particular as a component of the PaleoclimaticModelling Intercomparison Project(PMIP and PMIP 2; Joussaume et al., 1999;Crucifix et al., 2005; Braconnot et al., 2007a,b).The aim of these paleoclimatic data-modelcomparisons is twofold: (1) to “validate” theclimate models by examining their ability tocorrectly reproduce an observed environmentalFigure 3.10. Global lake status at 6 ka (6,000 years ago) showing the large region that extends from Africaacross Asia, where lake levels were higher than those of the present day as related to the expansion of theAfrican-Asian monsoon. Note also the occurrence of much drier than present conditions over North America.(The most recent version of the Global Lake Surface Database is available on the PMIP 2 web page http://pmip2.lsce.ipsl.fr/share/synth/glsdb/lakes.png.)95