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Perspective

Sensitivity and rapidity of vegetational response to abrupt climate change

Dorothy Peteet*

National Aeronautics and Space Administration͞Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025; and Lamont Doherty Earth Observatory, New Core Lab, Palisades, NY 10964

Rapid climate change characterizes numerous terrestrial sediment records during and since the last glaciation. Vegetational response is best expressed in terrestrial records near ecotones, where sensitivity to climate change is greatest, and response are as short as decades.

he last two decades of scientific re- stable isotopes in central North America vegetation was responding to a dramatic Tsearch have seen a pronounced in- (4), antiphasing between rainfall in North cooling. By the 1940s, as the of crease in studies of abrupt climate change. America’s Great Basin and Africa’s Rift palynology (the study of pollen and Understanding past ‘‘flips’’ in climate be- Valley (5), and patterns of vegetation re- spores) was established, many investigated comes a priority as we examine reasons for sponse in the central Appalachians (6). sites all over Europe showed signs of this climate change and the interrelationship These terrestrial patterns of shifts parallel pollen change from shrub͞tree to of the biosphere with the atmosphere- marine records from the North Atlantic and back again. Evidence for glacial ad- ocean-cryosphere. The realization that (7) and the tropical Atlantic (8). vances during the cold snap were mapped modern climate appears to be changing at in and even in , a rapid rate has challenged scientists to The ‘‘Bo¨lling’’ Abrupt Warming Shift. The first where glaciers actually reformed. By the look at past records of abrupt climate of the most pronounced events in the early 1980s, the was con- change. How rapidly can vegetation re- deglaciation sequence is the ‘‘Bolling’’ sidered a significant event that took place spond to climate shifts? What magnitude warming, which occurs Ϸ12,700 14Cyr 11,000–10,000 14C ago throughout of vegetational change has been seen in before (15,000 yr before Europe (12). When looking for the event the past? How widespread were these present). This major ‘‘step’’ appears in elsewhere in the world, this 14C in- changes and how do they compare with many pollen records from Ϸ41°N to 29°N terval is considered a radiocarbon ‘‘chro- shifts in the polar ice cores and the ocean? along the North Atlantic seaboard (6), as nozone.’’ Complications to these correla- A selection of vegetational records that increases in temperate forest (oak, hick- tions occurred with the recent discovery of reveal rapid shifts during the last degla- ory, beech, ash, hornbeam) pollen differences in the radiocarbon time scale ciation, the glaciation itself (marine iso- abruptly rise from sites in New York and and the calendar time scale. Several ra- tope 3), and the are ex- Connecticut and throughout the south- diocarbon ‘‘plateaus’’ exists in the inter- amined here. The most sensitive eastern U.S. to as far south as Florida (9). val, and thus the YD interval actually are from sites near an ecotone, a transition This North American temperature in- spans more than 1,500 calendar years. PERSPECTIVE Thus, in calendar years, it is considered to zone between vegetational communities. crease appears to parallel the European Ϸ rise in many of the same genera, as far be between 13,000 and 11,600 years Deglacial Pattern of Abrupt Climate Change. south as Monticchio, Italy (10). Evidence ago (1). This paper focuses on terrestrial (sedi- of even earlier deglacial warming on both The estimates of the magnitude of the ment) records of abrupt vegetational continents is apparent when one considers YD cooling range from 10°C in northern change, which actually represent the ear- the retreat of the Laurentide and Scandi- Europe to 2°C in southern Europe. By the liest detected evidence of rapid climate navian ice sheets and montane glaciers in end of the l980s, paleoclimate research in

shifts. European macrofossil investiga- order for the lakes and bogs that record ice cores also showed this as a SPECIAL FEATURE tions in Scandinavia about the turn of the the Bolling event to be ice-free. major climatic cooling event of from 7 to last century identified a series of late- 15°C, and the subsequent warming at its glacial climatic oscillations named the The Younger Dryas Cold Snap. The YD is the close took place in Greenland in a decade Bo¨lling (warming), Allero¨d (warming), best documented example of an abrupt or less (1). Older Dryas (cooling), and Younger Dry- climate change, primarily because of its North American YD Evidence. The first as (YD) (cooling). This same pattern of millennial duration and its extensive geo- strong evidence for the YD in North abrupt shifts in climate is seen in great graphic coverage. The Younger Dryas was America came from Atlantic in detail in the Greenland oxygen isotope first discovered around the turn of this the l980s, when stratigraphic sections and pattern (1). Some recent papers highlight- century in an examination of plant mac- lake records showed shifts in lithology, ing the pattern of abrupt late-glacial cli- rofossils in a stratigraphic section in Den- pollen, and macrofossils, suggesting a matic shifts in sediment records as similar mark. The tiny leaves of Dryas octopetala summer temperature decline of at least to this Greenland pattern include the Eu- (Rosaceae family) were found above re- 5°C (13, 14). In some cases, chironomids ropean pollen, beetle, and isotopic lake mains of shrubs and trees that indicate the records (2), midge (chironomid) larvae climate was warming after the (11). response to lake water temperatures esti- This return to Dryas, which is an arctic– *To whom reprint requests should be addressed. E-mail: mates in maritime Canada (3), shifts in alpine tundra indicator, indicated that the [email protected].

PNAS ͉ February 15, 2000 ͉ vol. 97 ͉ no. 4 ͉ 1359–1361 Downloaded by guest on September 25, 2021 suggest an air temperature decrease of as although the records are not well dated demonstrate the variety of atmospheric large as 6–20°C (3), implying a dramatic (4). However, the vegetational response is effects that took place. Longer terrestrial temperature decline and large sensitivity very minimal, probably because of the lack records such as those from Florida (28), of midge larvae to water temperature of ecotones, and the lack of good chro- coastal Washington (29), and Chile (30) change. The return to warmer conditions nology prevents estimates of response demonstrate that rapid shifts have taken was quite rapid at most sites. At the same times. place before and during the last glaciation, time, pollen, macrofossil, and accelerator On the western coast of North America, and that the magnitudes of some of these mass spectrometry (AMS) 14C ages from evidence for a YD cooling is found in events are as large as 6°C. Better resolu- southern New England documented the many coastal records (22, 23). On Kodiak tion and dating is needed to tie them to the occurrence of a YD average July cooling Island, AK, several AMS-dated sites marine and records. of 3–4°C and, at its close (the Holocene record a regional cooling correlative with boundary), a shift back to warm condi- the YD, and the striking lithology shows a The Holocene 8,200- Cold Event. A cool- tions within Ϸ50–75 years (15, 16). This pronounced warming at the – ing event during the Holocene, Ϸ8,200 estimate of a response time of the vege- Holocene boundary. The pollen and mac- calendar years ago (7,500 14C) is recorded tation to the Holocene warming is taken rofossil record reveals a YD increase in in Greenland ice cores (31) and is appar- from the 5-cm interval of a high sedimen- crowberry (Empetrum), which indicates ent in European sediment records (32, tation rate Linsley Pond, CT core section cooling, followed by the dominance of 33). A recurrence of spruce and the de- that represents 50 years of the entire YD ferns in the Holocene, indicative of cline of thermophilous species in a pollen core span of 1 meter. The spruce, warmth and moisture (24). The magnitude record from Browns Pond, VA records fir, , paper , and alder trees, of the temperature response is difficult to this event along the Atlantic seaboard (6), which had increased during the YD, were estimate, but the response appears quite and oxygen isotope shifts are seen in Min- suddenly replaced by oak and white . rapid. Supporting evidence from the Bris- nesota between 8.9 and 8.3 calendar years, Similar pollen and macrofossil stratigra- tol Bay region also suggests a cooler, drier before this event (34). Further detailed phy has been AMS-dated in two addi- YD (25). The possibility of North Pacific investigations are needed to document the tional sites in southeastern New York (17, circulation changes associated with the Holocene abrupt changes both in North 18). Oxygen isotopes in diatoms suggest Younger Dryas (i.e., Santa Barbara Basin, America and Europe, and elsewhere the YD temperature shift was as large as Sea of , Gulf of California) greatly around the globe. 6°C (19). Thus, different paleoindicators enlarges the geographic pattern of ocean give a YD temperature decline in eastern circulation change and calls into question Sensitivity to Rapid Climate Change. The best North America ranging from 3 to 20°C. the role of this major ocean in the climate examples of abrupt climate change re- To the south, in the central Appala- forcing for this event (26). corded by vegetational records suggest chians, an interesting pattern of vegeta- that, where a sharp ecotone exists, the tional change occurs during the late- Global Distribution of YD? Peteet (27) sum- pollen͞macrofossil records provide a sen- glacial and early Holocene. In Virginia, marized the possibility of scattered late- sitive and rapid response to a climate the YD is apparently marked by slightly glacial palynological evidence for the YD change. This is true of records in southern warmer, wetter climate as hemlock dom- around the globe and concludes that, at New England and the southern Appala- inates at Browns Pond (6). This pattern of this point, there is not enough evidence to chians, where a mixture of boreal and climate change can be consistent with a indicate that the YD is an obvious global deciduous species during the late-glacial cold reversal in the northern North At- event as defined by vegetational change. provided the mixture of species for imme- lantic region if one considers the steep Although the evidence for the northern diate response to a temperature change— temperature gradient that may have oc- hemisphere is very convincing, tropical either a rapid decrease (YD, 8,200-yr curred between cold subpolar waters and and southern hemisphere data is more event) or a rapid warming (Holocene the adjacent warmer subtropical waters controversial, primarily because the data change) that occurred within a century (6, close to the latitude of Virginia. This steep must support a regional vegetational and 15–18). This rapid response to the Holo- temperature gradient would favor storm climatic change with robust AMS chro- cene warming is also seen in coastal track frequency and consequently might nology. Until a clear regional response Alaska, where cold, windswept tundra is be responsible for this vegetational re- from more than one site is well dated, the rapidly replaced by tundra species that sponse of a wetter climate. The only other question remains an open one, and an favor moisture and warmth (24). In these published southeastern U.S. record that estimate of timing and magnitude of the cases, there is very little migrational lag may record a YD appears to be Jackson vegetation response to climate is difficult. because all of the response species are Pond, KY, which shows a slight cooling Areas such as southern South America are already in the region, unlike the midwest- with the recurrence of spruce (20). More especially critical for our understanding of ern U.S. These sensitive vegetational sites with detailed AMS are the leads and lags and responses of north- records then parallel the response to cli- needed to firmly establish the late-glacial ern and southern hemispheres. mate from the isotopic records—either timing and magnitude of climatic oscilla- from ice cores, lake carbonate records, or tion. Marine Stage 3 Rapid Climate Shifts. The diatoms. They also parallel the records Sites in the midwestern U.S. show a YD high-resolution Greenland ice cores indi- from animal responses: i.e., midge larvae oscillation, primarily through an increase cate that the Younger Dryas is just one of or beetles. However, the estimates of the in spruce in some sites, and the suggested many pronounced and rapid climate magnitudes of the temperature shifts cooling is Ϸ1–2°C in summer (21). Oxygen changes that have occurred in the last seem to vary widely among biological in- isotope shifts from the Great Lakes region 50,000 years. Major changes during these dicators. Thus, multiproxy, high-resolu- match the Greenland ice core and some earlier times such as dust flux increase and tion studies in sensitive ecotones are es- European lake records, suggesting the snowfall declines in Greenland as well as sential for a better understanding of Bo¨lling, Allero¨d, and YD climate changes, methane declines during the coolings abrupt events.

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