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Climate Science: Investigating Climatic and Environmental Processes
Millennial-Scale Variability (<104 years)
The North Atlantic Connection
Forcing Factors

Identifying Uncertainty
Abrupt climate change 8200 years agoDid 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.Image of northern hemisphere 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 d 18O 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.
1500 year cycles of ice rafting
Image from Ruddiman, 2001 used by permission of W. H. Freeman & Co.

 

 

FORCING FACTORS
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Internal influences on climatic variability at millennial scales include Dansgaard-Oeschger cycles which vary in the length of their intervals. Recent ice core studies examining changes in d 18O 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. Image of planktonic formaminifera
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 of climate signal and uncertainty
Image from Ruddiman, 2001 used by permission of W. H. Freeman & Co.



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