February 17, 2011 | 3
"Fire and Ice"
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
Robert Frost, American poet (1874-1963)
The climate of a region, as experienced by daily observations of a cool morning and hot midday, was for very long time considered simply the result of the height of the sun above the horizon. This idea forced a very simple view of the distribution of climates on Earth, to the poles temperature dropped, to the equator it raised, forming so large parallel climatic belts. Such a static view or the Earth also didn’t need or even allow climate changes in the past or in the future time.
With the establishment of the deep geological time by the first geologists and naturalists it became clear that not only the distribution of sea and land changed over time, but so did climate.
The German philosopher Gottfried Wilhelm Leibniz (1646-1716) imagined a once molten Earth, formed by the agglomeration of hot gases and dust, slowly cooling and developing a stable crust. According to Leibniz this cooling continued still today, enabling the formation of large ice caps on the poles. The past warmer climates postulated by this hypothesis seemed supported also by the discovery of fossil remains of presumed tropical animals in Europe and Siberia, like elephants, lions and even rhinoceroses.
Fig.1. Replica of bones of Cave bear (Ursus spelaeus) discovered in 1987 by an amateur fossil hunter in the Conturines Cave in the Dolomites (Alps), the highest situated locality were such fossils were found (2.700m a.s.l.). Today the surroundings of the cave are a barren and cold alpine landscape; unable to provide nutriment for such large animals, but when these bears lived here 100.000 years ago the climate must be warmer, and the vegetation lush. Similar fossil remains in caves all around Europe of supposedly tropical animals provided also first insights of past climatic changes.
The French naturalist Georges-Louis Leclerc, Comte de Buffon (1707-1788) linked the extinctions of these animal to past and profound climatic changes, in Europe presumably a sudden cooling event – "It’s impossible", he resumes, "to transform an elephant in a reindeer."
If such cooling continued, he foresaw a gloomy future for the planet in his work "Petrifications et fossils" (1786):
"the diminution of the waters, combined with the multiplication of organisms, will be able to retard by only a few thousand years the enveloping of the whole Earth by ice and the death of Nature from cold."
Also the great Scottish geologist Sir Charles Lyell (1791-1875) tried to solve the mystery of past climate changes, evidenced by more and more paleontological and geological clues. Lyell denied sudden changes, as they could not be explained at the time without invoking mysterious or unknown forces. Lyell also considered geological time as a cycle, idea adopted from astronomy and the celestial movements of the planets. He compares the history of the Earth and the climatic changes that have occurred with a "geologic year", with fall, winter, spring and summer – as ordered and similar to the movements of earth around the sun.
Animal and plant species were perfectly adapted to these "geological seasons". Lyell denied in principle the possibility of extinction, he regarded the apparent loss of species in the fossil record as an artefact of the very imperfect preservation of the stratigraphic column, also if a "geological" season ended, some animal and plant species did diminish in abundance (making it more improbable to become fossilized) and meanwhile others flourished, a pattern reversible at any time. According to Lyell so it was possible that one day:
"Then might those genera of animals’ return, of which the memorials are preserved in the ancient rocks of our continents. The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyl might flit again through the umbrageous groves of tree ferns."
That mammals during certain geological periods were not the dominant group in the fossil record was just the fault of the climate, unfavourable for their development, but favourable for reptiles or fish.
Fig.2. Lyell’s vision of cyclic time and climate inspired one of most famous caricatures in geology. In the same year in which the first volume of "Principles of Geology" was published (1830), in the introduction of a book about natural oddities (Francis Buckland’s "Curiosities of Natural History", edition of 1858) prominently figured "Professor Ichthyosaurus", lecturing to his fellow reptilian students. The caricature was drawn the geologist Henry de la Beche (1796 -1855), who imagined a palaeontology lesson in a distant future, in which, after a climate change, as inferred by the tropical vegetation in the background, giant marine reptiles discuss the inferiority of strange creatures with weak jaws and small teeth – the mammals, who have lost heir dominant role during the long gone ice age period.
Lyell in his "Principles of Geology" (first edition 1830-1832) explained these cyclic climate changes as a slow, steady process, controlled mainly by the distribution of land and sea on the globe and the differences absorption of heat and radiation by rocks and water. Land distributed mainly on the poles causes a global cooling, if land clusters around the equator, absorbing much of the solar radiation, the result was a global warming.
Fig.3. Global map as published by Lyell in his "Principles of Geology" (8th edition 1850) to illustrate the past climatic changes.
Lyell assumed that through sea level changes equivalent land areas (compared with the modern continents) can emerge or sink. He wrote:
"When land is massed in equatorial and tropical latitudes polar climates are mild. The land, heated to an excess under the equatorial sun, gives rise to warm currents of the air that sweep north. On the other hand, land massed around the poles produces the reverse effect. There is no land at the equator to soak up heat and no warm winds coming into Polar Regions."
Soon after Lyell’s book was published, the simple view of a steadily cooling or warming climate was challenged by the proposed influence of gases in the atmosphere or the number of sunspots on the temperature of Earth. But the most important discovery was the recognition of sediments from ancient glaciers extending far from the mountains or poles to the mid latitudes. Such glaciers and ice caps could be explained only by the new emerging theory of ice ages, characterized by suddenly occurring warm and cool climate in the geologically recent past.
In 1842 the French mathematician Joseph Alphonse Adhémar (1797-1862) elaborated a hypothesis to explain cyclic occurring ice ages. He calculated the variations of the "direction" and declination of Earth’s axis and the "movements" of Earth around the sun during the past, these parameters influence the time and amount of solar radiation that reach Earth, causing cyclic climatic changes. Adhémar proposed that in a period of 11.000 years the hemisphere that experiences a longer winter would develop in an ice age. But there was a major flaw in his idea, in 1852 the German Geographer and explorer Alexander von Humboldt (who contributed notably to the understandings of climate by his meticulous observations in South America) noted that Adhémar didn’t consider that even if one hemisphere experiences lower radiation, the opposite hemisphere experience an increase, so in the end the total sum of energy on the planet remains more or less unvaried.
Despite this first failure to explain the climate of Earth, the work of Adhémar had the important effect to inspire later research and naturalists.
In 1833, James Croll (1821-1890), son of a poor stonecutter of Perthshire in Scotland, purchased a copy of the "Penny Magazine", a magazine intended for children education. He was fascinated and began extensively to read about science: "At first I was totally confused, but then the beauty and simplicity of the ideas provided me with delight and surprise, and I began seriously to study the matter."
After troubling years, Croll found work as maintenance supervisor of the Andersonian College in Glasgow, where he had access to the library, a resource that he grateful exploited.
"At that time, the question of what could have triggered the ice age was much discussed among geologists. So in the spring of 1864, I directed my attention to this topic."
From 1864 onwards Croll corresponded with Charles Lyell on the connection between ice ages and variations in the Earth’s orbit. Croll used in his calculations an important factor that Adhémar did not yet know, the "movements" of the perihelion and aphelion along Earth’s ecliptic (the precession of the equinoxes). In 1875 he published his calculations in a work entitled "Climate and Time, in Their Geological Relations".
The geologist Archibald Geikie wrote about the work of Croll:
"The astronomical theory seems to me the best solution to the present ice age riddle. It bears in it all the decisive factors for the occurrence of alternating cold and warm periods, and accounts for the peculiar character of glacial and interglacial climates."
But there was still a problem, even if dating methods at these times were primitive and at best approximate, geological evidences supported a very young age of the mapped glacial deposits, but after Croll´s theory the last glacial period ended 80.000 years ago. When Croll died, highly respected, geologists considered his theory wrong.
Geikie resumed: "It may well be, that with certain modifications of his views we will solve the secret. But for now we must be continue to work and wait."
It was the Serbian Milutin Milankovitch (1879 – 1958) who despite recognizing the previous achievements noted also the insufficient data and errors made by his predecessors.
Between 1912 and the beginning of the first World War Milankovitch published some preliminary abstracts of his developing theory based on better measurements, concluding that all three astronomical factors, in contrast to previous authors, are important to explain Earth’s climate. Finally in 1920 he published his final draft of the theory "Mathematical theory of thermal phenomena caused by solar radiation", where he reassumed:
-Periodic Glaciations are caused by variations of astronomical parameters
-The three known astronomical parameters influence the amount of solar energy on Earth’s surface, especially during summer on the northern hemisphere
- It is possible to calculate these changes and so calculate the climate in the past.
Fig.4. Outcrop of the Trubi-Formation at Capo Spartivento (Calabria, region in southern Italy), a succession of marls from the Pliocene-Pleistocene transition (ca. 1,8 million years). The regular darker horizontal "Zebra-stripes" are caused by organic rich layers, thought to be formed by modifications in the biological productivity of the former sea in response of changes in climate and of the astronomical parameters – a "recent" example of the Milankovitch cycles.
Today other parameters are known to influence the climate of Earth as well, like the distribution of sea and land, the amount of dust, gases and vapour in the atmosphere, vegetation or the activity of the sun to mention some.
However like a pacemaker controls the beats of a heart, astronomical parameters influenced the rhythms of climate in geological past until the time a new parameter showed up – men and his industry.
CHORLTON, W. (ed) (1985): Ice Ages (Planet Earth). Time-Life Books: 176
CROLL, J. (1875): Climate and Time, in their Geological Relations. A theory of secular changes of the Earth’s Climate. D. Appleton and Company, New York: 630
FLEMING, J.R. (1998): Charles Lyell and climatic change: speculation and certainity. In: BLUNDELL, D.J. & SCOTT, A.C. (eds) Lyell: the Past is the Key to the Present. Geological Society, London, Special Publications, 143: 161-169
LYELL, C. (1850): Principles of Geology or, the modern changes of the Earth and its inhabitants – considered as illustrative of Geology. 8th revised edition, John Murray, London: 811
RUDWICK, M.J.S. (2005): Bursting the limits of time – The reconstruction of Geohistory in the Age of Revolution.The University of Chicago Press, Chicago, London: 708
About the Author: David Bressan is a freelance geologist based in the European Eastern Alps. He graduated with a project on Rock Glaciers dynamics and hydrology in the Ötztal-Alps, this phase left a special interest for quaternary deposits and modern glacial environments. Following the traces, studying old maps and reading historic reports on glaciers he became interested in the development of geomorphologic and geological concepts by naturalists and geologists trough time, so he combines curiosity, field trips and old depictions to blog about the History of Geology.
The views expressed are those of the author and are not necessarily those of Scientific American.
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