updated: May 02, 2007)
Continental Configuration and Ice Ages
As seen from the paleoclimate
record, the world is currently in an
interglacial period, which is part of the
cycle of ice ages that has been in progress for the past few million
years. These are greatly influenced by the
configuration of the
hemisphere has large continents surrounding the Arctic Ocean centered
on the North Pole. A large land mass at 55 degrees north is very
sensitive to temperature. When temperatures are below normal a large
ice sheet can form, while a slight warming can cause the entire ice
sheet to melt. The larger the land mass, the larger and more stable the
ice cap that will develop. When an ice caps starts
growing, it lowers local temperatures and reflects sunlight, causing
more ice to accumulate. This positive feedback led, for
to an ice cap over Canada larger than present day Antarctica.
The southern hemisphere has a small continent
centered on the South Pole, causing a permanent ice cap buildup and
contributing to global cooling in the long term. Ice cannot accumulate
in the ocean, so the spread of ice caps is limited to land and shallow
water. Wind and ocean currents circle the continent, isolating it from
the rest of the climate system. This configuration is very stable and
contribute to short term climate change.
Because the northern hemisphere is sensitive to climate change while
the southern hemisphere is not, it is the northern hemisphere that
drives global climate change.
The Milankovitch Cycles
cycles are changes in the Earth's orbit around the sun
that affect climate. The total amount of solar radiation
per year remains constant, but its distribution between hemispheres and
seasons changes. The relatively small changes caused by these orbital
cycles have a large influence because geographical instability
described above. There are three different cycles:
Earth's orbit around the sun varies from nearly circular to elliptical
100,000 year cycle. This can lead to a maximum 30% seasonal difference
in solar insolation when the orbit is at its most elliptical. At present the
eccentricity is relatively low (0.0174), and the Earth
receives 7% more solar energy in January (at perihelion, closest to the
than in July (at aphelion). The
orbit is gradually becoming more circular.
The Earth's orbital velocity is faster when closest to the sun. The
present northern winter (at periihelion) is four days longer than
|Obliquity is a change in the
tilt of the Earth's axis, between 22.1 and 24.5 degrees, in a 41,000
year cycle. Because this
changes, the seasons as we know them can become exaggerated. More tilt
means more severe seasons. We are presently about half way through a
cycle (at 22.4 degrees), and the axial tilt is gradually drecreasing.
A change in obliquity from 22 deg. to 24.5 degree increases
average insolation at latitude 65 deg. by 5 W/m2, and at the poles by
while insolation at the equator decreases by about 3.5 W/m2. [ref,
Obliquity acts in combination with eccentricity. A large tilt in
combination with an eccentric orbit can lead to large changes in
seasonal insolation, described below.
|Precession is a gradual change
direction of the Earth's axis, caused
by gravitational torques exerted by the Moon and Sun on the
Earth in a 21,000 year cycle. This affects the time of year a
season will occur. Precession is
causing the (northern) winter solstice to come later every year, and in
10,500 years winter will occur in July.
Precession affects whether the closest approach of the Earth's orbit to
the sun occurs in northern summer or winter. Given the very different
geography of the northern and southern hemispheres, this has a large
effect on climate.
Northern winters will gradually
become cooler as winter occurs when the Earth is farther from the sun,
and summers will become warmer. Since 1750 there has been a
0.4% increase in solar radiation (1.4 W/m2
the northern hemisphere
in April with a corresponding decrease in radiation in September [ref].
See this page
for a detailed description of the effect of orbital cycles on season
The graph below shows how the three Milankovitch cycles are changing in
time, starting from two hundred thousand years ago (on the right) and
to 100,000 years in the future. The trend is clearly towards
smaller obliquity and eccentricity, which means the effect of orbital
changes will be smaller. The changes may not be sufficient to start the
next ice age at the usual time, so it is possible that
the present interglacial period may last longer than previous ones.
There is a fourth orbital cycle not taken into account by the
Milankovitch theory: the inclination of Earth’s orbital plane
varies with a 100,000 year period (coinciding with Eccentricity) by up
to 3 degrees relative to its average position. If there is any
difference in the quality of radiation (eg. cosmic radiation) received
from the sun that depends on inclination, this could also affect
climate. But there is no actual evidence to show this makes any real
One Million Years of Milankovitch Cycles and Glaciation
years ago the ice ages corresponded to the 41,000 obliquity cycle, and
the pattern is still apparent during more recent ice ages which follow
the 100,000 year eccentricity cycle.
Observe that all three of these cycles will become relatively small in
the near future. Eccentricity is low now, and heading toward
zero, reducing seasonal variation in solar intensity.
is average but decreasing, reducing seasonal changes as well.
Conditions were similar 400,000 years ago, and the
period then was similar to the current one, with a slightly lower
maximum temperature and lasting longer than other interglacials.
Adding large amounts of carbon dioxide into the atmosphere will affect
the timing of ice ages in the future. For example, releasing
1,000 Gt of carbon, which is possible if current emission rates remain
the same for the next century, could delay the next ice age by 50,000
How Milankovitch Cycles Affect Temperature on the Earth's
The chart below shows the variations in solar intensity caused by the
Milankovitch cycles in the Arctic and Antarctic. The vertical scale is
the month of year, and we can see periodic
springtime warming of up to 60 watts per square meter (W/m2).
This should be compared with 3.7 W/m2
of direct forcing from a doubling of carbon dioxide levels. Although
the total amount of solar radiation over the course of a year does not
change much, the timing of the springtime highs corresponds well with
the start of the Eemian and Holocene interglacial periods, and periodic
warming events during the ice age. You can see these charts matched
against temperatures in Greenland
right to left, we can see the northern hemisphere spring time high
that ended the previous ice age and began the Eemian
interglacial period. This occurs at a time of high
when the northern hemisphere is tilted toward the sun at the time of
closes approach. About 10,000 years later (line B), or half a
Precession cycle, the southern hemisphere is tilted toward the sun, and
a southern springtime warm period continues the process of melting the
polar ice. The reduced ice cover reduces albedo, which causes
more solar energy to be absorbed. The Eemian warming comes to
relatively rapid end when unusually cold springtime temperatures occur.
A similar process repeats
itself 100,000 years later (lines C
and D) to
begin the present
(Holocene) interglacial period, except it is not followed by such cold
spring time conditions, so the interglacial is lasting longer.
Note that the solar anomoly
during the Eemian is more intense than the Holocene, and the Eemian was
interglacial. This is because Eccentricity was greater, and
Obliquity (the tilt of the axis)
was also greater then, thus amplifiying the seasonal warming.
To put an insolation anomaly of 60 W/m2
into perspective, the map below shows average insolation values for the
entire Earth. Each color band represents 20 W/m2
so the Milankovitch forcing shifts the insolation by three bands to the
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