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Reconstruction of past climates is possible via a number of techniques and provides vital information on historic
phases of rapid climate change. One particularly useful technique identifying rapid climate transitions is the
analysis of polar ice cores. Ice within cores can be analysed to reveal the ratio of differing forms of oxygen
known as isotopes - manifestations of the same element but with varying numbers of neutrons and, therefore,
weights. Two examples of oxygen isotopes are
18O and 16O, having ten and eight neutrons
respectively. During transport to polar regions, condensation and precipitation preferentially removes so-called 'heavier
water', containing 18O, from the air so that the vapour reaching high latitudes is
typically depleted of 18O. However, the
distribution of water isotopes has been well documented to show the existence of a direct relationship between average
18O content and the mean annual temperature at the precipitation site; in warmer climates, the ratio of
18O compared to 16O
rises. By analysing ice samples at progressively deeper depths, a continuous record of historical, implied temperatures can
be constructed.
Dansgaard-Oeschger Events
The diagram below shows
18O
levels extending back to ~50,000 years before present (b.p.) recovered from the
GISP2 ice core, recovered from the Greenland Ice Sheet.
Especially prominent in this record are the
seemingly
regular spikes in
18O
levels, marked with letters and numbers. These features are particularly frequent beyond
~25,000 b.p. when the earth was in the midst of the last glacial period. The spikes indicate excursions of
warmer weather when it is believed that the climate of Greenland changed rapidly - by as much as 8-16 deg. C in
as little as 40 years. These anomalous warm phases are called Dansgaard-Oeschger events and vary in length from
under one thousand to many thousands of years1. At the end of each event the climate cooled rapidly, although
at a slower rate than the initial warming. Experiments using climate models have shown that the geographical
pattern and rate of change characterising these events is consistent with the change we'd expect from
re-arrangement of the north Atlantic thermohaline circulation. But even so, why these events occurred with such
regularity is still unknown.
Source: Rahmstorf, 2003
Enlarge image
Although the frequency of Dansgaard-Oeschger events seems to have decreased as the last glacial period drew to a
close, a number of rapid changes appear to have punctuated this gradual warming. Two well-known episodes are
the Younger-Dryas and 8.2 kyr (kilo-year) cooling events, when the climate returned briefly to more glacial
conditions.
The Younger-Dryas Cool Event
The Younger-Dryas event can clearly be seen in the figure above as a dip in
18O
levels between ~12,000 and ~13,000 years b.p. This climate episode was characterised, over Greenland at least, by temperatures of
~15 degrees Celsius cooler than present values, and by a much windier, drier climate2. Like Dansgaard-Oeschger
events, the Younger Dryas anomaly is believed to have been linked with changes in the Atlantic Ocean circulation.
The cooling here is thought to have been driven by the influx of large volumes of fresh melt water originating
from the retreating ice sheets. The Younger-Dryas event apparently ended in spectacularly abrupt fashion
approximately ~11,600 years b.p. with ~7 deg. C warming over Greenland occurring in less than 15 years.
This abrupt warming may have been linked to a reactivation of the Atlantic thermohaline circulation and
additional warming caused by an increase in regional atmospheric water vapour causing an enhanced
(but totally natural) greenhouse effect.
The 8.2 kyr Cool Event
The so-called 8.2 kyr event represents a shorter, and more recent, episode of cooling, related to the sudden,
release of melt water from two glacial lakes, Agassiz and Ojibway. The volume of freshwater discharged into the
Atlantic Basin is thought to have been sufficient to severely weaken, if not stop, the Atlantic thermohaline
circulation, leading to a ~6 deg C cooling over Greenland. Recent work by RAPID scientists3 has enabled precise
dating and definition of the 8.2 kyr event following the production of a combined ice core record based on four
well-studied Greenland cores (
GRIP,
GISP2,
NGRIP
and
Dye3
). Dated oxygen isotope ratios from this
record are shown in the diagram below and reveal a general cooling episode of about 160.5 years duration (light blue) when
18O
levels were consistently below the average computed over the preceding 1,000 years. Within this general
cooling phase is a further central event (darker blue) when
18O
levels were even lower, lasting for 69 years during which
the maximum cooling would have been occurred.
Source: Thomas et al., 2006
(Note: time and depth run from right to left).
Enlarge image
There is some debate in the scientific literature concerning the evolution of events that constitutes the 8.2 kyr event,
especially concerning the apparent delay between the lake drainage and the cooling signal present in the ice record.
In fact recent work 4 using ocean sediment records supports the hypothesis of an initial cooling event circa
~8,490 years b.p. - almost co-incidental with the onset of drainage from Lakes Agassiz and Ojibwa. One hypothesis is that
this preliminary cooling may symbolize
the initial response of the climate system to the fresh water release and that the main 8.2 kyr
event visible in the ice record represents the final culmination of changes in Atlantic Ocean circulation, and therefore the largest
temperature change.
NOTES:
1: Rahmstorf, S. 2003, 'Timing of abrupt climate change: A precise clock', Geophysical Research Letters, 30, 10,
doi:10.1029/2003GL017115.
2: Taylor, K.C. et al., 'The Holocene-Younger Dryas transition recorded at Summit, Greenland', Science, 278, 825-827.
3: Thomas, E.R. et al., 2006, 'The 8.2 kyr event from Greenland ice cores', Quarternary Science Review, in press,
doi:10.1016/i.quascirev.2006.07.017.
4: Ellison, C.R.W. et al., 2006, 'Surface and deep ocean interactions during the cold climate event 8200 years ago',
Science, 312, 1929-1932.
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