[[Image:James Croll.jpg|thumb|right|150px|James Croll (1821-1890) Source:Irons, James Campbell, 1896. Autobiographical Sketch of James Croll with Memoir of his Lifeand Work. London.]]

James Croll (1821-1890) was a Scottish physical scientist who was the leading proponent of an astronomical theory of [[climate change]] in the nineteenth century. His fundamental insights into the interplay of [[astronomy|astronomical]] and [[geology|geological factors]] – into cosmic [[physics]] and [[climate dynamics]] -- were extremely influential. In 1875 Croll published his major book, Climate and Time, a work delayed several years due to ill health, but a work widely admired for the profound impression it produced on geologists around the world. The leading geologist of his day, [[Lyell, Charles|Charles Lyell]], revised his Principles of Geology in response to Croll’s theory.

In 1864, Croll published an article in the Philosophical Magazine “On the Physical Cause of the Change of Climate During Geological Epochs.” In this paper Croll introduced changes in the [[earth's orbit|earth's orbital elements]] as likely periodic and extraterrestrial mechanisms for initiating multiple [[glacial epoch]]s.

Croll proposed that the eccentricity of the [[earth’s orbit]] was sufficiently great to “account for every extreme of [[climatic change]] evidenced by [[geology]].” His theory of [[ice age]]s took into account both the precession of the [[equinox]]es and variations in the shape of the earth's orbit. It predicted that one hemisphere or the other would experience an ice age whenever two conditions occur simultaneously: a markedly elongate orbit, and a [[winter solstice]] that occurs far from the [[sun]].

These cosmical factors provided a mechanism for multiple [[glacial epoch]]s and alternating cold and warm periods in each hemisphere. In other words, when the Northern Hemisphere was in the grips of an ice age, the Southern Hemisphere would be in an interglacial. As the earth's orbital elements varied, this situation would eventually be reversed. Feedback mechanisms, such as [[radiative effect]]s of the ice fields, enhanced formation of [[cloud]] and [[fog]], changes in [[sea level]], and the mixing and redirection of warm and cold [[ocean current]]s would serve to enhance the climatic changes initiated by the orbital elements.

[[Image:Croll record.jpg|thumb|left|250px|Variations in the earth’s orbit for three million years before 1800 A.D. and onemillion years after it. (Source: Croll, 1875, following p. 312)]]

The astronomical theory of [[climate change]] emerged between 1864 and 1890 with the work of James Croll. However, because of uncertainties in the timing of [[ice age]]s and in the [[stratigraphic record]], and because Croll’s theory predicted glaciation in only one hemisphere, the theory was largely disregarded for at least three decades. The astronomical theory reemerged from [[eclipse]] and was formulated into a mathematical theory of insolation by [[Milankovi?, Milutin|Milutin Milankovi?]] between 1920 and 1941. But even the [[Milankovi? theory]] “had an uneven run” and was in partial eclipse until the 1960s, in part because of [[thermal lags in the climate system]] and in part because of the unexplained lack of continental [[glaciation]] prior to the [[Pleistocene]]. In 1976 the theory received new confirmation from the paleostratigraphic work of James Hayes, John Imbrie, Nicholas Shackelton, and others who documented the astronomical signals in a number of independent proxy climate records.

During Croll’s lifetime the [[Great Ice Age]] had been discovered and notions of multiple glacial and “[[Crollean]]” [[interglacial epoch]]s were being debated. Many of the major mechanisms of climatic change had been proposed, if not yet fully explored: changes in [[solar output]], changes in the [[Earth's orbital geometry]], geographical changes, and changes in [[atmospheric transparency]] and [[atmostpheric composition|composition]]. New climate theories were being introduced and new work was being done on [[heat budget]]s, [[spectroscopy]], and the [[carbon dioxide]] content of the atmosphere. The stratographic sequence had not been worked out and many geologists still thought that [[glacial drift deposit|glacial “drift” deposits]] had been carried by [[iceberg]]s. Through such tempestuous theoretical waters, Croll kept a steady course, negotiating between cosmic and terrestrial [[physics]] on the one hand (as exemplified by [[Herschel, Frederick William|Herschel]] and [[Kelvin, William Thomson|Lord Kelvin]]) and [[geology]] on the other, as practiced by [[Lyell, Charles|Lyell]], [[Darwin, Charles|Darwin]], and the Geikie brothers.

Editor's Note: This entry is adapted from [[user:james.fleming|James Rodger Fleming]], [http://www.meteohistory.org/2006historyofmeteorology3/3fleming_croll.pdf James Croll in Context: The Encounter between Climate Dynamics and Geology in the Second Half of the Nineteenth Century], History of Meteorology 3 (2006):43-53

Further reading

• Croll, James, 1875. [http://books.google.com/books?id=JBsAAAAAQAAJ&pg=PA1&dq=Climate+and+Time+in+Their+Geological+Relations Climate and Time in Their Geological Relations]. A theory of secular changes of the earth's climate. New York.
• Fleming, James Rodger, 1998. Historical Perspectives on Climate Change. New York. [{{ISBN| |1=0195078705}}]
• Irons, James Campbell, 1896. [http://books.google.com/books?id=VppNOpMbdLAC&pg=PA9&dq=Autobiographical+Sketch+of+James+Croll+with+Memoir+of+his+Life+and+Work Autobiographical Sketch of James Croll with Memoir of his Life and Work]. London.
• Lyell, Charles, 1866-68. [http://books.google.com/books?id=iroQAAAAIAAJ&pg=PA1&dq=Principles+of+Geology Principles of Geology], 10th ed. London.