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Sensitivity and Rapidity of Vegetational Response to Abrupt Climate Change

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Sensitivity and Rapidity of Vegetational Response to Abrupt Climate Change 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 times 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 science 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 tundra 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 Scandinavia and even in Scotland, 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 Younger Dryas 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 years ago throughout of vegetational change has been seen in before present (15,000 calendar 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 time 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 stage 3), and the Holocene are ex- Connecticut and throughout the south- diocarbon ‘‘plateaus’’ exists in the inter- amined here. The most sensitive histories 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 Greenland 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 Canada 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 ice age (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 ice core 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-Year Cold Event. A cool- tions within Ϸ50–75 years (15, 16). This pronounced warming at the Pleistocene– 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 boreal spruce, warmth and moisture (24). The magnitude record from Browns Pond, VA records fir, larch, paper birch, 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 pine. 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 Japan, 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).
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