John Tyndall (c.1822–1893), Irish physicist, mountaineer, and public intellectual, is best known for his work on the absorption of heat by gases such as water vapour and carbon dioxide in the atmosphere (and for explaining why the sky is blue). Seen in retrospect, he is a critical figure in the history of climate science. Yet this retrospective view hides a complex, and in many ways more interesting story.

In The Ascent of John Tyndall, the first major biography of Tyndall for more than 70 years, I unpick the motivations behind Tyndall’s work on the absorption of heat by gases, which started in 1859, and the manner in which he interpreted them.

Guest post by Roland Jackson

Tyndall had considered the topic for several years. He read Macedonio Melloni’s work on the absorption of heat by liquids and solids around 1850, and had frequently discussed the issue with friends. His work in mountain environments, studying glacier motion, rekindled that interest. He had studied air bubbles in ice, the formation of flower-shaped structures in ice by a focused beam of light, and thought about the conduction of heat through ice. He was intrigued by solar radiation, and now turned his attention to the atmosphere, to examine its interaction with solar and terrestrial radiation, and to investigate the remarkable conditions of temperature in mountain regions. His aim was to do for gases what Melloni had done for liquids and solids. As a physicist, there was a further driving force. Tyndall was convinced that not only the physical, but also the chemical composition of substances—and specifically their molecules—played a part previously unrecognized in radiation and absorption. He would be probing the nature of molecules themselves using radiation.

Samples of air in the laboratory absorb very small quantities of heat. Tyndall believed that no one, up to that point, had been able to demonstrate any absorption. Melloni, for example, had not been able to do so. Showing, as ever, considerable experimental skill, Tyndall invented a differential spectrometer to detect the absorption of heat by small quantities of gases held in a sample tube. He was successful almost immediately. On 9th May 1859 he tried various experiments without effect, but on 18th May he could write ‘the subject is completely in my hands’. A week later, effectively claiming his priority, he announced his results to the Royal Society, and then in a lecture at the Royal Institution. In fact, though there is no evidence that he or anyone else in Europe knew it, the American Eunice Foote had discovered the absorption of heat by carbon dioxide and water vapour in 1856, three years before Tyndall, and should be credited with the discovery. Her apparatus was much less capable of accuracy, and she was not able to follow it up, but her story is a fascinating one.

Tyndall took as his starting point the idea from Joseph Fourier, Claude Pouillet, and William Hopkins that the atmosphere could allow heat from the Sun to pass through it more easily than heat emanating back from the Earth. In his experiments, Tyndall demonstrated the ability of many individual gases to absorb heat, though the only one he specified in his report and lecture summary was coal gas. But he concluded: ‘Thus the atmosphere admits of the entrance of the solar heat; but checks its exit, and the result is a tendency to accumulate heat at the surface of the planet’. Tyndall had demonstrated and explained the physical basis of what we now call the greenhouse effect. He was keen to establish his priority, and published reports in continental journals, including Cosmos, Il Nuovo Cimento, and the Bibliothèque Universelle. It was only in the last of those that he specifically mentioned his discovery of the absorption of heat by water vapour and carbon dioxide.

What interested Tyndall most was water vapour, because of its evident linkage to weather and local atmospheric conditions. More than a year after his initial experiments, on 20th November 1860, Tyndall determined for the first time the substantial absorption of heat by water vapour compared to dry air. As he refined his apparatus for measuring the absorption of heat by water vapour and other gases, the strong absorption by water vapour struck him ever more forcibly.

The results of his work in the autumn of 1860 were written up into his first major paper on the absorption of heat by gases, published in 1861 and given as the Bakerian Lecture to the Royal Society. This is arguably the founding paper of climate science, seen in hindsight.

It was immediately apparent to Tyndall that the absorption and emission of heat by water vapour could explain differences between air temperatures at midday and evening, or the temperature at the top of a mountain compared to the bottom. But he also claimed directly that changes in the amount of water vapour, carbon dioxide, or hydrocarbons, all of which absorbed heat, could have climatic effects. He wrote: ‘if, as the above experiments indicate, the chief influence be exercised by aqueous vapour, every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid [carbon dioxide] diffused through the air’.

Though there was public discussion in the latter part of the nineteenth century about implications for the climate of burning huge quantities of coal, it was not until 1896 that the actual warming effect of carbon dioxide was estimated, by the Swedish chemist Svante Arrhenius. And it was not until 1938 that Guy Callendar made the quantitative connection between global warming and emission of the gas through human activity.

Tyndall never wrote about the impact of human activity on the climate. Indeed, regarding the burning of coal, he was more concerned that the US, with its vast supplies, would inevitably outcompete the UK economically.

Nevertheless, Tyndall’s accurate and extensive work, continued over many subsequent years, underpins our current understanding of the greenhouse effect, weather, and climate change. It seems fair to regard him as the founding figure in climate science.