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Chapter 16
Climate Changes:
Past and Future
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.
The geologic column.
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.
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.
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.
There is evidence that the period a.d. 900–1200 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 1°C.
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.
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.
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.
Three astronomical factors influence the
timing and intensity of the seasons:
eccentricity in the orbit,
Earth’s 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.
The eccentricity of Earth’s 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.
Obliquity is the tilt of Earth’s 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.
Precession is the change in the orientation of the Earth’s axis.
Combined with changes in the orientation of the elliptical orbit,
the result is a 23,000-year cycle in radiation.
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.
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.
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.
Numerical models indicate that increased
tropospheric aerosol contents have the net effect
of reducing surface temperatures globally.
Stratospheric 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.
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.
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.
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.
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-Atmosphere Interactions
Atmosphere-Biota Interactions
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
Remnant Landforms
Layers of sediment composed of large material can
indicate the occurrence of heavy precipitation in the past.
Past Vegetation
Pollen diagrams provide information on past vegetation at a site,
which is useful for determining past climates.
The final chapter examines
atmospheric optics.