JOHN LESLIE ON THE PALEOCLIMATIC SIGNIFICANCE OF SWISS MORAINES IN 1796

Sir John Leslie (1766-1832) was professor of mathematics and natural philosophy at the University of Edinburgh, preceding the pioneer Scottish glaciologist James David Forbes and succeeding John Playfair, author of Illustrations of the Huttonian Theory of the Earth (1802). Leslie was a contemporary of Joseph Fourier, and like him was devoted to the study of heat and its propagation. In 1804, Leslie challenged Buffon’s theory that the Earth is slowly cooling, arguing instead that it must “grow gradually warmer by the accession of the solar rays”. He calculated that the entire Earth is warming by one degree every 2405 years. It must be remembered that this was 20 years before Fourier’s (1824) concept of radiative energy balance. Indeed, the existence of infra-red rays (which the Earth emits) was only discovered in 1800.

Leslie supported his concept of global warming with geological evidence. In company with pioneer photographer Thomas Wedgwood, he had gone to the continent in 1796 and taken a coach over Grimsel Pass in the Swiss Alps, from the headwaters of the Rhône to those of the Aar. Leslie recognized that the innumerable ridges of loose angular stones, vegetated in proportion to their distance from active glaciers, mark the former extents of those glaciers at a colder time. He noted that “this remarkable fact is known to every tourist”. He concluded that the “line of perpetual congelation must have descended at least 2000 feet lower than its present station. At that remote period, the lake which now feeds the Aar was probably a glacier.”

It would be decades before the same conclusions were reached by Venetz (1822), Charpentier (1835) and Agassiz (1837). It is true that Hutton (1795) and Playfair (1802) had earlier ascribed the granite erratics on the Jura to glacial action, but neither had ever seen a glacier and their attention was fixed on timeless processes of erosion. Both attributed the former extent of Swiss glaciers to a greater elevation of the Alps (as did Charpentier), not to climatic change. Leslie had the advantage over later geologists that he was not encumbered with the (false) assumption, subsequent to Fourier (1824), that Earth’s surface temperature is steadily in decline, due to secular dimming of the sun. Leslie’s prescient intepretation of Swiss moraines was previously noted by de Beer (1932) and Cunningham (1990).