8.9: Natural Changes in Climate
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Earth’s climate goes through a variety of natural changes.
Long-term changes occur over millions of years (due to the
movement of Earth’s crust), or over hundreds of thousands of years
(due to changes in Earth’s orbit).
Within long-term changes, short-term variations in climate can be
caused by natural events such as volcanic eruptions.
Long-Term Changes Due to Continental Drift
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According to the theory of plate tectonics, Earth’s continents have
moved over the surface of the globe for hundreds of millions of
years.
Continental drift changes the distribution of land mass, and as a
result, also changes major air and ocean currents. These changes
have large effects on climate.
Figure 2 Continental drift affects climate because it changes the distribution of land around the
globe.
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For the last 800 000 years or more, Earth’s climate has cycled
between freezing ice ages and warmer interglacial periods.
Long-Term Cycles in Climate
Figure 4 Graph of changes in Earth’s average temperature over the past 400 000 years. The
values on the y-axis represent deviations from Earth’s average temperature today. Notice that
major changes in temperature happen in regular cycles. Warm interglacial periods occur about
every 100 000 years.
 According to the Milankovitch theory, these cycles are triggered by
three changes in Earth’s orbit around the Sun:
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Today, Earth is in a warm interglacial period.
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Eccentricity: The shape of Earth’s orbit varies in a cycle of 100 000
years.
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Tilt: Earth tilts back and forth over a cycle of about 41 000 years.
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Precession of tilt (wobble): As it spins, Earth slowly wobbles in a
cycle over 26 000 years.
Short-Term Variations in Climate
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Small variations in climate occur over tens of years to hundreds of
years.
These variations can be caused by volcanic eruptions, small
changes in the Sun’s radiation, and changes in the circulation of air
and ocean currents.
Volcanic eruptions spew rocks, dust, and gases high into the
atmosphere, blocking radiation from the Sun. As a result, Earth
temporarily cools down.
If the amount of radiation from the Sun drops slightly, Earth
climate cools down. Similarly, if the Sun radiates slightly more
energy, Earth’s climate warms up.
Changes to the ocean’s thermohaline circulation may cause abrupt
changes in climate, although this phenomenon is not fully
understood.
Some changes in air and ocean currents occur regularly, such as El
Niño
Figure 8 Red arrows represent warm water currents. (a) Normally, the west
coast of South America is cold and dry, due to a cold ocean current nearby. (b)
During an El Niño event, changes to prevailing winds affect the movement of
ocean water. The west coast of South America receives warmer, wetter
weather.
Check Your Learning, questions 1 – 8, pg. 353