Calspace Courses

 Climate Change · Part One
 Climate Change · Part Two

      Climate Change 2 Syllabus

    1.0 - The Ice Ages: An Introduction

  2.0 Discovery of the Ice Ages
         · 2.1 - Discovery of the Great Ice Age
         · 2.2 - Discovery of Multiple Ice Ages
         · 2.3 - Disc. of the Ice Age Record
         · 2.4 - Disc. of the Ice Age Cycles

    3.0 - Ice Age Climate Cycles
    4.0 - Climate Through the Last 1000 Years
    5.0 - Determining Past Climates
    6.0 - Causes of Millennial-Scale Change
    7.0 - Climate and CO2 in the Atmosphere
    8.0 - Recent Global Warming
    9.0 - Climate Change in the Political Realm
    10.0 - The Link to the Ozone Problem
    11.0 - Future Energy Use
    12.0 - Outlook for the Future

 Introduction to Astronomy
 Life in the Universe

 Glossary: Climate Change
 Glossary: Astronomy
 Glossary: Life in Universe

Discovery of Multiple Ice Ages

James Croll's calculations that attempted to explain Ice Ages via changes in the Earth’s orbit.
James Croll & the Earth’s Orbit
Louis Agassiz's concept of a "Great Ice Age" had to be modified in a number of ways. The ice sheets were not as large as he had thought, the ice age did not arrive as suddenly as he had proposed, and in fact there were several ice ages in succession, not just one giant one. The first evidence for this came from Scotland, where the geologist Archibald Geikie found plant fragments between layers of glacial till, and argued for sustained intervals of warm climate between different glacial ages. In other places geologists found soils and plant remains inter-layered between glacial deposits. A flurry of speculations followed about what kinds of processes might be responsible for producing multiple ice ages. Changes in carbon dioxide were proposed, as well as changes in the brightness of the sun. One of the more intriguing ideas came from the Scotsman James Croll, who proposed that changes in the orbit of Earth would result in changes in the severity of winters, which would generate appropriate changes in snowfall, and then feedback from whitening by reflection of sunlight during the rest of the year. All these various ideas introduced important new concepts to the understanding of climate change. And all of them were insufficient or wrong, at least as far as explaining the sequence of the ice ages, which had not yet been well established. For instance, Croll’s explanations lacked the data that they were aiming to explain. Another problem was that Croll's scheme made the last ice age much older than observed.

After Agassiz introduced his theory of the Great Ice Age, geologists labored hard in the field for many decades to refine the actual history of that astounding period. Soon it was found that there had been not just one ice age but several large glaciations, one following the other, separated by warm periods. The last one of these, though large, had not destroyed entirely the evidence for the earlier ones, some of which were even larger. From the ribbons of end-moraines left by the great ice sheets, and the deposits of wind-blown glacial dust (called "loess") there emerged the concept of four or five large glaciations during the ice-age time, during a period called the "Pleistocene."

Graph showing several European ice ages, their relative durations and the relative snow line during each. Note that during the glacial periods snow fell at lower altitude than during interglacial periods. Abbreviations: Glacial is “Gl.” and Interglacial is “Intergl.” (From: Penck and Brückner)
The Response of Penck & Brückner
By the turn of the century it was clear that there is no sharp climatic boundary between the "Tertiary" period preceding the Pleistocene, and the Pleistocene itself. The Pleistocene is set off by having fewer marine deposits and more glacial ones than the earlier time. But there is not much evidence for extinctions marking any boundary between the (warmer) Tertiary and the (cooler) Pleistocene. Once it was obvious that the Great Ice Age was in fact a period of repeated glaciations and warmings, the task was to identify these different glaciations, establish whether they were cyclic phenomena, and attempt to correlate them between the various regions on the same continent and between continents. As the famous Alpine geologists Albrecht Penck and Eduard Brückner noted in 1909, the collection of such data must precede the search for causes. Penck and Brückner responded to James Croll and his book, Climate and Time (published in 1875), which had stimulated so much discussion. Penck's position (and that of many of his colleagues) was that Croll's theory was useless in explaining the facts. In a similar vein, Gustav Steinmann, a leading alpine geologist of his time, gave this opinion:

"When we can show, as is in fact the case, that the events were contemporaneous not only for the various regions of the northern hemisphere but also for the southern one, it is then clear that the climatic problem of the ice age period is a general one [comprising the Earth as a whole] and we can eliminate thereby all those theories that explain glaciations from an alternating unfavorable effect on the two hemispheres by astronomical processes such as the changing eccentricity of the Earth's orbit, etc." (Steinmann 1916, p. 78)

After such dismissal, the brilliant speculations of Croll had no further impact on working geologists. In truth, Croll's approach was too far ahead of the evidence, and he gave an explanation for climate cycles before such cycles were known or suspected.

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