384
surprising systematic changes during the Holocene (Blunier et al. 1995)
which would rather hint at an externally forced cause (receding continental ice sheet give way to changing and evolving biosphere which influences
methane production, see Blunier (1995)). This clearly indicates the need
for a variety of climate proxies in order to characterize the dynamics and
variability during a given period.
Three events of abrupt reorganisations recorded in sea sediments and
ice cores, are superimposed on the longer term glacial-interglacial transition. The abrupt warming into the BlZlllingj AllerlZld, the cooling initiating the Younger Dryas and its termination all occur on time scales of a
few decades to a few years (Dansgaard et al. 1989; Taylor et al. 1993).
Model simulations have shown that such changes can be understood in
terms of abrupt reorganisations, i.e. switches from one mode of operation
to another, initiated by perturbations such as melting terrestrial ice sheets
(Wright and Stocker 1993). It is important to distinguish between abrupt
reorganisations and chaotic variability where the system resides for some
time in one dynamical regime and then switches to another (Lorenz 1963;
Lorenz 1990). Abrupt reorganisations are rapid changes between different
equilibrium states when system parameters are slowly changing (Stocker
and Wright 1991; Mikolajewicz and Maier-Reimer 1994; Rahmstorf 1995).
In the search for mechanisms of periodic climatic variability it is often
tempting to look for periodic processes in some forcing variables and then
postulate enhancing feedback mechanisms which cause a response of sufficient amplitude in the climate variable under consideration. Examples are
the various solar cycles such as the sunspot (10-11 yr), Hale (22 yr), and
Gleissberg (84yr) cycles. Although these time scales do occur in spectra
of various climate proxies abundantly (see below), it seems unlikely that
such cycles are due to direct solar forcing. The global sensitivity of the climate system to changes in the shortwave irradiation is estimated at about
0.14 KjWm- 2 (based on an AGCM simulation by Lean and Rind (1994))
with a spatially rather uniform response. The only effect on solar irradiation that has been directly measured is that ofthe 11-year solar cycle whose
peak-to-peak amplitude has been determined at about 2.5 Wm- 2 (ERBE
1990). This would result in a temperature variation of about 0.35 K which
would have to be detected as a globally uniform signal. Significant positive feedback mechanisms would have to operate in the climate system to
amplify such a signal to the amplitude seen in many climate records (order
of 1 K) and to impose regional patterns such as observed in the climatic
reconstruction (e.g. Briffa et al. (1992)).
surprising systematic changes during the Holocene (Blunier et al. 1995)
which would rather hint at an externally forced cause (receding continental ice sheet give way to changing and evolving biosphere which influences
methane production, see Blunier (1995)). This clearly indicates the need
for a variety of climate proxies in order to characterize the dynamics and
variability during a given period.
Three events of abrupt reorganisations recorded in sea sediments and
ice cores, are superimposed on the longer term glacial-interglacial transition. The abrupt warming into the BlZlllingj AllerlZld, the cooling initiating the Younger Dryas and its termination all occur on time scales of a
few decades to a few years (Dansgaard et al. 1989; Taylor et al. 1993).
Model simulations have shown that such changes can be understood in
terms of abrupt reorganisations, i.e. switches from one mode of operation
to another, initiated by perturbations such as melting terrestrial ice sheets
(Wright and Stocker 1993). It is important to distinguish between abrupt
reorganisations and chaotic variability where the system resides for some
time in one dynamical regime and then switches to another (Lorenz 1963;
Lorenz 1990). Abrupt reorganisations are rapid changes between different
equilibrium states when system parameters are slowly changing (Stocker
and Wright 1991; Mikolajewicz and Maier-Reimer 1994; Rahmstorf 1995).
In the search for mechanisms of periodic climatic variability it is often
tempting to look for periodic processes in some forcing variables and then
postulate enhancing feedback mechanisms which cause a response of sufficient amplitude in the climate variable under consideration. Examples are
the various solar cycles such as the sunspot (10-11 yr), Hale (22 yr), and
Gleissberg (84yr) cycles. Although these time scales do occur in spectra
of various climate proxies abundantly (see below), it seems unlikely that
such cycles are due to direct solar forcing. The global sensitivity of the climate system to changes in the shortwave irradiation is estimated at about
0.14 KjWm- 2 (based on an AGCM simulation by Lean and Rind (1994))
with a spatially rather uniform response. The only effect on solar irradiation that has been directly measured is that ofthe 11-year solar cycle whose
peak-to-peak amplitude has been determined at about 2.5 Wm- 2 (ERBE
1990). This would result in a temperature variation of about 0.35 K which
would have to be detected as a globally uniform signal. Significant positive feedback mechanisms would have to operate in the climate system to
amplify such a signal to the amplitude seen in many climate records (order
of 1 K) and to impose regional patterns such as observed in the climatic
reconstruction (e.g. Briffa et al. (1992)).