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U of M GEOG 5426 - Bradley Natural Climate Variability and Global Warming 2008

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CoverPages from Battarbee_&_Binney_Section1-2.pdfCh10_Bradley....10Holocene perspectives on future climate changeRaymond S. BradleyKeywordsENSO (or El Niño), monsoon (or Asian monsoon), climate forcing, droughtIntroductionWhatever anthropogenic climate changes occur in the future, they will be superim-posed on, and interact with, underlying natural variability. Therefore, to anticipatefuture changes, we must understand how and why climates varied in the past. Thisrequires well-dated records of forcing factors, as well as paleoclimate; both areavailable from a variety of natural archives. Relying on instrumental data to under-stand the spectrum of climate variability is completely inadequate; the record oflarge-scale (hemispheric or global) temperature extends back only ca. 150 yearsand the same can be said for most of the major modes of climate variability (Table 10.1). Even the longest instrumental records barely cover 300 years, and formost of the world such records are rarely longer than 120 years. This means thatour understanding of climate system variability is largely limited to the interannualto decadal scale. To examine longer term (centennial to millennial-scale) variabil-ity requires much longer datasets. The Holocene provides a particularly relevantperiod for such an endeavor, as large-scale boundary conditions (continental iceextent, topography, sea-level) have remained very close to modern conditions formuch of the Holocene, and low frequency (orbital) forcing is well understood forthis period. Thus, Holocene paleoclimate data are able to resolve the full spectrumof climate variability and to place the limited instrumental records in a long-termperspective. This is particularly germane to the issue of anthropogenic climatechange, as it provides a context for recent changes. Mann et al. (1999) argued that,for the Northern Hemisphere, the mean annual temperature of the past fewdecades of the 20th century was the highest for at least 1000 years, and this hypo-thesis has been supported by several later studies (National Research Council 2006).Moreover, Osborn and Briffa (2006) showed that both the magnitude and spatial9781405159050_4_010.qxd 6/3/08 3:59 PM Page 254Holocene perspectives on future climate change|255..extent of recent warming was unprecedented in the past millennium. Such studiesprovide a check on energy balance and general circulation models that seek to simulate temperature changes over multi-centennial periods in order to examinethe role of natural versus anthropogenic forcing (e.g. Crowley 2000; Ammann et al.2003; Goosse et al. 2005). All of these studies conclude that the rise in temperatureduring the 20th century (particularly the late 20th century) is unprecedented, andthe spatial pattern of warming cannot be explained by natural forcing alone; green-house gases provide the necessary additional forcing to account for the recentchanges in temperature. Thus, paleoclimate data contribute to both detection andattribution studies related to anthropogenic climate change.Paleoclimate data are also important for the broader debate about the role ofgreenhouse gases in recent climate change. Those who seek to dismiss concernsover greenhouse gases frequently point to episodes in the past when temperatureswere higher (even though these episodes were commonly just in a limited geo-graphic region and/or in a particular season). For example, critics sometimes pointto the early Holocene (a time when many Northern Hemisphere alpine glaciersdisappeared and Arctic sea-ice was of limited extent) as “evidence” that human-induced climate change is a fiction and recent changes are merely a manifestationof underlying natural variability. Paleoclimatic research, however, clearly showsthat orbital forcing (largely due to changes in precession) was responsible for thepattern of early Holocene warming. Furthermore, the early Holocene was a time ofsummer warmth mainly in the Northern Hemisphere; in that hemisphere, insola-tion receipts steadily declined during the Holocene. In Northern Hemisphere ..Table 10.1 Examples of some key climatic time series, based on instrumental records(Sources: and data Starts Indices StartsCentral England temperature 1659 Southern Oscillation Index (Tahiti– 1866Darwin pressure)Global and hemispheric 1850 Global SST ENSO index 1845temperaturesEngland and Wales precipitation 1766 Multi-variate ENSO 1950NAO 1821AMO 1860Northern Annular Mode/Arctic 1899OscillationPDO 1900Southern Annular Mode/Antarctic 1948OscillationIndian Monsoon 18449781405159050_4_010.qxd 6/3/08 3:59 PM Page 255..256|Raymond S. Bradley..winters, and in the southern hemisphere summer and autumn (from December toJune), insolation receipts in the early Holocene were below modern values, butincreased through the Holocene. The rapid warming of the late 20th century isunrelated to this forcing; rather, the rate, seasonal pattern, and geographic extentof recent changes all point to greenhouse gases as the primary cause. Paleoclimateresearch can thus help to differentiate the cause and effect of climate changes,thereby elevating the discussion of strategies needed to deal with current environ-mental problems above the noisy distractions of those more interested in divertingattention from the issues at hand.Although there has been an understandable emphasis on global (or hemis-pheric) temperature reconstructions, regional hydrologic variability is likely to beof most concern in the future, and so paleoclimatic reconstructions must pay particu-lar attention to documenting past changes in precipitation and/or regional-scale water balance (as discussed by Verschuren and Charman, this volume). TheHolocene record of climate variability is replete with examples of (largely unex-plained) hydrologic instability. Multi-decadal- to multi-century-length droughtsoften started abruptly, were unprecedented (in the experience of societies at thetime), and thus were highly disruptive to their agricultural, economic, and socialfoundations (AUP episodes: abrupt, unprecedented [in magnitude/duration], andpersistent). One of the best documented examples is from Quintana Roo (YucatanPeninsula), Mexico where there were a series of AUP droughts between ca. ad 800and ad 1000. These were accompanied by severe economic and

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