Ice core data - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 164

4.7 SUDDEN CLIMATE CHANGES
Ice core data provide incontrovertible evidence that there have been very strong
sudden climate changes during the various ice ages (see Figures 4.5 , 4.6 , and 4.12 ) .
According to Figures 4.5 and 4.6 , temperature excursions have occurred at
Greenland amounting to tens of degrees, up and down, within a few decades or
centuries. Such rapid climate changes cannot possibly be due to slowly changing
solar input and, hence, scientists have sought explanations outside the realm of the
astronomical theory, typically based on changes in ocean currents. There is an
extensive literature on these sudden climate changes.
Dansgaard-Oeschger events are rapid climate fluctuations involving a sudden
warming in a very short time (a few decades) followed by a gradual cooling back
to initial conditions over the next 1,500 years. Twenty-three such events have
been identified over the last glacial period (between 110,000 and 23,000 ybp ).
Sudden intense cold dry phases also occasionally affected Europe and the North
Atlantic region, and possibly many other parts of the world. These so-called
''Heinrich events'' were first recognized as the traces of ice surges into the North
Atlantic, based on ice-rafted debris found in high-latitude sediments (Heinrich,
1988). They also show up in the Greenland ice cores and some are also detectable
in the European pollen records and distant Antarctic ice cores. Figure 4.26 shows
numbered Heinrich events that occurred over the past 100 kybp . Wilson et al.
(2000) provide a good description of Heinrich events. They discussed two models
that have been proposed for their origin. One model is based on the accumulation
of geothermal and frictional heat at the base of the ice sheet, leading to large-scale
generation of large icebergs that cool the NH. The other model presupposes some
external cooling mechanism leading to expansion of the ice sheets to the point
where they break off at the edges and generate many large icebergs that cool the
NH.
A number of scientists have investigated these sudden climate changes and
provided rational explanations for them, mostly in terms of changes in ocean
flows. Section 8.6 discusses these models. Most of that work deals specifically with
rapid climate change events that occurred during the most recent ice age and its
aftermath. But if there are forces operating that can change the climate by such a
large amount in so short a time, it would seem likely that these forces may also be
involved in longer term glacial-interglacial cycles. This implication does not seem
to have been emphasized by modelers of short-term fluctuations.
Around 14,000 years ago, a rapid global warming and moistening of climates
began, perhaps occurring within the space of only a few years or decades. Con-
ditions in many mid-latitude areas appear to have been roughly as warm as they
are today, although many other areas—while warmer than during the Late Glacial
Cold Stage—seem to have remained slightly cooler than at present. Forests began
to spread back and the ice sheets began to retreat. However, after a few thousand
years of recovery, the Earth was suddenly plunged back into a new and very
short-lived Ice Age known as the ''Younger Dryas'', which led to a brief
resurgence of ice sheets (see Figure 4.4 ) . The main cooling event that marks the