Geoscience Reference
In-Depth Information
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
is reconstructed between the periods AD 400-1400 and AD 1400-2000 (Box
ocean core isotopic data suggest that the circa 1400 transition was marked by
warming sea surface temperatures (SSTs) along the central California coast
SST), and increasing winter precipitation with cooler summer temperatures
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
2003
and a foram Mg/Ca based SST reconstruction from near Mindanao in
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