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compiled a list of other global events that occurred at the same time as these
megal oods. He found that, around AD 1605, at least two major volcanic
eruptions occurred: the Huaynaputina volcano in Peru in 1600, and other
volcanic evidence revealed in an Antarctic ice core. He also discovered numer-
ous paleoclimate records worldwide showing that the years between 1600 and
1610 were unusually cold, particularly during the summer, with the summer
of 1601 being the coldest in the Northern Hemisphere of the past millen-
nium. h ese records were largely based on tree-ring records from California,
Idaho, the Canadian Rockies, Subarctic Quebec, the Gulf of Alaska, New
Zealand, Mongolia, and Norway. Ice-core records from both Greenland and
Antarctica also suggest unusually cold conditions. But was there a connec-
tion between the volcanic eruptions, Northern Hemisphere cooling, and the
California megal oods? We will return to this question in the next chapter,
where we discuss possible causes of past climate change.
the little ice age and the west
h ese megal oods occurred during the Little Ice Age, an event that, as described
above, let traces throughout the Northern hemisphere between the i t eenth
and nineteenth centuries. In the western United States, glaciers also advanced
in the Sierra Nevada, Cascades, and Rockies. Conditions became wetter in
the West, and l oods were larger and more frequent, as discussed above. In
the Sierra Nevada, abundant moisture allowed trees to grow faster, putting on
wider rings. East of the Sierra Nevada, Mono Lake rose to its highest level in
centuries. In the Great Basin, Owens and Pyramid lakes were also high.
Forest i res in California were reduced during the Little Ice Age. For
instance, though the Giant Sequoias in the southern Sierra Nevada bear scars
from i res over the past millennia, those scar records show a marked decline
in i re frequency at er AD 1600. h e records also closely track the summer
values of the Palmer Drought Severity Index (PDSI), a measure of the mois-
ture content of soil in the root zone. Tree-ring records contain a history of
the PDSI. A comparison between the PDSI and the i re-scar records shows
that three-fourths of the largest i re years occurred during the driest years.
Mean summer temperatures, the PDSI, and i re frequencies all peaked about
AD 1200 during the Medieval period, followed by a steady decline in these
factors (suggesting a cooler, wetter climate with fewer i res) during the Little
Ice Age (see i gure 27B).
