Geoscience Reference
In-Depth Information
bad can it get? How prolonged and severe the droughts? How massive the
l oods?
lessons from the past
h e truism that history repeats itself applies not just to the af airs of
humankind but to climate as well. h e climate history of the past 20,000
years reveals that the American West has been plagued by deep and pro-
longed droughts, as well as by enormous l oods, on a fairly regular basis.
Paleoclimate records from across the region, extracted from many sources
discussed in this topic—tree rings, sediment cores, lake levels, and sediment
geochemistry, among others—have been compiled over the years to produce
this history. Although some questions remain unanswered, the records have
informed scientists of the range of climate conditions the region has expe-
rienced, including the frequency and magnitude of extreme climate events.
h ese extreme events far exceed those experienced over the past century and
a half. How likely are such events to recur, and are we prepared for these
events today?
Droughts
During past periods of climatic warming, such as occurred during the mid-
Holocene and again in the late Holocene, conditions throughout the West
tended toward increasing dryness, with droughts that could stretch out for
centuries. Research from California and the Great Basin, for example, shows
that the mid-Holocene was dry for nearly 1,400 years. Humans occupied
the American West during this time, and the archaeological record shows
that the mid-Holocene warming coincided with mass migrations of native
populations from the desert interiors of the West to the coastal regions. h is
long episode of warmer and drier climate was followed by the Neoglacial—a
period of cooler and wetter climate.
In the late Holocene, beginning approximately 1,800 years ago, dry-
ing conditions returned, peaking during a particularly dry period from
AD 900 to 1400 (the so-called Medieval Climate Anomaly). Prolonged
droughts were a i xture during this drier millennium, as were intermit-
tent l oods. Combined, these extremes brought down l ourishing civiliza-
tions that, the archaeological records show, all but disappeared as their
