Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3and climate is negative (see also Wang etal., 2007; Notaro et al., 2008), such that asparsely or unvegetated state (i.e., a brownSahara) would tend to favor precipitationthrough the recycling of moisture frombare-ground evapotranspiration. In thisview, the negative vegetation feedbackwould act to maintain the green Saharaagainst the general drying trend relatedto the decrease in the intensity of themonsoon and amount of precipitation,until such time that interannual variabilityresults in the crossing of a moisturethreshold beyond which the green statecould no longer be maintained (see Cooket al., 2006, for further discussion of thiskind of behavior in response to interannualclimate variability (i.e., ENSO). In thisconceptual model, the transition betweenstates, while broadly synchronous (owingto the large-scale forcing), might be expectedto show a more time-transgressiveor diachronous pattern owing to theinfluence of landscape (soil and vegetation)heterogeneity.These two conceptual models of the mechanismsthat underlie the abrupt vegetationchange—strong feedback and interannual variability/thresholdcrossing—are not that differentin terms of their implications, however. Bothconceptual models relate the overall decreasein moisture and consequent vegetation changeto the response of the monsoon to the graduallyweakening amplification of the seasonal cycleof insolation, and both claim a role for vegetationin contributing to the abruptness of theland-cover change, either explicitly or implicitlyinvoking the nonlinear relationship betweenvegetation cover and precipitation (Fig. 3.11).The conceptual models differ mainly in theirdepiction of the precipitation change, with thestrong-feedback explanation predicting thatabrupt changes in precipitation will accompanythe abrupt changes in vegetation, whilethe interannual variability/threshold crossingexplanation does not. It is interesting to notethat the Renssen et al. (2006) EMIC simulationgenerates precipitation variations for westAfrica that show much less of an abrupt changearound 5 ka than did earlier EMIC simulations,which suggests that the strong-feedbackperspective may be somewhat model dependent.Figure 3.11. African humid period records (Liu et al., 2007;reprinted with permission from Quaternary Science Reviews).A recent analysis of a paleolimnological recordfrom the eastern Sahara (Kröpelin et al., 2008)shows a more gradual transition from the greento brown state than would be inferred from themarine record of dust flux, which also supportsthe variability/threshold crossing model.There is thus some uncertainty in the specificmechanisms that link the vegetation responseto climate variations on different time scalesand also considerable temporal-spatial variabilityin the timing of environmental changes.However, the African humid period and its rapidtermination illustrates how abrupt, widespread,and significant environmental changes canoccur in response to gradual changes in alarge-scale or ultimate control—in this case theamplification of the seasonal cycle of insolationin the Northern Hemisphere and its impact onradiative forcing.4.3 North American Mid-ContinentalHolocene DroughtAt roughly the same time as the Africanhumid period, large parts of North Americaexperienced drier-than-present conditions thatwere sufficient in magnitude to be registered ina variety of paleoenvironmental data sources.Although opposite in sign from those in Africa,these moisture anomalies were ultimately98