Geoscience Reference
In-Depth Information
climate: the extremes
Scientists began closely monitoring the West's climate at er the nineteenth
century, recording the region's weather variables: temperature, atmospheric
pressure, precipitation, and wind speed and direction. h ese measure-
ments provided monthly and yearly averages, dei ning the region's climate.
Climatologists have also examined extreme deviations from the mean in
order to understand the causes of these deviations and perhaps learn how to
predict them. h e measuring and monitoring of climate over the past cen-
tury has allowed climate scientists to assemble the pieces into a larger picture
that begins to explain what drives climate, including the critical interactions
between the Pacii c Ocean and its overlying atmosphere. As geographically
varied as the West is, climate in this region has been fairly consistent over the
past 150 years and, by and large, suitable for human habitation. h is period of
time would seem to be long enough for us to know what to expect from our
climate, both in normal years and in “extreme” years. However, as we shall
see later in the topic, this is not the case.
Extreme events have struck every few decades over the past century and a
half, some with catastrophic results. Even more extreme events have occurred
with some regularity in the distant past, though the intervals between them
ot en lasted a century or more. Understanding the causes of these events and
how they af ected the West in the past can help current residents anticipate
the ef ects when such events recur in the future—which they surely will.
Droughts
On one end of the climate spectrum is drought, when the wet season fails
partially or altogether. Nothing brings more fear to the hearts of water
planners in the American West than drought. Droughts lack the visual and
visceral images of other natural disasters; instead, they creep up on the land-
scape with no clear beginning or end. Meteorologists dei ne drought rather
vaguely as an abnormally long period of insui cient rainfall that adversely
af ects growing or living conditions. Such a bland dei nition belies the dev-
astation wrought by these unique natural disasters. In human history, whole
civilizations, even while at their peak, have toppled in the face of prolonged
drought. For humans, drought means failed crops, desiccated landscapes,
water rationing, decimated livestock, and, in severe cases, water wars, famine,
and mass migration.
