SOLAR FORCING OF THE HOLOCENE CLIMATE: A MODEL STUDY
S.L. Weber
Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands.
E-mail weber@knmi.nl
Estimated solar irradiance changes throughout the Holocene have been used to force an
atmosphere-ocean model. The solar forcing is based on C14 values (Stuiver et al., 1998) over
the last 10,000 year, scaled to the Lean et al. (1995) solar record (Crowley, pers.comm.). It
has decadal resolution. The aim is to assess the role of solar forcing in generating Holocene
climate variability. This extends earlier work (Cubasch et al., 1997; Drijfhout et al., 1999;
Rind et al., 1999) to longer timescales. Other short-timescale forcings (like volcanic dust,
GHG concentrations, aerosols) are neglected. We do include orbital forcing and compare the
present simulation to a 10,000 year 'control' experiment with orbital forcing alone (Weber,
2001). Some first results of the analysis were presented at the Holivar workshop on
'Holocene climate variability: comparisons of climate model simulations with paleoclimatic
reconstructions'. A more extensive analysis is in preparation.
Consideration of longer timescales (50 yr and longer) is especially of interest as many proxy
data have low resolution, thus only registrating the longer timescales (e.g. Bond et al., 2001).
In addition, the solar forcing spectrum is red and the climatic response is thought to be
stronger for increasing timescales.
The intermediate-complexity climate model ECBilt (Opsteegh et al., 1998) was used in the
present experiment. It is a coupled atmosphere/ocean/sea-ice model containing simplified
parameterisations of the subgrid-scale physical processes. ECBilt is computationally more
efficient than comprehensive GCMs. Therefore, it is possible to perform very long
simulations. ECBilt was applied earlier to the study of climate and sea-level variability in the
late Holocene (Schrier et al., 2002) and rapid transitions in the early Holocene (Renssen et al.,
2001).
The ECBilt simulation exhibits the main feature of climatic evolution during the Holocene,
that is Northern Hemisphere (NH) summers are warmer and there is an intensified
hydrological cycle in the early Holocene compared to present (Joussaume et al., 1999). The
driving force is the higher insolation in NH summer which leads to stronger heating over the
continents. Two research questions which can be addressed by the present simulation are (i)
whether the climatic response to varying solar intensity depends on the background climatic
state and (ii) whether it depends on the timescale.
Insolation and a number of characteristic climatic parameters are shown in Figure 1 for the
time period 9-1 kyr BP (Before Present). All graphs are detrended to remove the orbitalforced signal. To facilitate a visual comparison between the forcing and the climatic
parameters the records have also been low-pass filtered for timescales longer than 100 years.
The centennial-timescale variability that is evident in all climatic parameters is due to the
imposed external forcing as well as internally generated. Temperature (second graph) and, to
a lesser extent, precipitation (third graph) seem to correlate well with the insolation. The
lower two graphs give an index of the strength of the N. Atlantic overturning circulation and
the length of Nigardsbreen, a glacier located in southern Norway. Their relation to the
forcing is less clear.
Correlations for different timescales are now computed by applying a range of band-pass
filters to the records depicted in Figure 1. It is found that the correlation between
temperature/precipitation and insolation generally increases for increasing timescale
(correlations are low for short timescales because of the oceans damping any signal). The
maximum reached for precipitation is lower than that for temperature and it is reached at
longer timescales. The lag at which optimum correlations occur is a few years. We find that
the response is independent of the background climatic state.
Figure 1
The length of Nigardsbreen and two other glaciers has been computed with a glacier model
coupled off-line to ECBilt. The procedure is described by Weber and Oerlemans (2002).
Glacier length responds to insolation mainly through the summer temperature signal. For a
long response-time glacier (lags of about 70 yr) correlation increases for long timescales.
However, for a short response-time glacier (lags of 30-40 yr) correlations decrease for
timescales longer than about 300 yr because of negative feedbacks related to the dynamic
response of the glacier. Nigardsbreen expands dramatially, thus changing its behavior over
time from a short (thus fast) glacier in the early Holocene to a long (thus slow) one in the late
Holocene.
The present results indicate that in the case of a direct response correlations increase for
increasing timescale. In the case of feedbacks strongly modifying the response correlations
are maximum for a preferred range of timescales. The shape of the response function does
not depend on the time period, unless system characteristics change considerably over time
e.g. through strong glacier expansion. This implies that proxy data will only reflect solar
forcing at 100+ timescales, but possibly not at 500+. As correlations are typically nonstationary any link may be absent in shorter records.
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