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James Croll, FRS, (2 January 1821 – 15 December 1890) was a 19th-century Scottish scientist who developed a theory of climate variability based on changes in the Earth's planetary orbit.
In the 1850s he managed a temperance hotel in Blairgowrie, and was then an insurance agent in Glasgow, Edinburgh and Leicester. In 1859, he became a janitor at the museum of the Andersonian University in Glasgow. He was able to use the university library to get access to books, and taught himself physics and astronomy to develop his ideas.
From 1864, Croll corresponded with Sir Charles Lyell, on links between ice ages and variations in the Earth's orbit. He theorised that changes in the Earth's orbit could cause the Gulf Stream to be diverted, bringing less heat to the Arctic. More ice would then lead to more sunlight being reflected, causing a feedback loop. His ideas were published in the Philosophical Magazine in 1864, attributed to "James Croll, Anderson's University".
This led to a position in the Edinburgh office of the Geological Survey of Scotland, as keeper of maps and correspondence, where the director, Sir Archibald Geikie, encouraged his research. He published a number of books and papers which "were at the forefront of contemporary science", including Climate and Time, in Their Geological Relations in 1875. He corresponded with Charles Darwin on erosion by rivers.
In 1876, he was elected Fellow of the Royal Society, and awarded an honorary degree by the University of St Andrews. He retired in 1880 because of ill health.
Croll's work has been widely recognised in scientific circles and societies. The highest award of the Quaternary Research Association is the James Croll Medal.
Croll's theory predicted multiple ice ages, asynchronous in northern and southern hemispheres, and that the last should have ended about 80,000 years ago. Evidence was just then emerging of multiple ice ages, and geologists were interested in a theory to explain this. Geologists were not then able to date sediments accurately enough to determine if glaciation was synchronous between the hemispheres, though the limited evidence more pointed towards synchronicity than not. More crucially, estimates of the recession rate of the Niagara Falls indicated that the last ice age ended 6,000 to 35,000 years ago – a large range, but enough to rule out Croll's theory, to those who accepted the measurements.
Croll's work was widely discussed, but by the end of the 19th century, his theory was generally discredited. However, the basic idea of orbitally-forced insolation variations influencing terrestrial temperatures – now known generally as Milankovitch cycles – was further developed by Milutin Milankovitch and eventually, in modified form, triumphed in 1976. A number of scholars have argued that Croll's contribution was sufficiently fundamental as to describe the orbital cycles as Croll-Milankovitch Cycles.
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