Tree Rings and Climate: Sharpening the Focus - Dendroclimatology - page 337

Anomaly' (MCA), and he proposed a mechanism of changed atmospheric circula-
tion. These lake low stands corresponded with sustained periods of dryness recorded
in rings of lower forest border bristlecone pine in the nearby Great Basin (LaMarche
1974 ; Hughes and Graumlich 1996 ; Hughes and Funkhouser 1998 , 2003 ) . Graham
and Hughes ( 2007 ) used tree rings to achieve a more direct confirmation of Stine's
Mono Lake low stands by driving a hydrographic model of Mono Lake with recon-
structed inflow rates derived from tree-ring chronologies. Cook et al. ( 2004b ) , also
using tree rings and comparisons with other proxy records (e.g., lake level and wild-
fire reconstructions from fire scars in tree rings and charcoal in sediments), showed
that this period was indeed anomalous across much of western North America,
such that a markedly greater portion of the area tended to be experiencing drought
conditions in the centuries before the mid-second millennium AD than since then.
Recent Colorado River flow tree-ring-based reconstructions with good sample depth
through the past millennium identify extraordinary, multidecadal drought during the
mid-twelfth century (Meko et al. 2007 ) .
Taken together, these findings support Stine's (1994) designation of this period
as being anomalous in western North America. The question then arises, what
caused this? Graham et al. ( 2007 ) found that climate between approximately AD
500 and 1350 was marked by generally arid conditions on land in much of western
North America and cool sea surface temperatures along the California coast. They
were able to link this Medieval Climate Anomaly to large-scale circulation features
revealed by tree rings and by other proxy climate records such as coral bands and
ocean sediments. This was in part achieved by reference to analogous patterns seen
in transient climate model runs. According to Graham et al. ( 2007 ) , the Medieval
Climate Anomaly was followed by wetter conditions in western North America and
warming coastal SSTs during the transition into the Little Ice Age. In concert with
these midlatitude changes, they reported that proxy records from the tropical Pacific
Ocean show contemporaneous changes indicating cool central and eastern tropi-
cal Pacific SSTs during the MCA, with warmer than modern temperatures in the
western equatorial Pacific.
This pattern of midlatitude and tropical climate conditions is consistent with
the hypothesis that a dry MCA in the western United States resulted (at least in
part) from tropically forced changes in Northern Hemisphere circulation patterns
like those associated with modern La Niña episodes (Cook et al. 2007 ) . Building
on results reported by Graham et al. ( 2007 ) concerning the MCA/LIA transition
in the north Atlantic region, Trouet et al. ( 2009 ) combined tree-ring records from
north Africa with speleothems from northern Europe to reconstruct a major shift
in the North Atlantic Oscillation at the time of this transition. The Graham et al.
and Trouet et al. studies used relatively small numbers of proxy climate records,
including long tree-ring chronologies, along with climate models, to infer shifts
of climate regimes on very large spatial scales. Their results are largely consistent
with those obtained in a very different manner by Mann et al. ( 2009 ) . In this case,
the methods of statistical climate field reconstruction (CFR) were used to exam-
ine the differences between two reconstructed global temperature fields, one for the
MCA (here AD 950-1250), the other for the LIA (here AD 1400-1700). This was