Milankovitch Orbital Cycles

(last 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 present configuration of the continents.

The northern 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 example, 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 does not 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

Milankovitch cycles are changes in the Earth's orbit around the sun that affect climate. The total amount of solar radiation received 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:

The Earth's orbit around the sun varies from nearly circular to elliptical in a 90,000 to 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 sun) 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 summer.

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 tilt 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 annual average insolation at latitude 65 deg. by 5 W/m², and at the poles by 17 W/m², while insolation at the equator decreases by about 3.5 W/m². [ref, #193]

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 in the 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 ) in 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 length.

The graph below shows how the three Milankovitch cycles are changing in time, starting from two hundred thousand years ago (on the right) and extending 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 difference.

One Million Years of Milankovitch Cycles and Glaciation

Before a million 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. Axial tilt is average but decreasing, reducing seasonal changes as well. Conditions were similar 400,000 years ago, and the interglacial 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 years [ref].

How Milankovitch Cycles Affect Temperature on the Earth's Surface

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 and Antarctica respectively.

Reading from right to left, we can see the northern hemisphere spring time high (line A) that ended the previous ice age and began the Eemian interglacial period. This occurs at a time of high eccentricity 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 a 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 a warmer 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 south.

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