Geoscience Reference
In-Depth Information
important information for water planners about the long-term history of
stream l ow.
In the early 1940s, Schulman conducted a landmark study: a 600-year
reconstruction of Colorado River l ow using tree-ring records from long-
lived Douglas i rs. h e records revealed several sustained periods of low
l ow, which translate into drought, that recurred with some regularity. h e
reconstruction showed a particularly severe drought in the late 1500s, a
drought that has shown up in multiple records from throughout the West.
Schulman's reconstruction also showed that the late nineteenth century and
the early decades of the twentieth century were unusually wet. h is was a
critical period of expansion by American settlers, and climate conditions at
that time gave the settlers a false impression that the region was wetter than
it actually is and hence ripe for growth and the establishment of new farms.
Schulman was also interested in climate cycles—repeating patterns of
relatively benign conditions with reliable moisture, followed by prolonged
drought. He wanted more information about such cycles in the American
West to better anticipate the region's future. Schulman did indeed i nd
recurring phenomena, including one cycle that corresponded in length
(ca. 11.6 years) to already observed sunspot activity. In later chapters, we will
come back to these climate cycles, which have an important place in our
understanding of climate and how it changes with time.
natural repositories of western
climate change
Bristlecone pines are not the only—or even the most important—archives of
past climates. In fact, paleoclimatologists depend on many tools and archives
for their investigations. In recent decades, new technological advances have
allowed Earth scientists to tease out more and more from these natural
archives, ranging from the chemical and isotope compositions of fossils and
sediments to molecular fossils contained within organic matter preserved in
these sediments. Some are most useful for the recent past, others for events
that happened thousands or even millions of years ago. All these tools have
required extraordinary insights and hard work to develop, and their value
continues to grow as scientists i nd new uses for them in their research.
Just where are these archives of past climate located, and how do we
i nd them? Earth scientists are discovering that the modern American
