Summary of climate variations - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 207

values. The sea level stood 110m lower than today. The CO 2 and CH 4 content
of the air remained at the low values characterizing full glacial conditions.

. 14,500 years ago, the termination was in progress—glaciers throughout the

world began to retreat; polar temperatures began to rise; atmospheric dustiness
began to diminish; CO 2 and CH 4 began to rise. In the northern Atlantic basin,
this first phase of the termination manifested itself in the catastrophic breakup of
the large valley glaciers that extended northward from the Alps. Within a period
of a few hundred years, the valleys became ice free. However, climatic conditions
remained too harsh to permit the valleys to be reforested.

. 12,800 years ago, there occurred a sudden warming event—a major warming

event took place in the northern Atlantic region that created climatic conditions
similar to those of the present. Trees replaced shrubs. The interglacial had
seemingly arrived. The warm period lasted about 1,800 years but was interrupted
by a short sudden cooling event around 12,000 years ago, known as the
Allerød-Bolling event.

. 11,000 years ago, glacial conditions abruptly returned—cold prevailed for 1,200

years (this was the Younger Dryas).

. 10,000 years ago, the cold snap abruptly ended—since then interglacial

conditions have remained uninterrupted.

According to Broecker (2002), ''no equivalent of the Younger Dryas cold snap
appears to punctuate earlier terminations . It appears to be a one-time event

triggered by a sudden and very large release of meltwater stored in proglacial
Lake Agassiz.''
Wright (1896) studied the rates of ablation of Greenland and Swiss glaciers
during local summers and concluded: ''During the closing stages of the glacial
period, the ice sheets, both in America and Europe, may have melted away very
fast. If such ablation prevailed every summer for one or two centuries, it must
melt 600 to 1,200 meters of ice

...

...

.''
Most marine sediments are subject to mixing as they accumulate, so the time
scale is blurred by several thousand years. Hence, unlike ice cores, marine
sediments are unable to detect sudden climate changes, but they can provide a
smoothed record of past climate changes. Thus, in describing the last termination
and recent climate history of the past 20,000 years, we must rely mainly on
Greenland ice core data (see, e.g., Figures 4.2 , 4.3 , and 4.4 ). Broecker (2002) also
cited corroborating data from the sediments of Gerzensee (a small Swiss lake),
the distributions of various beetle species, and the high-resolution planktic
foraminifera speciation record in deep-sea sediments off the British Isles.
The sudden sharp changes in climate that occurred during the past ice age
( Figure 4.5 ), and especially in its aftermath ( Figures 4.3 and 4.4 ), have been the
subject of many investigations and discussions. Broecker (2002) devoted 26 pages
to this topic. It is theorized that the sudden cooling of the Younger Dryas was
produced by a large release of stored meltwater from proglacial Lake Agassiz that
gushed through the St. Lawrence lowlands into the northern Atlantic. The sudden
decrease in salinity triggered a shutdown of the ocean's deep-circulation system