Eunice Foote and the dawn of climate science
The American Eunice Foote is finally getting the recognition she deserves. In 1856 she showed, using sunlight, that gases including carbon dioxide and water vapour can absorb heat. She then suggested that changing amounts of carbon dioxide in the atmosphere could result in a change of climate. She was the first person to do either of these things. Her work was forgotten until about ten years ago, and is rightly generating renewed interest now. But exactly what she did or didn’t discover, and why, has become a hot topic. Foote is pitched against the Irish physicist John Tyndall, who detected the absorption by carbon dioxide and water vapour of what we now call longwave infrared radiation in 1859. From that he explained the physical basis of the greenhouse effect and made the link to climate change. Did he know of Foote’s work? Probably not, though it can’t be ruled out. Neither he nor any of his contemporaries in Britain or on the Continent mentioned her discovery in correspondence, so far as we know.
But what drove the research of these two, and what did they actually demonstrate? I had written previously about Tyndall’s experiments, and whether he knew of Foote’s work, but I wanted to know more about what inspired her experiments. A chance encounter on Twitter during lockdown led to a collaboration with Dr Joe Ortiz, as we joined together expertise in climate science and the history of science to take a closer look. Our new paper 'Understanding Eunice Foote’s 1856 experiments: Heat absorption by atmospheric gases' has just been published in Notes & Records.
Tyndall’s rationale was clear. He knew that solids and liquids, but not gases, had been shown to absorb heat. But he thought that gases, since they contained the same molecules as the liquid or solid, should also do so. For him, it was a problem in molecular physics. The Italian Macedonio Melloni had tried to detect the absorption of heat by gases, but not succeeded. Tyndall built some very sensitive apparatus and soon detected the absorption by several gases of heat radiated from a metal cube containing boiling water. He also realised, and explained, that the absorption of heat from the Earth’s surface by gases like water vapour and carbon dioxide in the atmosphere, and their subsequent radiation of heat, would result in an increase of the Earth’s temperature compared with the absence of an atmosphere. That’s what we now call the greenhouse effect. The existence of this effect was already known, and Tyndall referred to Joseph Fourier and Claude Pouillet, who had theorised it a few decades earlier. What Tyndall did was to detect and explain the basis of the physical mechanism.
The drivers for Foote’s work were different. Unlike Tyndall, she mentioned no previous scientists in her published paper. At no point did she describe or theorise the greenhouse effect. But she did show, using sunlight as a heat source, that carbon dioxide and water vapour absorb heat. She found the strongest effect with carbon dioxide, and wrote that if at an earlier period of the Earth’s history there had been a higher proportion of carbon dioxide than at present, an increased temperature of the atmosphere must have resulted. But why was she interested in carbon dioxide?
It is a contemporary report of her paper in Scientific American that gives the best clues to her motivations. Foote started by examining the effect of the density of the air on the absorption of ‘the sun’s rays’. She did not explain why, but the Scientific American article pointed to a suggestion by a correspondent that ‘that density of the atmosphere, and not the angularity of the sun’s rays, was the principal reason why it was warmer in valleys than on the tops of mountains’. Foote showed experimentally, unlike any other correspondents, that increasing the density did have an effect. She then moved on to examine the composition rather than the density of gases, using first moist air and then carbon dioxide. It was the latter that showed the greatest effect. Foote brought together both density and composition in her conclusion, writing: ‘An atmosphere of that gas would give to our earth a high temperature; and if as some suppose, at one period of its history the air had mixed with it a larger proportion than at present, an increased temperature from its own action as well as from increased weight must have necessarily resulted.’ So it was the nature and amount of the gas that mattered. The Scientific American article again gives the clue as to why carbon dioxide was significant: ‘It is believed and taught by geologists that during the period preceding the carboniferous era,– when the coal bed materials were forming – that the atmosphere of the earth contained immense quantities of carbonic acid [carbon dioxide], and that there was a very elevated temperature of atmosphere in existence, in comparison with that of the present day. Those who believe that this earth was once a fiery ball, attribute this ancient great atmospheric heat to the elevated temperature of the earth; but Mrs. Foot’s [sic] experiments attribute it to a more rational cause, and leave the Plutonists but a small foundation to stand upon for their theory.’
It is tantalising that Foote’s published paper does not mention these arguments, but the Scientific American article points strongly to her motivations. Foote’s paper was presented at the 1856 meeting of American Association for the Advancement of Science (AAAS) not by her, but by Joseph Henry, director of the Smithsonian Institution. It was not published or even mentioned in the proceedings of the meeting, but was printed instead in the American Journal of Science and Arts. The whole episode is curious. It is doubtless a reflection of the fact that she was a woman. It was rare indeed for women to contribute to AAAS meetings. Indeed, Eunice Foote’s second paper, the following year, was the first by a woman to be published in the proceedings. These were the only two papers that she wrote. They are the only two papers by any American woman in physics before 1889. Had Foote been welcomed into the scientific community, who knows what she might have achieved. She was a women’s rights campaigner and inventor, and deserves far more recognition.