161
1.2 The Paleo-Record
Over the past twenty five years, remarkable records of the climate changes
that occur over the glacial/interglacial cycles have been amassed (e.g. Fig.
3 from the Greenland ice cores). The Greenland ice sheet is ideally situated to record variations in the formation of North Atlantic Deep Water (NADW) which occurs in the adjacent Greenland, Norwegian, and
Labrador seas. Climate changes on time scales of decades to hundreds of
thousands of years are present in this record.
The Greenland ice cores show that the cold climate of the last ice age
was punctuated with occasional warm ("interstadial") periods lasting from
several hundred to several thousand years (Dansgaard et al., 1993). The
transitions between the warm and cold periods were abrupt - the most
recent warmings following the last glacial and Younger Dryas cold periods
took place in less than fifty years (Alleyet al., 1993). These DansgaardOeschger (D-O) events have been well correlated with foraminiferal indicators of sea surface temperatures buried in the North Atlantic ocean
sediments (Bond et al., 1993) implying that these oscillations occurred
over the entire North Atlantic and were not simply confined to the region
of Greenland. The global nature of the events is revealed by their methane
signal (Chappelez et al., 1993), also present in the Greenland ice cores,
and the presence of muted features corresponding to many of the events
in the Vostok Antarctic ice core (Dansgaard et al., 1993). Recently, the
foram record has been extended back to the Eemian period (the interglacial previous to our own; McManus et aI, 1993; Keigwin et al., 1994).
While confirming the D-O events during glacial times, it appears that the
last warm period (the Eemian), which perhaps most closely corresponds
to our own interglacial period, exhibited stable North Atlantic sea surface
temperatures.
1.3 Mechanisms
It will be the basic purpose of this set of lectures to evaluate the current
theories for the mechanisms for decadal-to-centennial variability. Any hope
of eventually predicting aspects of the climate years in advance will depend
on understanding the precise mechanism the real climate system uses to
generate this variability.
We may categorize these mechanisms as follows:
1. Decadal-to-centennial variability of the forcing functions external to
1.2 The Paleo-Record
Over the past twenty five years, remarkable records of the climate changes
that occur over the glacial/interglacial cycles have been amassed (e.g. Fig.
3 from the Greenland ice cores). The Greenland ice sheet is ideally situated to record variations in the formation of North Atlantic Deep Water (NADW) which occurs in the adjacent Greenland, Norwegian, and
Labrador seas. Climate changes on time scales of decades to hundreds of
thousands of years are present in this record.
The Greenland ice cores show that the cold climate of the last ice age
was punctuated with occasional warm ("interstadial") periods lasting from
several hundred to several thousand years (Dansgaard et al., 1993). The
transitions between the warm and cold periods were abrupt - the most
recent warmings following the last glacial and Younger Dryas cold periods
took place in less than fifty years (Alleyet al., 1993). These DansgaardOeschger (D-O) events have been well correlated with foraminiferal indicators of sea surface temperatures buried in the North Atlantic ocean
sediments (Bond et al., 1993) implying that these oscillations occurred
over the entire North Atlantic and were not simply confined to the region
of Greenland. The global nature of the events is revealed by their methane
signal (Chappelez et al., 1993), also present in the Greenland ice cores,
and the presence of muted features corresponding to many of the events
in the Vostok Antarctic ice core (Dansgaard et al., 1993). Recently, the
foram record has been extended back to the Eemian period (the interglacial previous to our own; McManus et aI, 1993; Keigwin et al., 1994).
While confirming the D-O events during glacial times, it appears that the
last warm period (the Eemian), which perhaps most closely corresponds
to our own interglacial period, exhibited stable North Atlantic sea surface
temperatures.
1.3 Mechanisms
It will be the basic purpose of this set of lectures to evaluate the current
theories for the mechanisms for decadal-to-centennial variability. Any hope
of eventually predicting aspects of the climate years in advance will depend
on understanding the precise mechanism the real climate system uses to
generate this variability.
We may categorize these mechanisms as follows:
1. Decadal-to-centennial variability of the forcing functions external to