Climate Changes:

1
Chapter 16 Climate Changes Past and Future
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Climate change can be defined as a change in any
statistical property of the atmosphere, such as
a change in mean temperature. However, changes
in climate may occur even though the mean values
of precipitation, temperature, and wind remain
the same over time.
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The geologic column.
4
For most of its life, Earth has been largely free
of permanent (year-round) ice. It is a warm
planet, punctuated by perhaps seven relatively
brief ice ages. The warm times persist for
hundreds of millions of years to billions of
years, whereas the ice ages last on the order
of tens of millions of years to perhaps a hundred
million years. Oscillations in temperature and
ice cover are called glacial/interglacial cycles.

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In the depths of the last glaciation, around
20,000 years ago, land ice covered much more area
as seen in the map above. Sea level was about 120
m lower than it is now, so that a land bridge
existed between Siberia and Alaska.
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Warming began about 15,000 years ago, interrupted
about 2,000 years later by the Younger Dryas, a
time when colder conditions returned for about
1,200 years. 11,800 years ago another period of
abrupt warming began bringing climate into the
present interglacial.
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There is evidence that the period a.d. 9001200
was warm in the North Atlantic. This Medieval
Warm Period, coincides with the Viking settlement
of Greenland. The so-called Little Ice Age, from
1450 to 1850, was a cold period for western
Europe as alpine glaciers advanced and
temperatures fell by about 0.5 to 1C.
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Persistent oscillations emerge for every time
period going back 500,000 years. These
millennial-scale oscillations appear at
intervals of 6000, 2600, 1800, and 1450 years.
Such oscillations suggest that the
Earth-atmosphere system has a tendency to flip
back and forth between warm and cold states,
independent of long-term climate.
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The Maunder Minimum was a period of minimal
sunspot activity between about 1645 and 1715,
which coincided with one of the coldest periods
of the Little Ice Age. However, there have been
episodes in which variations in sunspot activity
did not coincide with climate change.
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The quasi-biennial oscillation (QBO) refers to a
pattern of stratospheric winds over the Tropics
that tend to reverse their direction in
approximately two-year cycles. When the QBO is
in its west-to-east mode there appears to be a
relationship between the number of sunspots and
winter conditions over northern Canada as surface
pressure rises and falls with sunspot
number. When the QBO is in its east-to-west
phase, no such connection is evident.
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Three astronomical factors influence the timing
and intensity of the seasons eccentricity in
the orbit, Earths axial tilt off the
perpendicular to the plane of the orbit, and
the timing of aphelion and perihelion relative
to the timing of the equinoxes. These three
factors make up the Milankovitch cycles.
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The eccentricity of Earths orbit changes
cyclically with a cycle of about 100,000 years
being especially prominent. Over about the last
15,000 years, there has been a steady decrease in
eccentricity, which will continue for 35,000
years.
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Obliquity is the tilt of Earths axis, which also
varies cyclically with a dominant period of about
41,000 years during which it varies between 22.1
and 24.5 off the perpendicular. The most
recent peak in obliquity occurred 10,000 years
ago, thus, we are about midway in the half
cycle from maximum to minimum obliquity.
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Precession is the change in the orientation of
the Earths axis. Combined with changes in the
orientation of the elliptical orbit, the result
is a 23,000-year cycle in radiation.
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The breakup of Pangaea (the early supercontinent)
and the slow movement of the resultant continents
undoubtedly caused major climatic changes because
all the factors that affect climate variables
were themselves greatly affected by the movement
of the continents.
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Atmospheric turbidity refers to the amount of
suspended solid and liquid material (aerosols)
contained in the air. Aerosols directly affect
the transmission and absorption of both solar and
infrared radiation. Aerosols can also
affect climate indirectly as cloud condensation
nuclei.
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Ship tracks appear as white streaks embedded in a
low-level cloud deck of speckled light gray
clouds. The image shows an area just offshore of
the U.S. with small amounts of cloud-free ocean.
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Numerical models indicate that increased troposphe
ric aerosol contents have the net effect of
reducing surface temperatures globally. Stratosph
eric aerosols tend to be smaller and have lower
terminal velocities. Because they are small,
the reduction in solar radiation reaching the
surface exceeds the gain in longwave radiation.
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Since the middle of the nineteenth century, there
has been an exponential increase in the input of
carbon dioxide to the atmosphere by fossil fuel
consumption. However, carbon dioxide is only
one of several anthropogenic greenhouse gases
that absorb outgoing longwave radiation. Methane,
nitrous oxide, and chlorofluorocarbons are also
effective absorbers whose contents are currently
increasing in the atmosphere.
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There is a constant exchange of carbon dioxide
between the ocean and atmosphere with the
ocean acting as a net sink for the greenhouse
gas. Photosynthesis by marine plants removes
carbon and settling plant and animal remains
transfer carbon downward. If the removal rate
were to decrease, there would be an accelerated
increase in atmospheric carbon dioxide levels.
21
Feedbacks refer to the interconnections of
different components of the atmospheric systems.
In a simple system that consists of only two
variables, changes in either one affect the state
of the other. If there is a change in the first
variable, it will produce a change in the
second, which will in turn affect the first
variable.
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Negative feedbacks inhibit further change and
are self-regulating. When the second variable
responds to initial change in the first
variable, its response will suppress further
change in the first. Positive feedbacks amplify
change in the initial variable. The response of
the second variable causes the initial change to
grow in a snowball effect. Among the feedback
mechanisms affecting climate are Ice-Albedo
Feedback Evaporation of Water Vapor Ocean-Atmosphe
re Interactions Atmosphere-Biota Interactions
23
Insights into paleoclimates or past climates are
based on information left in the geological and
biological records. There are a number of
techniques for studying past climates. Oceanic
Deposits Ice Cores Remnant Landforms Past
Vegetation Relic Soils
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Remnant Landforms
Layers of sediment composed of large material
can indicate the occurrence of heavy
precipitation in the past.
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Past Vegetation
Pollen diagrams provide information on past
vegetation at a site, which is useful for
determining past climates.
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The final chapter examines atmospheric optics.