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192 CHAPTER 6. FORSCHUNGSSTELLE “RADIOMETRIE”
6.1.2 Possible solar origin of the glacial 1,470-year climate cycle demonstrated
in a coupled climate model
Participating scientists H.Braun, M.Christl, A.Mangini, B.Kromer (AdW Heidelberg), K.Roth
(IUP Heidelberg), S.Rahmstorf, A.Ganopolski (PIK Potsdam), C.Kubatzki (AWI Bremerhaven)
Abstract It is shown that an intermediate-complexity climate model reproduces abrupt glacial
warmings (Dansgaard-Oeschger events) with a spacing of 1470 years when forced by freshwater cycles
of about 87 and 210 years, i.e. with periods close to known solar cycles. Thus, the glacial 1470-year
climate cycle could be caused by the Sun despite the lack of a 1470-year solar period.
Figure 6.1: Forcing and model response. The applied freshwater forcing (left panel, in mSv [1 Sv =
106 m 3 /s]) is the sum of two sinusoidal cycles with periods of 210 years and 86.5 years, respectively.
These periods are close to well-known solar cycles. In response to this forcing, the applied climate
model can show abrupt warm events in Greenland (Dansgaard-Oeschger events) with a period of 1470
years (right panel, in ◦ C). The dashed lines in both panels are spaced by 1470 years.
Background Many paleoclimatic archives show
a quasi-cycle of about 1470 years in the Last
Glacial. This pattern is manifested in rapid
climate shifts, the so-called Dansgaard-Oeschger
(DO) events. To explain these, various concepts
have been proposed, including internal oscillations
in the climate system and external (e.g. solar)
forcing. However, while pronounced solar cycles
of about 87 and 210 years are well-known [Peristykh
and Damon 2003, Wagner et al. 2001], a
1470-year cycle has not been found [Stuiver and
Braziunas 1993]. This has been considered as a
main argument against solar origin of the glacial
1470-year climate cycle.
Funding This work was funded by the Heidelberg
Academy of Sciences.
Methods and results To test if the lack of
a 1470-year solar cycle indeed argues against solar
origin of the DO events, we used the climate
system model CLIMBER-2 of the Potsdam Institute
for Climate Impact Research (PIK). In earlier
simulations with the model, abrupt glacial warming
events were already simulated which reproduce
many features of the observed DO events
[Ganopolski and Rahmstorf 2001, Ganopolski and
Rahmstorf 2002]. In the model, DO events represent
rapid transitions between a stadial (cold)
and an interstadial (warm) mode of the North Atlantic
thermohaline circulation (THC), triggered
by a threshold process.
In our model study, a freshwater forcing was applied
which is the sum of two sinusoidal components
with periods 1470/7 (=210) and 1470/17
(about 86.5) years (left panel in figure 1). The amplitudes
of both forcing components are small (10
mSv, i.e 10.000 m 3 /s), that is, about 5 cm/year
in the surface freshwater flux. Although the total
forcing does not explicitly have a spectral component
of 1470 years, it repeats with this period due
to the combined effect of the applied cycles.
The forcing can excite DO-like events in the North
Atlantic region (right panel in 6.1). Within a large
forcing-parameter range, these events are spaced
by 1470 years. This timescale is robust when
the phases, amplitudes, and periods of the two
forcing cycles are changed over some range. If
instead of two sinusoidal cycles a more realistic
forcing is applied with spectral properties close
to that observed in solar proxies, a regular 1470year
model response is still found. The stability
of the simulated 1470-year cycle results from two
well-known properties of the THC: its long characteristic
timescale, and the non-linearity (i.e. the
threshold character) inherent in the transitions
between the two modes of the THC. For Holocene
conditions, DO events do not occur in the model.
To conclude, our results show that the lack of
a 1470-year solar cycle does not by itself argue
against a solar origin of the glacial 1470-year climate
cycle.
Main publication Braun et al. [in press]