12.3 Earth’s Orbital Variations and Climate
Essential Question:
How do changes in the Earth’s tilt, the shape of Earth’s orbit,
and the wobble of Earth’s rotational axis affect climate?
Reading 1: Earth’s Tilt (Obliquity)
Earth’s axis is tilted about 23.5◦ from its flat
plane of travel around the Sun. You probably
know that this tilt causes the seasons.
During our Northern Hemisphere summers,
the North Pole is tilted toward the Sun. In
this position, the Northern Hemisphere
receives the maximum amount of solar
radiation. More radiation (or energy) makes
it warmer. During our winter, the North Pole
is tilted away from the Sun, and thus receives a minimum amount of solar radiation, making it colder. Note that
the Southern hemisphere has opposite seasons: when the Northern Hemisphere is in winter, the Southern
Hemisphere is tilted towards the sun and experiences summer.
If Earth’s axis were not tilted, there would be no seasons. The amount of solar radiation a place on Earth
receives would be the same all year.
But, did you know that the tilt of the axis changes over time? The tilt varies between about 22.1◦ and 24.5◦. This
variation occurs on a cycle of around 41,000 years. If Earth’s axis is tilted more, the seasons become more
extreme: summers are warmer and winters are colder. If the amount of tilt is less, the seasons become milder:
summers are cooler and winters are warmer.
Cool summers are thought to lead to growth and advance of glaciers. During a cool summer, the snow that fell
during winter doesn’t fully melt away. Snow and ice can then continue to pile up near the pole(s), eventually
forming huge ice sheets. As the snow and ice keep building up, their white surfaces reflect more sunlight to
space, which causes even more cooling. This may be how ice ages start.
Reading 2: Shape of Earth’s Orbit (Eccentricity)
Earth revolves around the Sun in an elliptical path. The shape of this
path changes over time. Sometimes it is close to a perfect circle, and
at other times it is slightly more elliptical (oval, or egg-shaped).
The change in shape of the Earth’s orbit means that Earth’s distance
to the Sun changes by a small amount. As Earth’s orbit changes
shape, the amount of solar radiation Earth receives also varies by a
small amount. When the orbit is almost circular, the distance
between the Earth and the Sun is nearly equal all year. However,
when the orbit is more elliptical, the distance between Earth and the
Sun varies by up to 12%. But these changes in solar radiation due to
distance are relatively small—about a 6.7% difference now, up to
about 23% at maximum eccentricity.
In the picture below, changes in orbit
shape are greatly exaggerated!
~100,000 year cycle
Eccentricity also affects the length of the seasons. The Earth moves slower when it is further from the sun, so a
high eccentricity will cause the Earth to be further away from the sun for more days during the year. It will also
change the length of seasons by several days.
Data show that there is a strong connection between glacial and interglacial periods on a cycle of 100,000 years.
This matches the pattern of change in the shape of Earth’s orbit, so there might be a connection between climate
change and orbit shape.
Reading 3: Earth’s Wobbly Axis (Precession)
Earth’s axis of rotation wobbles like a spinning top. The
Earth spins around on its axis once every 24 hours, but
the direction the axis points in space wobbles, making
an almost perfect circle. The Earth takes about 23,000
years to complete one full wobble.
How does wobble affect Earth’s climate? The wobble
affects the axis and the orbit of Earth. Because of the
wobble, Earth’s axis points in different directions at
different times. Right now, Earth’s axis is pointed toward
the North Star, Polaris. At other times, the axis has pointed in other directions, such as toward the star Vega.
Earth is currently closest to the Sun during our winter in the Northern Hemisphere. Remember it is winter
because the Northern Hemisphere points away from the sun, so we get much less solar energy in winter.
Because Earth is closest to the Sun at this point in its orbit, though, it receives slightly more solar radiation than it
would if it were further away. This makes our Northern winters a little warmer than average. It also makes our
summers a little colder, meaning we now have slightly milder seasons in the Northern Hemisphere.
Summers currently occur in the Northern hemisphere when Earth is furthest from the Sun. In about 11,500 years,
the Earth’s wobble will cause the earth’s axis to rotate 180°. Because of this, the Earth will instead experience
winters when it is farthest from the Sun, and summers when it is closest. This will make our seasons more
extreme than they are now.
Another way to think about this is that the shortest day of the year will keep moving. In 11,500 years, the
shortest day will be June 21. At that time, the middle of summer and the longest day of the year will be
December 21. In another 11,500 years, conditions will return to our current pattern.
Reading 4: Ice Ages
We mentioned earlier that colder summers are thought to cause glaciers to grow. Each of the three cycles above
– tilt, orbital shape and wobble – can lead to changes in how warm summers are. If two or three of these cycles
align to create cooler summers, ice in glaciers and at the poles can grow even faster. Over time, these growing
glaciers and ice sheets can create what we call an ice age, where large areas of the Earth (such as almost all of
Canada) are under hundreds to thousands of feet of ice. These three cycles may also combine to create warmer
summers, melting glaciers and ice sheets, resulting in one or more ice-free poles. We call relatively ice-free
periods “Interglacials”.
View the animations at the website linked below to see how these three cycles work.
http://www.wwnorton.com/college/geo/egeo2/content/animations/18_2.htm