High-Resolution Paleoclimatology - Dendroclimatology - page 12

period; compositing climatic conditions following such events increases the signal-
to-noise ratio, giving a clearer view of the climate system response to such events.
Thus Fischer et al. ( 2007 ) were able to show that summer conditions in Europe have
tended to be both cold and dry after major tropical volcanic eruptions; but in winter,
a positive NAO circulation has generally been established, resulting in mild, wet
conditions in northern Europe and well below average precipitation in the Alps and
Mediterranean region.
Dendroclimatic research has been especially important in documenting the fre-
quency, geographical extent, and severity of past drought episodes, as well as
periods of unusually high rainfall amounts; such studies have been especially exten-
sive in the United States (e.g., Stahle and Cleaveland 1992 ; Hughes and Funkhouser
1998 ; Cook et al. 2004 ) . These studies have shown that there has often been a
strong connection between severe droughts in the southwestern United States and
the occurrence of La Niña episodes, although the precise geographical pattern of
each drought has varied over time (Stahle et al. 2000 ; Coleetal. 2002 ) . Tree-
ring research has also been applied to reconstructing modes of circulation in the
past, such as the North Atlantic Oscillation (Cook et al. 1998 ; Cullen et al. 2001 ) ,
Pacific Decadal Oscillation (Gedalof and Smith 2001 ) , and Atlantic Multidecadal
Oscillation (AMO) (Gray et al. 2004 ) . In all of these cases, the paleoclimatic recon-
structions have expanded our understanding of the spectrum of variability of these
modes of circulation and provided insight into how large-scale teleconnections (and
interactions between Atlantic- and Pacific-based circulation regimes) may lead to
persistent, large-amplitude anomalies over North America and other regions.
Great strides have been made in constructing hemispheric- and global-scale
patterns of past climate variability by combining many different types of high-
resolution paleoclimatic records, using a variety of statistical methods (Mann et al.
1998 , 1999 , 2005 ; Moberg et al. 2005 ; Rutherford et al. 2005 ) . These studies have
demonstrated the importance of volcanic and solar forcing, and of the increasingly
dominant effects of anthropogenic forcing over the last 150 years. Nevertheless,
such studies rely largely on the most extensive database of paleoclimatic recon-
structions that is currently available—that provided by dendroclimatology. On the
one hand, this is good because the physiological basis for how trees respond to cli-
mate is well understood, thanks to decades of careful studies, and tree rings provide
the most accurate chronologies available. However, the use of tree rings in long-
term paleoclimate reconstructions is dogged by questions of uniformitarianism (a
question not unique to dendroclimatology, of course), but more significantly by the
difficulty of resolving the full spectrum of climate variability from overlapping, rela-
tively short, tree-ring series. This matter can be resolved by obtaining longer records
where possible, expanding the tree-ring database to improve data density back in
time, and developing new statistical approaches; all these methods are necessary
to ensure that long-term paleoclimatic reconstructions are as reliable as possible.
New isotopic and image analysis techniques applied to tree growth may add further
information about past climate variations in regions that were formerly off-limits to
dendroclimatologists, thereby extending the geographical domain for large-scale cli-
mate reconstruction. New proxies, especially from lake sediments and speleothems,