Helpful Internet reading:

• Michael J. Pidwirny, 2000, Cause of Climatic Change <http://www.geog.ouc.bc.ca/physgeog/contents/7y.html> in Fundamentals of Physical Geography.
• William W. Locke, 1999, Glaciers With Time in Glaciers and Glaciology Online Course, Department of Earth Sciences Montana State University
• Climate Change, Past and Future: The Ice Ages. Calspace Course, University of California, San Diego
• Causes of Climate Change, ARIC Global Climate Change Student Guide, Manchester Metropolitan University

Glaciation is triggered by one or more mechanisms that reduces or changes the amount and distribution of solar energy reaching the earth's surface. Any unifying theory of glaciation must explain the following:

• Occurrence and distribution of ice-age events in the Geologic Past (Precambrian, Ordovician, Permian, and Quaternary)
• Cyclic nature of glacial stages and interglacials within each major event
• Contemporaneous glaciation in both the north and south hemispheres
• Sawtooth pattern of climatic change. Glaciers take a long time to build up and disintegrate rapidly.

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 Driving forces/triggering events

• Plate Tectonics
  • Responsible for positioning land masses in high or low latitudes
    • Traditionally ice-ages were thought to occur when land masses were concentrated at high latitudes. Why?
  • Controls to some extent the exposed area and elevation of continents
    • Exposed Area: Continents have a higher albedo, that is they reflect more solar radiation. Therefore the greater the continental area the cooler the earth.
    • Elevation: If continents are higher there will be more area at higher altitudes and intersecting the regional snowline.
  • The positioning of land masses greatly affects oceanic circulation. Ocean circulation is responsible for redistributing much of the earth's heat.
  • High mountain ranges change atmospheric circulation.
  • Rates of sea-floor spreading influences atmospheric CO₂ (greenhouse gas)
    • high rates of sea-floor spreading
      • CO₂ is released into the atmosphere and climate is warmer (i.e. Cretaceous)
    • low rates of sea-floor spreading
      • Sea level is lower, more CO₂ is consumed by weathering and the climate is cooler

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    Precambrian glaciation: The Late Proterozoic glaciation clearly contradicts the hypothesis that glacial events occur when continental masses are largely concentrated at high latitudes. Paleomagnetic study of Late Proterozoic tillites show that glaciation occurred close to sea level in the tropics.

    Read Hoffman and Schrag, 2000, Snowball Earth <http://www.sciam.com/print_version.cfm?articleID=00027B74-C59A-1C75-9B81809EC588EF21>

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• Cosmic Clouds: Every 300,000 years the solar system passes through clouds of intergalactic dust that may reduce the total amount of solar radiation.
• Long term changes in solar output: An unlikely explanation in view of the fact that solar output has increase 30% over the last 2 billion years.

Within each ice-age event there are typically several glacial stages, lasting around 20-100ka when continental glaciers advance and subsequently retreat. Just in the last ice age spanning the past 2.5 ma there have been up to 50 stages of global cooling. The causes of repetitive glacial stages is debatable. However, the Milankovitch theory is the most commonly accepted explanation.

 

• Astronomical Theory of glaciation: Milankovitch cycles: astronomic cycles that when combined appear to correlate with and explain glacial cycles. (Effects on solar radiation are typically calculated for the northern hemisphere)
  • Changes in the Earth's tilt (22.1-24.5); controls the intensity of the seasons (±40,000 year cycles)
  • eccentricity of the earth's orbit; high eccentricity; Earth further from sun (±100,000 and ±400,000 yr. cycles). Adds to or reduces amount of seasonal insolation received.
  • Wobble of the Earth's axis (precession of the equinoxes); determines the timing of perihelion and aphelion relative to the seasons. (±21,000 year cycles)

    After Scott Rutherford, 2000, Milankovitch Cycles: Spectral Analysis, URI Graduate School of Oceanography Narragansett, Rhode Island

  Problems with Milankovitch theory:

  • Predicts cycles but not glacial conditions
  • predicted decrease in temperature is less than 1°C whereas a decrease of about 7C° is required. Measured change ranges from 10-20°C
  • Glacial stages should alternate between hemispheres
  • The weakest event (eccentricity) seems to have the greatest effect

  History of the Astronomical Theory

  • Joseph Alphonse Adhemar, French mathematician
    • 1842 published Revolutions of the Sea where he related glaciations to the 22,000 year precession of the equinoxes and eccentricity of the Earth's Orbit, and the fact that the Sun is not located exactly at the center of the focus. The hemisphere tilting away from the sun during aphelion would experience longer winters (by about six days) and hence glaciation.
    • Problems:
      • The decrease in solar energy experienced by one hemisphere tilting away from the sun is balanced by the amount of radiation receive in the other hemisphere. Therefore there is no change to the total amount of radiation received By the Earth.
      • Glacial events would have to alternate between hemispheres every 11,000 years. At that time the data (and the technology to obtain the data) supporting his hypotheses was unavailable.
      • Adehemar could not include changes in the eccentricity of the Earth's orbit or axial tilt because they were in the process of being discovered by the French astronomer Urbain Leverrier.
  • James Croll Scottish laborer and self taught natural scientist
    • 1864 published a paper in the Philosophical Magazine attributing glacial stages to changes in orbital eccentricity.
    • 1875 Croll published Climate an Time where he also included the 22°-25° variation in axial tilt. Croll was limited by the fact the timing of both these changes was still largely unknown.
  • Milutin Milankovitch Serbian Astronomer
    • Spent 30 years calculating radiation curves for the northern hemisphere based on astronomical cycles. In 1930 he published Mathematical Climatology and the Astronomical Theory of Climatic Change in in Kopen's Manual of Climatology. His mission would not have been possible if Ludwig Pilgrim had not published (1904) a detailed mathematic study outlining the variations in precession, eccentricity, and tilt over the last million years.

  Related sites:

  • Milankovitch Cycles: Changes in Earth-Sun Interaction by Matt Rosenberg
  • William W. Locke, 1999, Milankovitch Cycles in Glaciers and Glaciology Online Course, Department of Earth Sciences Montana State University

• Volcanic eruptions and meteor impacts

  These events would increase the concentration of atmospheric dust and reduce global temperatures.

  • problems:
    • By themselves these events are considered to be too short termed. However, they could trigger a series of positive feedback mechanisms which could promote glaciation.
    • These events, which largely random and non-cyclic, have not been shown to correspond to known glaciations.
  • Supervolcanoes could trigger big freeze, Thursday, 3 February, 2000, BBC Online Sci/Tech

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• Changes in the solar constant:
  • problem: Cycles are too short-termed and do not correspond to glacial cycles.

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• Cosmic dust and 100,000-year tilt cycles of the earth's orbital plane
  • A New Theory of Glacial Cycles, Richard Muller's site outlining the history of his and Gordon Mac Donald's hypothesis. Include links to online versions of their paper in addition to reviews.
  • New Ice Age Theory Gets Chilly Reception (APnet)
• Fluctuations in greenhouse gasses (CO₂ and methane)

Positive feedback mechanisms (augmentation):

A positive feedback mechanism is one that amplifies a change. Such feedback mechanism can result in a minor change or event resulting in a widespread cooling.

• Amplifying processes - critical to producing glacial events, but least understood
• Increased snow cover results in increased albedo--glaciers advance
• Changes in oceanic circulation
  • Increased sea ice deflects surface currents: (e.g. Gulf stream deflected away from higher latitudes)--glaciers advance
  • Decreased ocean salinity decreases bottom water circulation--glaciers advance. Increased ocean salinity increases bottom water circulation.
• Lowering of sea level
  • Higher % of exposed continent results in higher albedo--glaciers advance
  • Grounding lines retreat seaward enabling glaciers to advance
  • decreased ocean circulation--less heat transported N
• Changes in CO₂ (decreased during glaciation--reasons why are hotly debated)--Decreased CO₂--glaciers advance.
• Ice thickness increases, more area is above the regional snowline--glaciers advance.
• Changes in atmospheric circulation--greater orographic precipitation--glaciers advance
• Higher amounts of atmospheric dust--higher during glaciation  

What are the positive feedback mechanisms mentioned in these articles?

• Earth Enters the Big Thaw Tuesday, 7 March, 2000, BBC Online Sci/Tech
• Arctic 'now adding to global warming' Wednesday, 7 February, 2001, BBC Online Sci/Tech
• Ice Sheets Play Important Role in Climate Change , Clark, et al.., 1995, Eos Vol. 76, No. 27, Jul 4, 1995, pp. 265, 270.
• Melting Permafrost May Accelerate Global Warming, UNEP Scientist Warn, United Nations Environment Programme, 2/2001

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Negative feedback mechanisms:

• When a continental glacier becomes quite large a number of negative feedback mechanisms aid its disintegration.
  • Orographic precipitation is confined to the margins and the center of the ice sheet becomes starved--glacier retreats.
  • Isostatic depression decreases elevation of the ice surface relative to the regional snowline. More ice lies within the ablation zone--ice retreats.
  • Melting glaciers cause rise in sea level, grounding lines retreat landward causing rapid calving--ice retreats
  • CO₂ is consumed through the weathering of silicate minerals which is in part temperature dependent. Lower temperatures cause lower rates of reaction and the progressive build up of CO2.
  • Lowering of sea level decreases pressure on the ocean floor inducing the release of methane from gas hydrates (clanthrates). Methane is 10-25% more effective than CO2 as a green house gas.

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Questions

• Why ice sheets take tens of thousands of years to build up but take only one to two (?) thousand years to disintegrate?
• What factors limited the size the ice sheet?
• What are the negative feedback mechanisms that cause such rapid deglaciation? 

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Relevant Sites

• **** Abrupt Climate Change, Inevitable Suprises, National Academic Press online book, 2002
• * AGU Science form everyone: Climate and global change
• ** Can Global Warming Cause Ice Ages, WHOI
• Climate Change Some Basics, 1997, Jan Schloerer
• Global warming could trigger ocean current collapse, 1997, Environmental News Network
• Lamont's Broecker Warns Gases Could Alter ClimateOceans' Circulation Could Collapse, 1997, Laurence Lippsett, Columbia University Record,  v. 23, n. 11
• Methane Hydrates and Climate Change
• Schmidt, G.A., and D.T. Shindell 2003. Atmospheric composition, radiative forcing, and climate change as a consequence of a massive methane release from gas hydrates. Paleoceanography 18 , no. 1, 1004, doi:10.1029/2002PA000757.
• Shindell, D.T., G.A. Schmidt, M.E. Mann, D. Rind, and A. Waple 2001. Solar forcing of regional climate change during the Maunder Minimum. Science 294 , 2149-2152.
• **The Big Chill Discussion on the causes of glaciation from Nova Online, Cracking the Ice Age
• Why do glaciers occur? Brief article by Doug Latimer, Rocky Mountain Hiking, WorldWeb Travel Guide

[Glacial and Quaternary Geology] [extended GeoIndex][QkRef][Geological Sciences] [Degree Programs] [Salem State College]
Lindley Hanson (email) Last Modified 1/27/03