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A bolide did it! No... not really. (3)
Iâm sure those of you who are actually interested and have lasted this far
are wondering where Iâm going with all of this. Of course, keep in mind that
opinions differ on the finer points, and the picture of the variability in
salinity, air temperature, and deep water formation is not yet well understood
(and of course, climate science is ultimately the battle between computer
models), but without further delay, hereâs the pointâ
Ice cores through the Greenland and Antarctic ice sheets provide a continuous
record of atmospheric conditions over their respective areas extending back
over 400,000 years. Cores through deep-sea sediments in the North Atlantic give
information about sea-surface temperatures and salinity, the production of
North Atlantic deep water, ice volume in glaciers, and the production of
icebergs. Abrupt temperature variations are recorded over the past 100,000
years. Many times during the last ice age, temperatures near Greenland warmed
rapidly over periods of 1 to 100 years, followed by cooling over long periods
of time. Example: 11,500 years ago, temperatures over Greenland warmed by 8
degrees Celsius in only 40 years in three steps, each spanning 5 years. Much of
the northern hemisphere warmed and cooled in phase. This is called a
Dansgaard/Oeschger event. Icebergs carry glacial debris out to sea, depositing
it on the ocean floor as they melt. Via ocean sediment cores, one can se!
times when large numbers of icebergs were released into the North Atlantic.
These are called Heinrich events.
The correlation of Greenland temperatures with iceberg production is directly
related to the Meridional Overturning Circulation. When the icebergs melted,
fresh water surged into the ocean, increasing the stability of the water
column, and thus shutting down the production of North Atlantic Deep Water.
This in turn greatly reduced the transport of warm water to the North Atlantic,
effectively producing a very cold northern hemisphere climate. The melting ice
pushed the polar front, the boundary between cold and warm water in the North
Atlantic, further south than its current position. The location of the front
can be determined from ocean sediment cores. When the Meridional Overturning
Circulation shuts down, heat that is normally carried to the North Atlantic
from the tropics, is available to warm the southern hemisphere, thus explaining
the periods of Antarctic warming seen in ice cores.
It works like thisâ The switching on and off of the Meridional Overturning
Circulation has a large hysteresis. There are two stable states. State one is
the current circulation. State two is when deep water is produced mostly near
Antarctica, and upwelling occurs in the far North Pacific (as it does today)
and in the far North Atlantic. Once the circulation is shut off, the system
switches to the second stable state. The trouble is that the return to normal
salinity does NOT cause the circulation to turn back on. Surface waters must
first become saltier than average for the first state to return.
Heinrich events precede the largest Dansgaard/Oeschger events that have been
document. Here is what seems to happen: The Heinrich event shuts off the
Atlantic Thermohaline Circulation, which leads to a very cold North Atlantic.
This is then followed about 1,000 years later by a Dansgaard/Oeschger event
caused by rapid warming. The Dansgaard/Oeschger â Heinrich events have global
influence, and they seem to be related to warming events seen in Antarctic ice
cores. This means that temperature changes are out of phase in the two
hemispheres. When Greenland warms, Antarctica cools.
All of that to get to thisâ A weakened version of this process with a period
of roughly 1,000 years may be modulating present-day climate in the North
Atlantic, and it looks as if it was responsible for the Roman Warm Period, the
Dark Ages Cold Period, the Medieval Warm Period, and finally the Little Ice
Age. There is also the Younger Dryas Event that approximately marks the
boundary between the Holocene and Pleistocene Epochs. Although deglaciation had
been taking place for at least 4,000 years, a rapid cooling in climate took
place at about 10,000 to 11,000 years ago. This cooling event may well have
been caused by a surge of glacial meltwater. During the early stages of
deglaciation, much of the meltwater from the Laurentide Ice Sheet emptied
primarily into the Gulf of Mexico. But by about 11,000 years ago, the ice
margin had retreated enough to allow the opening up of drainage into the St.
Lawrence Seaway near Newfoundland. The subsequent outflow of meltwater would
created an immense blob of low-salinity water in the North Atlantic, shutting
down the Thermohaline Circulation and causing some serious cooling; enough
cooling in fact to send the North Atlantic polar front as far south as to
approximately 45 degrees N, only 5 or 10 degrees north of the glacial maximum
But oh, thereâs moreâ Weâve directly witnessed the effects of the
Thermohaline Circulation shutting down. In the late 1960s, a huge blob of
near-surface fresher water appeared off the east coast of Greenland, probably
as a result of a large discharge of ice into the Atlantic that took place in
1967. Called the Great Saline Anomaly, it drifted southward, settling into the
North Atlantic in the early 1970s. There, it interfered with the Thermohaline
Circulation, arresting deep water formation in the Labrador Sea. It continued
to drift in a counterclockwise fashion around the North Atlantic, re-entering
the Norwegian Sea in the late 1970s and finally vanished. The effect of all of
this was very cold winters, particularly in Europe, but only for a few years
due to the blobâs relatively small size. The best part? Something just like
this, though on a much more massive scale, is occurring right now in the North
Atlantic. Over the past 30 years, the equivalent of a 10 foot!
hick layer of fresh water has been building up in the North Atlantic,
presumably due to the melting of Arctic ice.
Looks like we could be in for some cold weather. When? No one knows at which
point the Thermohaline shutdown could actually start. I just hope the bolide
thatâs on the way has really good timing.