When I first worked through the implications of a crustal shift
around 13,000 years ago, I was able to acquire the ice core data and
it became clear that the event itself was real and that it was
outright climate change in particular but arguably not climate
caused.
What we have here clearly spelled out is the evidence of a switch
been hit twice. The first time, 18000 years ago, the crust
rebalanced naturally causing a regime change in the climate that was
real but hardly ended the Ice Age itself. Of course precipitation
flows changed and the pole itself was shifted. This happened
naturally because of the ice build up on two continental masses
asymmetrically positioned on the poles.
This provided the knowledge needed to trigger the Pleistocene
Nonconformity in which a comet was directed to impact the Northern
Ice Cap and shift the crust into its present position and end the
Northern Ice Age. This occurred about 12900 years ago. For the next
thousand years the Greenland cores observed extreme cold which ended
abruptly with the likely draining of Lake Agassiz or some similar
mass melt event such as the mass release of ice out of the Arctic or
Hudson Bay. Any of these would plausibly allow a rapid increase in
Greenland temperatures.
I am now more comfortable with the earlier event as the natural
demonstration made possible the deliberate movement of the later
event and we have a time frame for it all that is congruent.
The magnitude of the freeze is not so important as the sharpness of
the recovery that ultimately resolved into the Holocene. After that
it likely took a couple of thousand years for the bulk of the
remaining ice on land to disappear and set the stage for continental
agriculture around 10,000 years ago.
ScienceDaily
(June 25, 2012) — Ice samples pulled from nearly a mile below
the surface of Greenland glaciers have long served as a historical
thermometer, adding temperature data to studies of the local
conditions up to the Northern Hemisphere's climate.
But the method --
comparing the ratio of oxygen isotopes buried as snow fell over
millennia -- may not be such a straightforward indicator of air
temperature.
"We don't believe
the ice cores can be interpreted purely as a signal of temperature,"
says Anders Carlson, a University of Wisconsin-Madison geosciences
professor. "You have to consider where the precipitation that
formed the ice came from."
According to a study
published June 25 by the Proceedings of the National Academy of
Sciences, the Greenland ice core drifts notably from other records
of Northern Hemisphere temperatures during the Younger Dryas, a
period beginning nearly 13,000 years ago of cooling so abrupt it's
believed to be unmatched since.
Such periods of speedy
cooling and warming are of special interest to climate scientists,
who are teasing out the mechanisms of high-speed change to better
understand and predict the changes occurring in our own time.
In the case of the
Younger Dryas, average temperatures -- based on the Greenland ice --
plummeted as much as 15 degrees Celsius in a few centuries, and
then shot back up nearly as much (over just decades) about 1,000
years later.
"In terms of
temperature during the Younger Dryas, the only thing that looks like
Greenland ice cores are Greenland ice cores," Carlson says.
"They are supposed to be iconic for the Northern Hemisphere, but
we have four other records that do not agree with the Greenland ice
cores for that time. That abrupt cooling is there, just not to the
same degree."
Working with
UW-Madison climatologist Zhengyu Liu, collaborators at the National
Center for Atmospheric Research and others, Carlson found their
computer climate model breaking down on the Younger Dryas.
While it could
reliably recreate temperatures in the Oldest Dryas -- a similar
cooling period about 18,000 years ago -- they just couldn't find a
lever in the model that would simulate a Younger Dryas that matched
the Greenland ice cores.
"You can totally
turn off ocean circulation, have Arctic sea ice advance all the way
across the North Atlantic, and you still will have a warmer climate
during the Younger Dryas than the Oldest Dryas because of the carbon
dioxide," Carlson says.
By the time the
Younger Dryas rolled around, there was more carbon dioxide in the air
-- about 50 parts per million more. The warming effects of that much
CO2 overwhelmed the rest of the conditions that make the Oldest and
Younger Dryas so alike, and demonstrates a heightened sensitivity for
Arctic temperatures to rising greenhouse gases in the atmosphere.
The researchers zeroed
in on the Northern Hemisphere's temperature outlier, Greenland ice
cores, and found that the conversion of oxygen isotope ratio to
temperature typically used on the ice cores did not account for the
sort of crash climate change occurring during the Younger Dryas. It
assumes prevailing winds and jet streams and storm tracks are
providing the moisture for Greenland precipitation from the Atlantic
Ocean.
"The Laurentide
ice sheet, which covered much of North America down into the northern
United States, is getting smaller as the Younger Dryas approaches,"
Carlson says. "That's like taking out a mountain of ice three
kilometers high. As that melts, it allows more Pacific Ocean moisture
to cross the continent and hit the Greenland ice sheet." The two
oceans have distinctly different ratios of oxygen isotopes, allowing
for a different isotope ratio where the water falls as snow.
"We ran an oxygen
isotope-enabled atmosphere model, so we could simulate what these ice
cores are actually recording, and it can match the actual oxygen
isotopes in the ice core even though the temperature doesn't cool as
much," Carlson says. "That, to us, means the source of
precipitation has changed in Greenland across the last deglatiation.
And therefore that the strict interpretation of this iconic record as
purely temperature of snowfall above this ice sheet is wrong."
By the study's
findings, Greenland temperatures may not have cooled as significantly
as climate headed into the Younger Dryas relative to the Oldest
Dryas, because of the rise in atmospheric carbon dioxide that had
occurred since the Oldest Dryas.
"You can say at
the end of the Younger Dryas it warmed 10, plus or minus five,
degrees Celsius. But what happened on this pathway into the event,
you can't see," Carlson says. It's a fresh reminder from an
ancient ice core that climate science is full of nuance, according to
Carlson.
"Abrupt climate
changes have happened, but they come with complex shifts in the way
climate inputs like moisture moved around," he says. "You
can't take one difference and interpret it solely as changes in
temperature, and that's what we're seeing here in the Greenland ice
cores."
The National Science
Foundation and Department of Energy funded the research.
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