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Box 7.5 Western United States droughts in medieval times
linked to changes over the Pacific basin
Box Fig. 7.9 Reconstructed Nevada division 3 July-June precipitation (smoothed with 50-
year Gaussian filter). Gray line marks the transition at ~AD 1400
Several multicentury shifts in precipitation over the central Great Basin
in the United States are seen in an ~8000-year reconstruction from the
bristlecone pine chronology at Methuselah Walk, and in an ~1800-year recon-
struction based on this and five other chronologies (Hughes and Funkhouser
1998 ) . A remarkable, but not unique, transition from drier to wet conditions
is reconstructed between the periods AD 400-1400 and AD 1400-2000 (Box
Fig. 7.9 ) . We set out to find an explanation for this transition (Graham et al.
2007 ) . Proxy evidence from tree-ring and pollen-based reconstructions, and
ocean core isotopic data suggest that the circa 1400 transition was marked by
warming sea surface temperatures (SSTs) along the central California coast
(Kennett and Kennett 2000 ; Box Fig. 7.10 , plot labeled Santa Barbara Basin
SST), and increasing winter precipitation with cooler summer temperatures
from southern and central California into the Great Basin (Box Fig. 7.10 , top
plot). In today's climate, such changes are associated with El Niño episodes,
suggesting the possibility that El Niño-like changes in tropical Pacific SSTs
may have played a causal role in producing the mid-latitude changes sug-
gested by the proxy records. A coral-based Niño-3.4 SST reconstruction from
Palmyra Atoll in the central tropical Pacific (Box Fig. 7.10 , plot 5; Cobb et al.
2003 and a foram Mg/Ca based SST reconstruction from near Mindanao in
the northwest equatorial Pacific (Box Fig. 7.10 , middle plot; Stott et al. 2004 )
support the idea of a trend towards more El Niño-like conditions at the circa
1400 transition. Analysis of the proportion of terrestrial material in a marine
core taken off the coast of central Peru indicates a contemporaneous increase
in river discharge associated with high-flow events (Rein et al. 2004 ) . This
 
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