Modeling Abrupt Climate Change as the Interaction Between Sea Ice Extent and Mean Ocean Temperature Under Orbital Insolation Forcing - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 57

Dansgaard et al., 1993; Blunier and Brook, 2001]. These
extraordinary records show abrupt regional temperature
increases of up to 15°K occurring in just a few decades,
both during the last glacial period and in the transition to
the present interglacial [Severinghaus and Brook, 1999;
Clark et al., 2002; Alley et al., 2003]. During the last ice
age, a repeating pattern of abrupt warming, gradual cool-
ing, and abrupt cooling resulted in trains of pulse-like
warming events lasting 1

In this chapter, we use conceptual models based upon
simple stochastic differential equations to show that sea ice
extent and mean oceanic temperature are key variables in the
development of the D-O and that both can be effectively
driven by changes in the orbital insolation to create temper-
ature variations fully consistent with the data of the last ice
age. One of these conceptual models [Saltzman et al., 1981]
consists of a self-sustained relaxation noisy oscillator that
reasonably reproduces the past 100 kyr of D-O climate 10 kyr (1 kyr = 1000 years) with

a distinctive sawtooth appearance, commonly known as the
Dansgaard-Oeschger (D-O) oscillations (Figure 1). Abrupt
climate change is believed to be the result of the inherently
nonlinear nature of the climate system (e.g., instabilities,
feedback, and threshold crossings), but despite decades of
vigorous research, the causes of abrupt climate change
remain elusive, since neither the physical mechanisms in-
volved nor the nature of the nonlinearities are well under-
stood [Clark et al., 2002; Alley et al., 2003; Rial et al.,
2004; Wang and Mysak, 2006; Wunsch, 2006; Ditlevsen
et al., 2007].

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fluctuations. The model, heretofore referred to as sea ice
oscillator (SIO), depends on two internal parameters, the free
frequency of the oscillator, and the intensity of the positive
feedback which controls the abruptness of temperature
change, itself a consequence of the rapid response of sea ice.
SIO best reproduces the D-O if the free oscillation period is
set around ~1.5 ka, a likely free frequency of the climate, as
suggested by the work of Bond et al. [1997]. At least two
types of abrupt events can be identi

ed in the model results:
internal, probably caused by convection/diffusion processes
in the ocean, and external, as insolation forces the climate

Figure 1. The Dansgaard-Oeschger (D-O) temperature fluctuations (also called D-O oscillations) as recorded in the North
Greenland Ice Core Project (NGRIP) ice core [North Greenland Ice Core Project Members, 2004]. The orbital induced
summer insolation at 65°N [Laskar et al., 2004] appears to influence the long period of the record through amplitude
modulation, but it is not clear whether it controls the high frequencies, too. We shall suggest that it does. The ice volume is
from the LR04 stack [Lisiecki and Raymo, 2005].