Geoscience Reference
In-Depth Information
of many previous interglacial periods, some scientists believe that it could last
thousands of years longer, perhaps extended by the addition of heat-trapping
carbon dioxide and methane into the atmosphere when humans began large-
scale agriculture and, most recently, the burning of fossil fuels. In truth, no
one knows precisely when the next ice age will return.
h e Quaternary has been divided into more than twenty glacial periods,
separated by interglacial periods. h e dif erent glacial states are marked
by changes in oceanic currents, changes in winds and airborne dust, and
shit s in storm tracks inl uencing when and where precipitation falls. Both
temperature and carbon dioxide levels have l uctuated between glacial and
interglacial states (see i gure 16A), based on ice core records discussed in
the previous chapter. During glacial periods, sea levels have fallen by up to
120 meters (390 feet) below its present, interglacial level as huge volumes of
water were transferred from the oceans to the continents to be stored as ice
in massive ice sheets. h e lowered sea levels exposed a land bridge between
Asia and North America.
When the last glacial period of the Quaternary reached its peak about
20,000 years ago, one of its ice sheets—the Laurentide—was two miles thick
and covered most of what is, today, eastern and central Canada, extending
southward into Indiana, Illinois, Ohio, and Pennsylvania. On the western
half of Canada was the Cordilleran ice sheet that reached from British
Columbia as far south as Seattle, Washington. h ese ice sheets grew so
high that scientists believe they altered atmospheric circulation. Mountain
glaciers also expanded over the upper reaches of the Cascade, Rocky, and
Sierra Nevada ranges.
Our story focuses on the climate history of the American West since the
Last Glacial Maximum. Within this relatively short time period, climatic
extremes were greater than anything we can imagine from our limited
historical experience. Today we live in a time of unusually benign weather,
and many “normal” conditions, as viewed over the past 20,000 years, seem
inconceivably harsh.
At er the peak of the last ice age, the climate began to warm at last, and
the ice began to melt, carving new rivers and leaving behind mountain-
ous loads of debris in moraines. Fit een thousand years ago, enormous
lakes formed across the Great Basin of modern-day California, Nevada,
Utah, Oregon, and Colorado. Some of these lakes were tens of thousands
of miles in area and hundreds of feet deep, covering vast basins between the
mountains.
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