Abstract:
Younger Dryas Data - Paleoclimate Evidence of Rapid Climate Change
Evidence of past climate change provides a historical perspective from which we may view our present and future climate. Records derived from ice cores, lake, bogs, ocean sediments, and glacial moraines indicate that our climate has changed considerably over the last 20,000 years. As paleoclimate techniques have improved and ... enhanced resolution magnifies our view, it is now obvious that the climate has changed frequently and rapidly in the past, both from warm to cold conditions and from cold to warm. The most prominent of these reversals is the Younger Dryas cooling that was first discovered in Europe, and occurred refers to the Arctic-Alpine flower, Dryas. Fossils of this plant and other tundra indicators were found sandwiched between sediments that indicated warmer, forested environments.) We may ask, does closer scrutiny of this event tell us anything that is relevant to our future? We know that describing the past patterns of change will aid us in understanding how rapidly vegetation can respond to climate change, and ultimately, what may be causing change.
Studies at GISS have resulted in the discovery of a similar event in North America. This event has now been examined in three lakes and swamps in southern New England, using a combination of pollen stratigraphy, macrofossil stratigraphy, and accelerator mass spectrometry (AMS) techniques. We are looking closely at the patterns of vegetational change in the rapid warming and cooling modes.
In the northeastern U.S., the Younger Dryas event is regionally apparent from pollen records that indicate a change from a mixed hardwood deciduous forest (oak, ash, spruce, fir) to a boreal assemblage (spruce, fir, larch, birch, alder) as seen in pollen zones A-3 and A-4 in the first figure. Subsequently, the pollen assemblage dramatically reverses, as the boreal components disappear and, first, white pine, then oak, dominates. This pattern of pollen assemblage zones is recorded in over 20 sites in the northeastern U.S. Three questions were asked concerning this change in regional pollen stratigraphy to determine whether this change represented a climatic change similar to that found in Europe: (1) Does this palynological change represent a real change in vegetation, as opposed to a change in wind direction? (2) Was the vegetational change due to changes in climate? (3) What were the timing and rate of the vegetational change?
To determine whether the palynological change represented a true vegetational change, sediment samples were screened for macrofossils (leaves, needles, seeds), which give a more precise determination of the species involved. In addition, macrofossils are important because, although pollen may blow long distances, the macrofossils indicate the actual vegetation at the site. The results of this detailed analysis indicated that the distribution of boreal trees indeed expanded during this interval, and that trees with such northerly distributions as paper birch replaced warmer species such as white pine. The winters during this time must have been severe, as increased alder and paper birch indicate disturbance. The magnitude of the change in the NY-CT-NJ region was a cooling of about 3 to 4 xC annually, and the snow depth was probably considerably more than present. Extreme winters probably left gaps in the forest that were filled by light-demanding paper birch and alder. Determination of the timing of the dramatic northeastern U.S. cooling and the subsequent warming is important for correlation with the European event, as well as for comparison with ice core and marine records, and other regional land records. The macrofossils were submitted for AMS dating to Lawrence Livermore Laboratories in California, where analysts there are able to determine the (14)C age of a single spruce needle. Results of the dating indicated that the cooling occurred between 11,000 and 10,000 years ago, which proves that the event here in southern New England is the same as that in Europe, in the Greenland ice cores, and in North Atlantic marine records.
From the 5-cm detailed analysis of the sedimentation rates in the sediment cores, estimates of the regional vegetational change rates were made. The best estimates are that the climate change at the onset of the cooling took place over about 100 to 200 years. The close of the Younger Dryas cooling, typified by a vegetation change from boreal trees to a warmer white pine and oak forest, took place over an interval of between 50 and 150 years. From the macrofossil changes, it is evident that trees such as spruce, fir, and paper birch became locally extinct at the time of this rapid warming, and have remained absent from this region for the past 10,000 years. This dramatic and rapid change is comparable to that predicted for future greenhouse warming, and is comparable to the report of a very sharp transition (50 to 100 years) for this period in the Greenland ice cores.
With two data points in-hand, it is now reasonable to inquire as to the global distribution of this event. At GISS, we are continuing to assess the available evidence for this reversal worldwide through continued literature compilation as well as field work. If indeed the methane and carbon dioxide decreases in the ice cores are linked in age to the Younger Dryas interval, it means that the event was global, because these gases are distributed rapidly in the atmosphere. Based upon North Atlantic evidence of cooling, in 1986 we reported the GCM results of the effects of a colder North Atlantic over land regions adjacent to the North Atlantic, as shown in the second figure, which agrees with terrestrial paleoclimatic evidence from the North Atlantic region. However, we are continuing to assess reports of the Younger Dryas in locations further afield, specifically with a research program of glacial geology and palynology at high latitudes, including Kodiak Island, Alaska.
Results from Kodiak Island suggest that glacial advances and vegetational changes characterized the interval from 11,000-10,000 years BP. We are currently identifying the vegetational changes during this period, but it appears that a major regional cooling did take place roughly during this time interval, as evidenced by the decline of warmth-demanding ferns and the increase of tundra. Additional AMS dating of macrofossils should tell us whether a major change took place that is correlative with the Younger Dryas of Europe and eastern North America.
We are continuing our modeling efforts from the latest data-model interaction. We have conducted additional GCM experiments using new marine data that indicate a 2 xC cooling in North Pacific sea surface temperature during the Younger Dryas. The resulting annual air temperature difference between today and this experiment is quite large throughout the Northern Hemisphere, including the air temperature over North America and Europe. This result is significant because it points to the tremendous impact of the large area of the North Pacific Ocean on northern hemispheric climate when the temperature is lowered, as less moisture in the air results in less of a greenhouse warming effect.
We continue to delve into the questions of the distribution, timing, magnitude, and vegetational expression of this rapid and major climate event of the past. By analyzing its effects regionally and globally, we will be in a better position to define the cause of such rapid climate changes, and our future.
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