|Causes of Climate
- Long-term (millions of years) climate
changes are linked to plate tectonic processes
- Short-term (thousands of years) changes
are associated with changes in the earth’s orbit
cause of such
global climate changes has to be processes that operate on a global
scale over very long time intervals. The most likely causes are associated
with the changing locations of continents and oceans (plate tectonics)
that would in turn affect atmospheric and oceanic circulation patterns.
For example, ice ages have occurred at four widely separated times in
the earth’s history from the Precambrian (700 million years ago) to the
recent past (~10,000 years ago). The
causes of such events are thought to be linked to global scale
processes. Some potential requirements for such long-term cooling events
- Continents near poles: Continental ice sheets that
are characteristic of ice ages can only form on land. Glaciation on the
ancient supercontinent, Pangaea, occurred when several continents were
grouped around the South Pole;
- Uplift associated with continental collisions:
One tectonic element that is thought to have contributed to global
cooling beginning ~40 million years ago is the formation of the Himalaya
Mountains. Some scientists have suggested that the mountains changed the
regional (maybe even global) atmospheric circulation patterns that
contributed to a cooler climate;
- Reduction in greenhouse gas concentrations:
Increased rainfall following uplift may have stripped carbon dioxide
from the atmosphere to be used in chemical weathering. The presence of
less carbon dioxide resulted in a reduction in global warming (i.e. a
Times in the geologic past when temperatures where much higher than
today are related to periods of more rapid plate movements and greater
volcanic activity. Both processes produced greater volumes of greenhouse
gases that caused long-term warming of the atmosphere.
Short-term climate fluctuations that occur on cycles
lasting thousands of years are related to
variations in the earth’s orbit around the sun. These variations
(called Milankovitch cycles after the astronomer who
identified them) cause the amount of insolation (incoming solar radiation)
to vary with time.
- The eccentricity of the earth’s orbit. The exact
path of the orbit around the sun changes with time and may become
eccentric (more circular) or more eccentric (more elliptical). These
changes occur on a 100,000 year cycle.
- Changes in the tilt of the earth’s axis.
The tilt of Earth's axis is currently
tilted at 23.5 but axial tilt ranges from approximately 22-25 degrees
over a 41,000-year cycle. Decreasing tilt reduces the contrast of
insolation associated with the seasons, increasing tilt exaggerates
seasonal differences. Lesser tilt promotes the buildup of ice at the
poles, greater tilts allow for more insolation during polar summers,
causing more snow melt.
- The precession of the earth on its axis:
The earth "wobbles" on its axis (precession), changing the direction of
axial tilt. Precession
on a 26,000 year cycle - the length of time taken for the axis to trace
a complete loop. Half-way through the precession cycle, Earth would be
tilted away from the Sun during the "Summer" solstice (Northern
Hemisphere) and the Sun would be overhead at the Tropic of Capricorn.
Maximum precession therefore results in a switch between Summer and
Winter seasons, with the warmest months occurring in what we now call
Winter and cooler months during our the middle of the year.
|Precession results in
the seasons alternating position as the Sun will be overhead at the
Tropic of Capricorn (Southern Hemisphere summer) during the middle
of the year (Northern Hemisphere summer)
All of these factors contribute to changes in the amount of solar
radiation that reaches the earth and the amount of heat that is
transferred from the sun. During ice ages these cycles correlate well with
jumps from cold intervals (glacials) to warmer intervals (interglacials).
However, the variations in solar radiation are not considered sufficient
to account for the magnitude of observed temperature variations.
Cooler temperatures during the glacials were associated with lower
concentrations of carbon dioxide (a heat trapping gas) and higher levels
of atmospheric dust (blocked incoming sunlight). What is less clear is how
these factors are linked to variations in the earth’s orbit. Climate
fluctuations that occur on an even shorter time scales (decades to
centuries) may be linked to variations in sun spot activity
or catastrophic volcanic eruptions.