I have a nit to pick with science writers and other journalists. If you have read my series on the geological history of Arizona (see links below), you may have noticed that the Earth has plunged into an ice age every 145 million years or so. But wait, haven’t ice ages occurred much more frequently? No. There is confusion because the term “ice age” is frequently misused in the vernacular by journalists (and often by many geologists who should know better). What they really mean is a “glacial epoch.” So what is the difference? Strictly speaking, an ice age consists of several glacial epochs separated by warmer interglacial periods. We are currently in an ice age that began in the Pleistocene Epoch 2.6 million years ago. There is ice covering the polar regions, and there are still continental glaciers. For the past 11,700 years, planet Earth has experienced a warmer interglacial period between the glacial epochs, called the Holocene.
A glacial epoch is a time during which much of the earth’s surface is covered by glaciers and continental ice sheets. The frequency, intensity, and duration of glacial epochs are related to the position and orientation of the earth with respect to the sun. The location of the continents also influences the severity of glacial epochs because continents confine ocean currents. For the last 500,000 years of our current ice age, the glacial-interglacial cycle has had a periodicity of about 100,000 years. Prior to 500,000 years ago, the glacial-interglacial cycle was 41,000 years. As explained below, it seems that one Milankovitch cycle became dominant over another. (There is some recent proposed research that aims to blame the change on greenhouse gases.)
Although our current ice age started about 2.6 million years ago, the planet began cooling from the Cretaceous Period hot house about 50 million years ago. Ice ages seem to be related to the position of the solar system within the galaxy. Ice ages have occurred whenever the solar system passes through one of the five known spiral arms of our galaxy, which occurs at intervals of about 145 million years (± 10 million years).
What do stars have to do with ice ages? The hypothesis, greatly simplified, is this: Star density within the spiral galactic arms is much greater than in the galactic disk, hence, the flux of cosmic rays is much greater. Cosmic rays penetrating our atmosphere collide with molecules in the air and produce ionization. The ionized particles attract water and produce more clouds than normal. The clouds reflect sunlight which causes cooling. There is both observational and experimental evidence to support this hypothesis. Cosmic ray flux can be deduced from the so-called cosmogenic nuclides, such as beryllium-10, carbon-14, and chlorine-36, as measured in ancient sediments, trees, shells, and in meteorites. The geologic reconstruction of temperature is based on oxygen-18 isotopes from fossils and cave stalagmites. Also, glaciation leaves distinctive deposits and land-forms.
In the graph below, the top panel shows several calculated cosmic ray flux reconstructions. In the bottom panel, that curve (red) is flipped to represent the cooling effect. Notice that the cosmic ray flux coincides with the geologic reconstruction (black) of ice ages. (The green “residual” curve represents the mathematical variance between models and observations.)
Glacial epochs within ice ages seem to be controlled by the relationship of the earth to the sun. There are three main variations called Milankovitch cycles (after Serbian geophysicist Milutin Milankoviæ who first calculated the cycles): Orbital Eccentricity, Axial Tilt, and Precession of the Equinoxes. All these cycles affect the amount and location of sunlight impinging on the earth. The following explanation of these cycles is summarized from The Resilient Earth:
Eccentricity cycle of 100,000 years
Earth’s orbit around the sun changes from measurably elliptical to nearly circular in a cycle that takes about 100,000 years. When Earth’s orbital eccentricity is at its maximum, seasonal variation reaches 20-30%. Also, a more eccentric orbit will change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes. The variation in eccentricity doesn’t change regularly over time, because the Earth’s orbit is affected by the gravitational attraction of the other planets in the solar system.
Where we are now: Earth’s current orbital eccentricity is near its minimum, i.e., relatively circular. Presently, Earth’s distance from the Sun at perihelion, in January, is 91 million miles. Earth’s distance from the Sun at aphelion, in July, is 95 million miles. This difference between the aphelion and perihelion causes Earth to receive 7% more solar radiation in January than in July (but the northern hemisphere is tipped away from the sun in January).
Axial Tilt cycle of 41,000 years
The second Milankovitch cycle involves changes in tilt of the axis of rotation relative to the orbital plane. Tilt varies on a 41,000 year cycle from 22.1 degrees to 24.5 degrees. The smaller the tilt, the less seasonal variation there is between summer and winter at middle and high latitudes. For small tilt angles, the winters tend to be milder and the summers cooler. Cool summer temperatures are thought more important than cold winters for the growth of continental ice sheets. This implies that smaller tilt angles lead to more glaciation.
Where we are now: Currently, axial tilt is approximately 23.45 degrees, reduced from 24.50 degrees just a thousand years ago.
Precession cycle of 23,000- 25,800 years
The third cycle is due to precession of the spin axis. As a result of a wobble in Earth’s spin, the orientation of Earth in relation to its orbital position changes. This occurs because Earth, as it spins, bulges slightly at its equator. Since Earth’s equator is not in the same plane as the orbit around the sun, gravitational attraction of the Sun, Moon and other planets on Earth’s equatorial bulge tries to pull Earth’s spin axis into perpendicular alignment with Earth’s orbital plane. Earth’s rotation is counterclockwise [viewed from above the north pole]; gravitational forces make Earth’s spin axis move clockwise in a circle around its orbital axis. This phenomenon is called precession of the equinoxes because, over time, this backward rotation causes the seasons to shift.
The full cycle of equinox precession takes 25,800 years to complete. Due to the eccentricity cycle, Earth is closest to the Sun in January and farther away in July, but the northern hemisphere is tilted away. Due to precession, the reverse will be true 12,900 years from now. The Northern Hemisphere will experience summer in December and winter in June. The North Star will no longer be Polaris because the axis of Earth’s rotation will be pointing at the star Vega instead.
Individually, each of the three cycles affect solar insolation patterns. When taken together, they can partially cancel or reinforce each other in complicated ways.
Glacial epochs can be triggered when tilt is small, eccentricity is large, and perihelion, when Earth is closest to Sun, occurs during the Northern Hemisphere’s winter. Perihelion during the Northern Hemisphere winter results in milder winters but cooler summers, conditions that keep snow from melting over the summer. Deglaciation is triggered when perihelion occurs in Northern Hemisphere summer and Earth’s tilt is near its maximum. There are other factors which act to enhance the forcing effects of the cycles. These include various feedback mechanisms such as snow and ice increasing Earth’s albedo and changes in ocean circulation.
The sun itself goes through cycles of solar intensity and magnetic flux. When the cycles are in a strong phase, the amount of cosmic rays entering the atmosphere is reduced, there are fewer clouds to block the sun, so it is warmer. When solar cycles wane, as is beginning to happen now, more cosmic rays enter the atmosphere and produce more clouds which block the sun, so it becomes cooler.
The number of sunspots (hence magnetic flux) varies on an average cycle of 11 years. There are also 87-year (Gliessberg) and 210-year (DeVriess-Suess) cycles in the amplitude of the 11-year sunspot cycle which combine to form an approximately 1,500-year cycle of warming and cooling. So far, there is no evidence that atmospheric carbon dioxide has anything to do with the cause of ice ages or glacial epochs.
The current state of solar cycles indicates that we are more likely to be entering a cooling phase similar to the so called “Little Ice Age” (there’s the term misused again) that we experienced from about 1450 to 1850 AD.
References for this post:
Hoffman, D.L. and Simmons, A., 2008, The Resilient Earth.
(See website also:http://theresilientearth.com/)
Shaviv, N.J., 2003, The spiral structure of the Milky Way, cosmic rays, and ice age epochs on Earth, New Astronomy 8, 39.
Shaviv, N.J., and Veizer, Jan, 2003, Celestial Driver of Phanerozoic Climate, GSA Today, July 2003.
Veizer, Jan, 2005, Celestial Climate Driver: A Perspective from Four Billion Years of the Carbon Cycle, Geoscience Canada, V. 32, no. 1.