6.II: The Ice Ages

The “ice age” is actually a series of glacials and interglacials going back 3,000,000 years. The cool and fluctuating climate of the Pleistocene has been one of the most influential forces of nature shaping the evolution of humans and other living things. 1

A.  The Rule: Glacials

B.  The Exception: Interglacials

C.  Citations

A. The Rule: Glacials

Shortly after 3 million years ago, Earth’s cooling climate crossed a critical threshold.  The polar ice caps grew, extending well into the temperate zones.  The Earth entered an ice age.  This Quaternary ice age was not the first or the most severe.  Geologists know of four earlier major ice ages.  In our logarithmic history, though, the others were just blips in deep time.  The Quaternary ice age completely dominates our study of the past few million years.  Technically, we are still in it.

What causes the polar ice caps to grow?  Chapter 7 discussed several long-term contributing factors related to plate tectonics, ocean currents, and the composition of the atmosphere.  Once global conditions are right, there are two major proximate causes: one on land and another at sea. 

A glacier is simply a perennial accumulation of snow on land.  If more snow falls in the winter than melts the following summer, then that snow cover persists year-round.  Next year, it grows a little larger.  After many years, the deep layers harden into solid ice.  As a glacier grows to massive scale, its sheer weight gives it a life of its own.  Glaciers can creep down slopes, carve fjords, displace boulders, calve icebergs into the ocean, and even deform the crust of the earth beneath them.  Northern glaciers are mostly predisposed to form in Canada and the Rockies, Greenland, northern Europe, western Siberia, the Himalayas, and the Alps.  In the southern hemisphere, they are almost entirely restricted to Antarctica but can also form in the Andes.   

Meanwhile, the surface of the ocean can freeze over to form sea ice.  It takes protracted cold weather to do this, but it does not require any precipitation.  Colder temperatures make sea ice deeper and more durable and enable it to persist at lower latitudes. 

Ice sheets on land and sea alike feed themselves with a positive feedback loop.  White ice reflects much more sunlight than the darker soil or water beneath it.  As the icy Earth absorbs less energy, temperatures drop even further and enable the ice to encroach closer to the equator.  At times, ocean waters have frozen fully down both coasts of Canada and around Europe as far south as the British Isles.  Glaciers have reached lowlands as far south as 40º N, the latitude of the central United States.  Sure, Nebraska gets snow every winter.  But can you imagine visiting Nebraska in July and finding it covered with ice a mile thick?  That’s the difference between winter weather and an ice age.  

An ice age has an enormous impact on life.  A glacial ecosystem can support little more than microbes, moss, algae, worms, and some small birds. 2 Most native terrestrial life forms must surrender their habitats to the ice sheets.  Sea ice supports a much richer food web (polar bears, walruses, penguins) but is less stable and covers a limited area. 

The impact doesn’t end at the ice’s edge.  An ice age results in dry climate worldwide.  The more water gets locked away in glaciers and sea ice, the less is available to recirculate into the atmosphere.  Furthermore, glaciers lock up large amounts of water on land that would otherwise melt and find its way to the ocean.  As a result, sea levels fall, sometimes changing the shapes of continental shorelines. 

B. The Exception: Interglacials

Fortunately for us, the Quaternary is barely an ice age.  When conditions conspire to produce particularly warm northern summers / southern winters, summertime melting exceeds winter freezing and the ice momentarily retreats back to its polar realms.  As you might guess, we are lucky enough to be in one of those exceptional periods right now, an interglacial.  The pattern of the Quaternary has been 100,000-year glacial periods punctuated by 10,000-year interglacials.  These climatic fluctuations are caused by complex planetary wobbles known as Milankovitch Cycles,named after the amateur astronomer who spent three decades of his spare time calculating their effects by hand.  In plain English, we might call them the shape of Earth’s orbit, the angle of Earth’s tilt, and the direction of Earth’s tilt. 

Earth’s orbit, like every planet’s, is an ellipse or a “stretched circle”.  Eccentricity describes the amount of stretch in the ellipse.  Under the gravitational influence of Jupiter and Saturn, Earth’s orbit fluctuates from minimum eccentricity to maximum and back to minimum again in 100,000-year cycles.  When the orbit is at its most eccentric, the seasons are at their most extreme and Earth’s average distance to the sun is minimized. 3 These are prime conditions for glacial thaw.  The glacial fluctuations of the past million years closely match this 100,000-year period, but Earth’s eccentric variation is too small to account for interglacials on its own.

Earth eccentricity orbit Milankovitch cycle ice ages Pleistocene Quaternary
Exaggerated change in Earth’s orbital eccentricity. An eccentric orbit brings Earth slightly closer to the sun on average, and more importantly it makes the seasons more extreme. 4

Nothing in the solar system is perfectly symmetric or aligned.  Earth has an equatorial bulge. 1 The planes of Earth’s orbit, Earth’s equator, and the moon’s orbit are all slightly offset from one another.  As a result of all these imbalances, the gravity of the moon and sun tug on Earth’s orbital axis and slowly twist it around in a cone.  This causes a 25,000-year cycle in stellar north, the direction in which Earth’s North Pole points toward the stars.  Sometimes the North Star is Vega, ¼ of the night sky away from Polaris!  For purposes of climate change, the important issue is that northern summertime occurs when the North Pole faces the sun.  The twisting axis causes summer to occur at varying points around Earth’s orbit – sometimes when Earth is closer to the sun and sometimes when it’s farther away.  Astronomers call this seasonal effect the precession of the equinoxes. When northern summertime occurs close to the sun, we get the unusually warm northern summers that promote interglacials.

Earth precession equinox Milankovitch cycle ice age
Precession of the equinoxes. Line A shows Earth’s axis today. Lines A’, A’’ etc. show the precession of the axis around a cone in a 25,000 year cycle, causing the seasons to shift around Earth’s orbit. 5

Due to the laws of physics, precession of the equinoxes causes yet another cycle, a precession of obliquity. 6 Obliquity measures the angle at which Earth’s axis (through the poles) is tilted from its orbital axis (perpendicular to its orbit around the sun).  This angle makes a 41,000-year cycle from about 22º to 25º and back again.  A steeper obliquity causes greater seasonal variations and more intense northern summers, so it is the higher obliquity that favors interglacial thaws.

Earth obliquity precession ice ages Milankovitch cycles
Obliquity. The tilt of Earth’s axis (red) oscillates within this narrow range. 7

The latter two cycles do not affect the total amount of solar energy to reach Earth, but rather the distribution of that energy around the globe and throughout the year.  That’s how touchy the climate is!  The patterns of sunlight make a difference because of Earth’s own asymmetries.  The Arctic and Antarctic zones are polar opposites in more ways than one.  The North Pole is in the center of a small ocean surrounded by land.  The South Pole is situated on a small continent surrounded by water.  It is much harder to melt glaciers in the southern hemisphere, which has land at the most extreme latitudes.  In fact, Antarctica has been permanently glaciated since Chapter 7.  The Arctic Ocean has limited area to form sea ice, but when ice does form there it is landlocked and stable.  The Antarctic Ocean is unbounded, so the ice that forms there is unrestrained from drifting northward and melting.  The Gulf Stream delivers copious precipitation to the Arctic Circle, while Antarctica is one of the most arid regions on Earth.  That is fortunate, because if the South Pole got as much snow as the North, most of the world’s fresh water supply would now be locked up in the southern polar cap!  Early studies of the ice ages focused exclusively on the northern hemisphere because that was where the scientists lived and made their discoveries.  More recent evidence suggests that southern sea ice has had particularly strong influence on the ice ages of the past million years. 8

The Milankovitch Cycles are too complex to fully detail here.  There are a few other cycles that Milankovitch did not know about.  They all interact with each other, sometimes reinforcing and sometimes cancelling each other out.  Sunlight further interacts with terrestrial conditions such as currents, atmosphere, geography, and even life.  The takeaway point is that these past three million years have been 80 – 90% ice ages.  The climate that we call “normal” today only occurs in exceptional times called interglacials.  Interglacials tend to begin suddenly, last about 10,000 years, and then gradually succumb to 100,000 more years of ice. 9

Back to Section 6.I: Introduction & Geological / Archaeological Terms

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Continue to Section 6.III: Early Humans

C. Citations

  1. Ice age image by Mauricio Antón, CC BY 2.5 (https://creativecommons.org/licenses/by/2.5), https://commons.wikimedia.org/wiki/File:Ice_age_fauna_of_northern_Spain_-_Mauricio_Ant%C3%B3n.jpg
  2. Arwyn Edwards, “Glacier ecosystems”, AntarcticGlaciers.org (3/03/2014), http://www.antarcticglaciers.org/glacier-processes/glacier-ecosystems/ (saved 2/03/18, accessed and archived 11/02/19).
  3. Eccentricity image by user RJHall, CC-BY-SA-2.0 license, https://commons.wikimedia.org/wiki/File:Kepler3.gif (accessed and saved 2/03/18, archived 11/02/19).
  4. Chris Colose, “Milankovitch Cycles”, Skeptical Science (7/22/2011), https://www.skepticalscience.com/Milankovitch.html (accessed and saved 2/3/18, archived 11/02/19).
  5. Precession image “Fig. 55. – Conical Motion of Earth’s Axis”  from Charles Augustus Young, Manual of Astronomy, p. 145, Ginn and Company (1902, 2ed Boston, 1912), via archive.org, https://archive.org/details/manualofastronom00younrich/page/144 (accessed, saved, and bookmarked 11/02/19).
  6. Guy Worthey, “Astronomy: precession of earth”, Washington State University (9/12/2000), http://astro.wsu.edu/worthey/astro/html/lec-precession.html (accessed and saved 2/03/18, archived 11/02/19).
  7. Obliquity image by Robert Simmon, from Steve Graham, “Milutin Milankovitch (1879 – 1958)”, Earth Observatory p. 2 (3/24/2000), in the public domain as the work of the US government agency NASA GSFC, https://earthobservatory.nasa.gov/features/Milankovitch/milankovitch_2.php (saved 2/03/2018, accessed and archived 11/02/19).
  8. Jung-Eun Lee et al., “Hemispheric sea ice distribution sets the glacial tempo”, Geophysical Research Letters 44(2):1008-14 (1/27/2017), http://onlinelibrary.wiley.com/doi/10.1002/2016GL071307/full (accessed 1/28/18, saved 11/02/19).
  9. National Oceanic and Atmospheric Administration, “Glacial-Interglacial Cycles” (date unknown), https://www.ncdc.noaa.gov/abrupt-climate-change/Glacial-Interglacial%20Cycles (accessed and saved 1/28/2018, archived 11/02/19).
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