High-Resolution Paleoclimatology - Dendroclimatology - page 8

such records into 12 monthly increments is not appropriate (cf. Thompson et al.
2000 a ). Similarly, hydrological studies in the Arctic have shown that in some lakes,
much of the runoff and associated sediment may be transferred into the lake over the
course of only a few weeks. For example, measurements at Sophia Lake (Cornwallis
Island, Nunavut, Canada) showed that 80% of the runoff and 88% of the annual
sediment flux occurred in the first 33 days of the 1994 melt season (Braun et al.
2000 ) . This sediment was subsequently distributed across the lake floor, forming an
annual increment (varve), but the climatic conditions that mobilized the sediment
were brief and perhaps unrepresentative of the summer season (and the year as a
whole). Other studies of arctic lakes indicate that watersheds containing glaciers
provide more continuous runoff and associated sediment flux throughout each sum-
mer, and thus provide a better proxy for summer climatic conditions (e.g., Hardy
et al. 1996 ) . Thus, understanding the environment from which the proxy archive is
extracted is critically important for proper interpretation of the paleoclimate record.
Process-based studies (often derided as simply 'monitoring') have also provided
insights into climatic controls on corals, showing strong nonlinearities at high water
temperatures (Lough 2004 ) . In situ measurements within caves, aimed at gain-
ing a better understanding of paleoclimate records, are now also being carried out
(e.g., McDonald et al. 2004 ; Cruzetal. 2005 ) . By comparison, dendroclimatology
is far advanced because ecophysiological studies of tree growth have a long his-
tory. Consequently, factors influencing tree growth increments are well understood
(Fritts 1996 ; Schweingruber 1996 ; Vaganov et al. 2006 ) , providing a very strong
foundation for paleoclimatic studies using tree rings.
1.6 Uniformitarianism
Perhaps because of the rapidity of recent climate change, many archives are no
longer responding to climate in a manner that typifies much of the past. This phe-
nomenon was first noted by Briffa et al. ( 1998 ) , who showed that some trees that
were formerly strongly influenced by temperature were no longer so influenced,
or at least not to the same extent. Figure 1.2 shows the geographical distribution
of this effect. Briffa et al. ( 2004 ) speculated that this response might be related to
recent increases in ultraviolet radiation resulting from the loss of ozone at high ele-
vations. Others have argued it might reflect the fact that trees in some areas have
reached a threshold, perhaps now being affected more by drought stress than was
formerly the case. Whatever the reason, it raises the question of whether such con-
ditions might have occurred in the past, and if so, whether it would be possible
to recognize such a 'decoupling' of the proxy archive from the ('normal') climate
driver. Paleoclimate reconstruction is built on the principle of uniformitarianism, in
which the present is assumed to provide a key to the past. If modern conditions (dur-
ing the calibration period) are not typical of the long term, this assumption will be
invalid. It is thus important to resolve the reasons for such changes and determine if
additional parameters (such as cell growth features) might provide clues about when
such stresses may have overwhelmed the typical climate response.