High-Resolution Paleoclimatology - Dendroclimatology - page 10

On a related point, it is clear that many natural archives are being detrimentally
affected by recent changes in climate. Thus, many high-elevation ice caps in the
tropics have been affected by surface melting and strong sublimation, so that the
recent isotopic record has been degraded or even lost entirely (Thompson et al.
2000 b ). Similarly, corals in many areas were greatly affected by exceptionally high
sea surface temperatures associated with the 1997-1998 El Niño (Wilkinson et al.
1999 ) . Many century-old Porites colonies in the Great Barrier Reef were killed at
this time.
1.7 Frequency Response
High-resolution records may have certain low-frequency characteristics that differ
from the spectrum of the climatic environment in which they are situated. Such
effects may be due to long-term biological growth (in the case of trees, and per-
haps corals), compaction (ice, sediments), non-climatic changes in depositional
environments (lake sediments, speleothems), and other proxy records. This issue
is especially important as efforts are made to extend paleoclimatic reconstructions
further back in time, to reveal changes in climate over thousands of years. Sediments
are certainly affected by compaction, but this effect can be relatively easily corrected
for by examining changes in density. This is also true in ice cores. Diffusion of iso-
topes within firn leads to a reduction in the amplitude of isotopic values that must
also be considered. Deposition rates in speleothems are determined by radiocar-
bon or uranium series dates, and such analysis is generally sufficient to determine
if deposition has been continuous over time. Certainly, there are no compression
issues to be concerned with here, so in that sense speleothems do offer a very good
option for identifying low-frequency changes in climate. This is illustrated well in
the Dongge Cave record of Wang et al. ( 2005 ) (Fig. 1.3 ) . The record shows an under-
lying low-frequency decline in monsoon precipitation, related to orbital forcing, on
which decadal- to centennial-scale variations are superimposed, which appear to be
(at least in part) related to variations in solar irradiance.
The issue of determining low-frequency changes in climate has been most prob-
lematical in dendroclimatology. The biological growth function of trees must first
be removed before climatic information can be extracted. When this procedure is
done, some low-frequency information may be lost. Furthermore, since most tree-
ring series are short, assembling a composite long time series from many short
records makes it even more problematical to obtain low-frequency information
over timescales longer than the typical segment length (Cook et al. 1995 ) . New
approaches to standardization of tree-ring series have been developed, and these
help to preserve more low-frequency information than do more traditional methods.
However, such approaches require very large datasets and so cannot be applied in all
cases. Another approach involves combining different proxies, some that may con-
tain more low-frequency information with others that capture well higher-frequency
information, so that together they cover the full spectrum of climate variability