Geoscience Reference
In-Depth Information
eleven
Why Climate Changes
cycles and oscillations
Just as the records of past peoples help us understand human
society, the records of past climates help us learn how the Earth
system works. And just as modern political scientists can test
their ideas against the history of humans, Earth system scientists
can test their models against past climate changes.
Richard Alley, h
e Two-Mile Time Machine
I n the past sev er a l ch a pter s, we have highlighted the work of
paleoclimatologists and the tools that have allowed them to re-create the
features and patterns of ancient climates in the American West. One reason
for their success is the variety of tools and climate archives at their disposal—
from the analyses of tree rings, lake levels, and ox ygen isotopes as indicators of
climate wetness and temperature to the interpretation of coarse sediment lay-
ers in l oodplains, wetlands, and coastal sediments as indicators of past l oods.
In this chapter, we explore in more detail the proposed mechanisms that
are responsible for past variations in climate and the persistent tendency of
climate to follow patterns and oscillations.
Over very long timescales (millions of years), climate change is caused
by forces within the earth that drive the very slow movements of tectonic
plates—processes that build mountains and spread ocean basins apart. Here,
we focus on those processes that occur on relatively short timescales (years,
decades, centuries, or millennia). Understanding the causes of climate varia-
tions and oscillations over longer time periods gives climate scientists insights
into how the climate works, the full range of climates possible, and what we
may expect in the future.
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