382
ability. First, a non-linear system can exhibit self-sustained oscillations in
response to external forcing. Their characteristic time scale is determined
by feedback mechanisms internal to the system. Second, external forcing
can generate abrupt reorganisation of the system, i.e. switches from one
equilibrium state to another. While these switches are caused by 'external'
perturbations (external for the ocean, i.e. for example atmosphere or ice
sheets), the evolution of the system occurs on a time scale determined,
again, by internal mechanisms.
The four features of climate change are shown in Fig. 2:
1. EXTERNALLY FORCED CHANGES,
2. SELF-SUSTAINED OSCILLATIONS,
3. NON-DETERMINISTIC (CHAOTIC) VARIABILITY,
4. ABRUPT REORGANISATIONS.
Various proxy and direct climatic data from the ice, sea and lake sediment
archive illustrate the climatic evolution during the last 40,000 years. The
transition from the last glacial to the Holocene is an externally forced
change due to changes in the distribution of solar radiation and the operation of a number of feedback mechanisms (greenhouse gases, albedo).
This is manifested in the gradual increase of 8 18 0 starting around 20,000
BP and in many proxy records from various archives (Imbrie et al. 1992;
Imbrie et al. 1993) and strong support for the Milankovic theory of climate
change.
Self-sustained oscillations of the ocean-atmosphere-terrestrial ice sheet
system are likely to be responsible for the climatic swings between milder
and colder phases (Dansgaard/Oeschger cycles) during the glacial and it
appears that they involve large changes of ice sheet volume which, when
melting, influence significantly ocean circulation (Bond et al. 1992; Bond
et al. 1993; Paillard and Labeyrie 1994).
Non-deterministic (chaotic) variability is visible indirectly in the 8 18 0
record of the last 10,000 years (Holocene) which fluctuates about a well defined mean value; variations are likely due to changes in the hydrological
cycle. Classically, this dynamical behaviour is thought to be most relevant for mesoscale processes (and hence comparatively short time scales)
in atmosphere and ocean but there are first indications that also on the
decadal-to-century time scale the concept of non-deterministic evolution is
ability. First, a non-linear system can exhibit self-sustained oscillations in
response to external forcing. Their characteristic time scale is determined
by feedback mechanisms internal to the system. Second, external forcing
can generate abrupt reorganisation of the system, i.e. switches from one
equilibrium state to another. While these switches are caused by 'external'
perturbations (external for the ocean, i.e. for example atmosphere or ice
sheets), the evolution of the system occurs on a time scale determined,
again, by internal mechanisms.
The four features of climate change are shown in Fig. 2:
1. EXTERNALLY FORCED CHANGES,
2. SELF-SUSTAINED OSCILLATIONS,
3. NON-DETERMINISTIC (CHAOTIC) VARIABILITY,
4. ABRUPT REORGANISATIONS.
Various proxy and direct climatic data from the ice, sea and lake sediment
archive illustrate the climatic evolution during the last 40,000 years. The
transition from the last glacial to the Holocene is an externally forced
change due to changes in the distribution of solar radiation and the operation of a number of feedback mechanisms (greenhouse gases, albedo).
This is manifested in the gradual increase of 8 18 0 starting around 20,000
BP and in many proxy records from various archives (Imbrie et al. 1992;
Imbrie et al. 1993) and strong support for the Milankovic theory of climate
change.
Self-sustained oscillations of the ocean-atmosphere-terrestrial ice sheet
system are likely to be responsible for the climatic swings between milder
and colder phases (Dansgaard/Oeschger cycles) during the glacial and it
appears that they involve large changes of ice sheet volume which, when
melting, influence significantly ocean circulation (Bond et al. 1992; Bond
et al. 1993; Paillard and Labeyrie 1994).
Non-deterministic (chaotic) variability is visible indirectly in the 8 18 0
record of the last 10,000 years (Holocene) which fluctuates about a well defined mean value; variations are likely due to changes in the hydrological
cycle. Classically, this dynamical behaviour is thought to be most relevant for mesoscale processes (and hence comparatively short time scales)
in atmosphere and ocean but there are first indications that also on the
decadal-to-century time scale the concept of non-deterministic evolution is