Volcanic markers and solar trends over the Last Millennium: Can we
compare Tree Ring Chronologies with Global Climate System Models? A
critical evaluation.

Caspar M Ammann, Robert Tomas, Eugene Wahl, (1) 
Philippe Naveau(2) and Hee-Seok Oh(3)

(1) National Center for Atmospheric Research
Climate and Global Dynamics Division
Paleoclimatology
Boulder, CO 80307-3000   USA

(2) Department of Applied Mathematics
University of Colorado at Boulder
Boulder, CO  80309-0526   USA

(3) Department of Mathematical and Statistical Sciences
University of Alberta
Edmonton, Alberta, T6G 2G1   CANADA

Abstract 

The study of tree ring chronologies can offer one of the most
continuous perspectives on climate over the past centuries and
millennia. Since a great number of locations aroun d the globe
occupied by humans are associated with forests, the spatial coverage
of this climate proxy is both exceptional and highly relevant. Thus,
tree ring records provide a unique archive to study climate with its
variability over the recent past as well as for investigating impacts
of the climatic variations on human societies. Future climates are
currently being assessed for a range of scenarios around
anthropogenically-altered concentrations of atmospheric constituents
(i.e., greenhouse gases, aerosol). Since the resulting climate change
occurs superposed on natural climate variations, it is important to
understand the 
Previously, the sensitivity of the climate system to particular
radiative forcings has received most attention. Solar irradiance
changes (including the distribution over the solar spectrum) and
radiative perturbation from volcanic haze after large explosive
eruptions are seen as dominant natural forcings (Lean et al. 1995,
Crowley 2000, Ammann et al. 2003). A significant portion of past
climate variations can be explained by these forcing components when
focusing on the hemispheric or global scales. Recent research,
however, also suggests that at least some regional aspects of climate
response could be associated with forcings as well. Regional
expressions of climate variability are mostly dominated by internal
modes of climate variability. If climatic response to external forcing
could project onto and thus modify the temporal behavior of specific
internal modes in a predictive fashion, one might be able to study the
forcing and impact link in the past and evaluate if climatic trends
indeed can be associated with particular forcings. In order to study
this in the past, we are on one side highly dependent on continuous
and well-dated climate proxy series. Tree ring chronologies around the
world provide a tremendous resource for studying such links. The
discipline has and is continuously providing further insight into the
climate response to external forcings (e.g., Douglas 1928, Fritts
1972, LaMarche 1978, Briffa et al. 1998), despite some limitations
(Briffa et al. 1996, Vaganov et al. 1999). On the other hand, we also
require a better physical understanding of mechanisms that operate in
the climate system that can translate the forcing into a spatial
structure in terms of response. Such insight can be gained from
climate models, in particular coupled Ocean-Atmosphere General
Circulation Models, or Climate System Models that include as complete
of a representation of the climate system as possible.

Here we present transient simulations of the past Millennium with the
fully coupled National Center for Atmospheric Research Climate System
Model (CSM) version 1.4 that was forced with explosive volcanism and
solar irradiance changes as well as the ice core-derived history of
atmospheric composition. The goal is to compare the model climate with
tree ring based reconstructions for different places around the
globe. More specifically, we search for common imprints of external
methods for the signal extraction. A statistical multi-state space
model (Naveau et al. 2003) is used to capture climate responses to
explosive volcanism that occur on short, interannual, time
scales. This approach provides an accurate estimator of the timing and
duration of the climate perturbation by an eruption. This will not
only allow for a more objective estimation of the associated peak
amplitude (cooling) and the subsequent time evolution of the signal,
but at the same time it will provide a measure of confidence through
the posterior probability for each cooling event. Tree ring series
simulation of such events has made much progress in recent years
(Stenchikov et al. 1998, Ammann et al. 2003). Next to the comparison
of individual events, we also evaluate to what degree a Generalized
Extreme Value distribution that well describes the volcanic forcing
magnitudes can also be found in the climatic signals of both model and
tree ring records. Short of applying a physiological tree growth
model, this test provides insight into linearity issues of cooling
estimates in years with strong volcanic perturbations in the
atmosphere.  We use non- decimated discrete wavelet transform analyses
(Oh et al. 2003) to study solar modulated climate variations in proxy
records as well as the experiments with the coupled model. We
highlight some important aspects of coherency and stationarity for the
solar variations at various decadal to centennial time
scales. Finally, we conclude by comparing the success of signal
detection between the model and tree ring chronologies and discuss
direct radiative effects as well as signals arising indirectly from
internal mode responses (e.g., Stenchikov 2004, Adams et al. 2003).

 
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have proven very sensitive to this forcing (Hirschboeck 1980, Loughrrigo and Jacoby 1999) and
and Fritts 1987, Briffa et al. 1998, D
rlds¢ using well adapted statistical
forcing in proxy and model