Book 2.indb - US Climate Change Science Program

Abrupt Climate Changelong-lived greenhouse gases, land-coverchange, and aerosols (Forster et al., 2007,Fig. 2.20b);• projected increases in anthropogenic radiativeforcing from 2000 to 2100 of around6 W m –2 (Meehl et al., 2007, Fig. 10.2).In the early Holocene, annual-average insolationforcing anomalies (at 8 ka relative topresent) range from –1.5 W m –2 at the equatorto over +5 W m –2 at high latitudes in bothhemispheres, with July insolation anomaliesaround +20 W m –2 in the mid-latitudes of theNorthern Hemisphere (Berger, 1978; Berger andLoutre, 1991). Top-of-the-atmosphere insolationis not directly comparable with the concept ofradiative forcing as used in the IPCC FourthAssessment (Committee on Radiative ForcingEffects on Climate, 2005), owing to feedbackfrom the land surface and atmosphere, but therelative size of the anomalies supports the ideathat potential future changes in the controls ofclimate exceed those observed over the past millennium(Joos and Sphani, 2008). Consequently,a longer term focus is required to describe thebehavior of the climate system under controlsas different from those at present as those of the21st century will be, and to assess the potentialfor abrupt climate changes to occur in responseto gradual changes in large-scale forcing.The controls of climate during the 21st centuryand during the Holocene differ from oneanother, and from those of the 20th century, inimportant ways. The major contrast in controlsof climate between the early 20th, late 20th, and21st century are in atmospheric composition(with an additional component of land-coverchange), while the major contrast between thecontrols in the 20th century and those in theearly to middle Holocene were in the latitudinaland seasonal distribution of insolation. In theNorthern Hemisphere in the early Holocene,summer insolation was around 8% greaterthan present, and winter about 8% less thanpresent, related to the amplification of theseasonal cycle of insolation due to the occurrenceof perihelion in summer then, while inthe Southern Hemisphere the amplitude of theseasonal cycle of insolation was reduced (Webbet al., 1993b). In both hemispheres in the earlyHolocene, annual insolation was greater thanpresent poleward of 45°, and less than presentbetween 45°N. and 45°S., related to the greatertilt of Earth’s axis than relative to today. Theenergy balance of the Northern Hemisphereduring the early Holocene thus features a largeincrease in seasonality relative to that of the20th century. This contrast between the pastand future will increase throughout the 21stcentury owing to the ongoing and projectedfurther reduction in snow and ice cover in theNorthern Hemisphere winter.Consequently, climatic variations during theHolocene should not be thought of either asanalogs for future climates or as examples ofwhat might be observable under present-dayclimate forcing if records were longer, butinstead should be thought of as a “naturalexperiment” (i.e., an experiment not purposefullyperformed by humans) with the climatesystem that features large perturbations of thecontrols of climate, similar in scope (but notin detail) to those expectable in the future. Inparticular, the climates of both the Holoceneand the 21st century illustrate the response ofthe climate system to significant perturbationsof radiative forcing relative to that of the 20thor 21st century.4.1 Examples of Large and RapidHydrologic Changes duringthe HoloceneFrom the perspective of the present and with afocus on the northern mid-latitudes, the strikingspatial feature of Holocene climate variationswas the wastage and final disappearance of themiddle- to high-latitude North American andEurasian ice sheets. However, over the muchlarger area of the tropics and adjacent subtropics,there were equally impressive hydrologicchanges, ultimately related to insolation-drivenvariations in the global monsoon (COHMAPMembers, 1988; Liu et al., 2004). Two continental-scalehydrologic changes that featuredabrupt (on a Holocene time scale) transitionsbetween humid and arid conditions were thosein northern Africa and in the mid-continentof North America. In northern Africa, the“African humid period” began after 12 kawith an intensification of the African-Asianmonsoon, and ended around 5 ka (deMenocal etal., 2000; Garcin et al., 2007), with the markedtransition from a “green” (vegetated) Sahara,to the current “brown” (or sparsely vegetated)93