Geoscience Reference
In-Depth Information
We s t s u f ered severe drought between AD 1021 and 1051 as well as from AD
1130 to 1170, AD 1240 to 1265, and AD 1360 to 1382.
Medieval Climate Extremes
h e Medieval drought included not only severe drought, as detected in the
San Francisco Bay marsh records, tree rings, and tree stumps from Mono
Lake and elsewhere in the Sierra, but also severe l oods. In fact, several stud-
ies suggest that conditions were exceptionally wet in the interval between
the two Medieval megadroughts (850 to 750 years ago). For instance, Carson
Sink lakes in the Carson Desert of the Great Basin of Nevada reached their
highest levels between 915 and 650 years ago. Using water balance models,
researchers at the Desert Research Institute calculated that these lakes would
have required four times the modern stream inl ow for 70-80 years in order
to reach such high stands. More evidence comes from the White Mountains
of eastern California, where bristlecone pine growth rings indicate this to
be the wettest period in the past 1,000 years. Tree rings from the southern
Sierra Nevada also suggest exceptionally wet conditions during this period
between the droughts.
Extremes in wet and dry during the Medieval period were not the only
anomalies. h e trees of the Sierra Nevada tell us that, for at least the lat-
ter part of the period, conditions were also warmer than average. Connie
Millar, a research scientist for the U.S. Department of Agriculture, discov-
ered the remains of trees that had been buried by the ash of a nearby volca-
nic eruption in the high peaks of the eastern Sierra Nevada 650 years ago.
Several species of these trees were growing much higher in the mountains
than they do today, suggesting that temperatures in eastern California were
more than 3°C (5.4°F) warmer toward the end of the Medieval drought
than today.
Millar's results were supported by those of dendrochronologist and paleo-
ecologist Lisa Graumlich when she was the director of the Laboratory for
Tree-Ring Research at the University of Arizona. Analyzing tree rings from
foxtail pines and western junipers in the southern Sierra Nevada, she found
that the period between 900 and 625 years ago exceeded twentieth-century
warmth.
Other scientists from the University of Arizona, including Connie
Woodhouse and David Meko, studied trees in the Colorado River Basin.
h
eir tree-ring data indicated a particularly warm and dry period during
