Climate
Science: Investigating
Climatic and Environmental Processes Millennial-Scale
Variability (<104
years) The
North Atlantic Connection
Forcing Factors Identifying
Uncertainty Did
the influx of freshwater 8,200 years ago from large lakes in what is now
northern Canada help trigger the coldest climate event in the Earth's
climate system in the past 10,000 years? That such a cold event occurred
is well documented by Baldini (2002)
and others, including Von
Grafenstein (1998),
whose data in the figure to the left shows snow accumulation and isotopically
inferred temperature records from the Greenland GISP2 ice core and fossil
shells in the sediments of Lake Ammersee, southern Germany. One
theory put forth by Barber, et. al.(1999)
as to what triggered this 400 year period of cooling is that two gigantic
glacial lakes in Canada's Hudson Bay region some 8,200 years ago broke
open when an ice dam from a remnant of the Laurentide Ice Sheet collapsed.
The flow of lake water rushing through the Hudson Strait and into the
Labrador Sea is estimated to be about 15 times greater than the current
discharge of the Amazon River. Also see Abrupt Climate
Change.
Another
climate mystery that scientists have puzzled over in recent years is why
during the the"middle Holocene" (roughly 7,000 to 5,000 years
ago), temperatures seemed to be warmer than even present day temperatures.
Indeed, some of the paleoclimatic data suggest that temperatures were
several degrees Celsius hotter than today. With the growing concern about
the potential for global warming,
such information is of great interest to climate scientists.
It now appears that temperatures were generally warmer, but only in the
summer in the northern hemisphere. The cause? Changes in the Earth's orbit
that operate slowly over thousands and millions of years that change the
amount of solar radiation reaching each latitudinal band of the Earth
during each month. (See The
Ice Age online slide set and Climate Science
100,000 Years for more on orbital forcing.) Such orbital changes can
be calculated, and what they indicate is that the northern hemisphere
should have been warmer in the summer and colder in the winter than at
present during the mid-Holocene.
The
North Atlantic Connection
The North Atlantic
has been the focus of extensive research in recent years into climate
variability at the millennial scale. Scientists have been able to
determine that during warm periods, debris from rocks are carried
by rafts of ice into the North Atlantic. When the ice melts, the
debris is deposited in ocean sediments. During colder periods, debris
from the ice rafting is no longer found in sediments. Researchers
have also found millennial scale
cycles in tree rings (Bond, 2001)
and in patterns of storminess in northeastern United States (Noren,
2002). See Dansgaard-Oeschger cycles
for more information.
First discovered by marine geologist Hartmut Heinrich in the 1980s,
large episodes of ice rafting have been called Heinrich Events.
(See online slideshow about such events for more.) Comparing the
d18O ice core record with the analysis of sediment cores
from the North Atlantic show repeated cycles of slowly developing
glacial conditions followed by abrupt shifts back to warmer conditions.
Image from Ruddiman, 2001
used by permission of W. H. Freeman & Co.
FORCING
FACTORS
Internal
influences on climatic variability at millennial scales includeDansgaard-Oeschger
cycles which vary in the length of their intervals.
Recent ice core studies examining changes in d18O
indicate oscillations of close to 1500 years in length
(Ruddiman,
2001).
How
Measured Scientists examining climate variability of 1000 years or
more have come to rely on studying the d
O18 values taken from ice cores and the shells of planktonic
foraminifera from marine sediments. The sediments also
reveal lithics (rock particles) and dust that offer additional
clues of the climate variations thousands of years ago. Example of planktonic
foraminifera from NOAA Paleoclimatology Program
Identifying Uncertainty
Scientists studying climate, particularly paleoclimates, must always
bear in mind the unknowns and uncertainties involved with the data, particularly
when attempting to match up records from different types of proxies or different
regions. The challenge is especially difficult when correlating millennial
scale oscillations when the resolution of the data is low and uncertainty
is high.
Image from Ruddiman,
2001 used by permission of W. H. Freeman & Co.
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