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  • Roland Jackson

Did Eunice Foote discover the 'greenhouse effect'?

The story of the American Eunice Foote and the discovery of the greenhouse effect has become a contested issue in the history of climate science, particularly pertinent at this time of the new IPCC report, the UN Climate Change Conference (COP26), and the broader issue of the proper recognition of women’s contributions to scientific discovery. Here's my latest take on this tangled web.

So, who was Eunice Foote? She was born in 1819 to Theriza and Isaac Newton, supposedly a distant relation to the Isaac Newton, and she attended Troy Female Seminary in New York state. The school had been founded by Emma Willard, who believed in educating young women in all subjects, including science. In 1841, Eunice married the judge and inventor Elisha Foote. They lived briefly in Seneca Falls, where they were neighbours and friends with the suffragette and abolitionist Elizabeth Cady Stanton. Both Footes were involved in the women’s rights movement. They attended the famed Seneca Falls Convention and signed the 1848 Declaration of Sentiments, a document protesting women’s disenfranchisement. I can’t show you an image of Foote, as no photograph of her has yet been identified, although if her relative Liz Foote finds one news will appear here.

As well as introducing her, I had better define what I mean by the greenhouse effect. Although the term greenhouse effect was not used until it was coined by Nils Ekholm in 1901, it refers to the process initiated by the absorption, by gases such as carbon dioxide and water vapour, of longwave infrared radiation from the Earth’s surface. Those gases radiate in turn, into the atmosphere and towards the Earth’s surface. The result is that both the surface of the Earth and the atmosphere become warmer than they would be if there were no such gases preventing the radiation from escaping directly into space. The important point is that the greenhouse effect is not directly caused by incoming solar radiation, which mostly passes into and through the atmosphere to be absorbed at the Earth’s surface or simply reflected back into space. And incidentally, an actual greenhouse works by a completely different mechanism, mainly of preventing heat loss by convection. I don’t think anyone is suggesting that the Earth is surrounded by a glass sphere. The term is of course metaphorical.

The existence of what we now call the greenhouse effect was first theorised by Joseph Fourier in the 1820s. Fourier calculated from theory that the earth should be cooler than it is, given incoming radiation, and suggested that the atmosphere might act as an insulator. In 1824, he wrote that 'the temperature [of the Earth] can be augmented by the interposition of the atmosphere, because heat in the state of light finds less resistance in penetrating the air, than in repassing into the air when converted into non-luminous heat'. In other words, the solar radiation mostly gets through, but radiation from the Earth’s surface, heading out to space, is more readily absorbed by the atmosphere. Claude Pouillet developed Fourier’s work mathematically. In 1836, he wrote that 'the atmospheric stratum...exercises a greater absorption upon the terrestrial than the solar rays'. He further suggested that water vapour and carbon dioxide might trap heat, thus warming the atmosphere. So in a sense I have already answered the question I posed. Eunice Foote didn’t discover the greenhouse effect. But it’s not quite as simple as that, because it depends what one means by ‘discovered’.

The person generally credited with discovering by experiment the basic mechanism of the greenhouse effect is the Irish physicist John Tyndall, working at the The Royal Institution of Great Britain in London. In doing so he referenced the prior work of Fourier and Pouillet. Tyndall used a copper cube containing boiling water at 100oC as his heat source, which emitted, in modern terms, longwave infrared radiation, like the Earth’s surface does. In 1859, he demonstrated the absorption of this radiation by gases including carbon dioxide and water vapour, and he showed that these gases also radiate heat. He wrote, in a sense rephrasing Fourier and Pouillet: ‘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’. And he continued in 1861: ‘if, as the above experiments indicate, the chief influence be exercised by aqueous vapour [that is, water vapour], every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid [that is, carbon dioxide] diffused through the air. And he went on: Such changes may in fact have produced all the mutations of climate which the researches of geologists reveal’.

So, Tyndall experimentally detected the absorption and radiation of longwave infrared by certain gases. He then used that both to explain the basis of the greenhouse effect and to relate it to possible climate change caused by water vapour and carbon dioxide.

But was he in fact the first to do that? This is where Foote comes in.

Foote’s experiments were carried out in 1856, three years before Tyndall’s. And they had a different motivation. Tyndall was driven by a desire to show that gases absorb radiant heat, or longwave infrared, like liquids and solids do. To his knowledge, this had not previously been demonstrated, although some like the Italian Macedonio Melloni had tried. Tyndall thought that absorption would depend on the molecular constitution of the gases, and his motivation was very much one of fundamental physics. It was certainly not motivated by a desire to understand climate change, and indeed he subsequently took no further interest in the topic, except in the context of meteorology, which is another story.

Foote’s experiments, by contrast, were driven by a desire to make sense of the effect of solar radiation on atmospheric temperatures. During one of the many Twitter spats that take place around the nature and significance of Foote’s work, I exchanged thoughts with Joe Ortiz, a climate palaeontologist in the US, and we wrote a paper about the motivation, design and interpretation of her experiments.

Her 1856 experiments seem to have been stimulated by letters in Scientific American that discussed theories about how the Sun heats the Earth, based on the question of why the tops of mountains are generally colder than valleys. Some believed it was the angle of the sun’s rays that affected the heating, while others believed it was the density of the air at different altitudes that determined temperature.

Foote explored these and related questions using two identical glass cylinders, each containing identical thermometers. Using an air pump, she exhausted air from one cylinder and added air into the other. After the temperature equalized, she placed both jars next to each other in the sun and recorded the resulting temperature every two to three minutes. She also conducted the experiment with both jars in the shade as a control. In comparing the temperature changes, she observed that, 'the [thermal] action increases with the density of the air, and is diminished as it becomes more rarified.' She then repeated the experiment, comparing moist to dry air by adding water to one cylinder and dehydrating the other using calcium chloride. From this test, she discovered that damp, humid air became significantly hotter than dry air.

Last, she measured the effect of different gases against 'common air', the ambient atmosphere, and found: 'the highest effect of the sun’s rays… to be in carbonic acid gas.', or in our terms carbon dioxide.

She then suggested that additional carbon dioxide in the atmosphere would cause global warming, 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 necessarily have resulted.'

So she does seem to have been the first person to detect the strong heat-absorbing properties of carbon dioxide and to suggest a link between carbon dioxide levels and climate change. Those are major discoveries that need proper recognition. But she made no attempt to explain the mechanism, unlike Tyndall, and referred to no previous work, such as that by Fourier and Pouillet. Moreover, it is not clear what is actually doing the heating in her experiments. She was using visible light not infrared, which is responsible for the greenhouse effect. It’s unclear at the moment to what extent the gases in her experiment were heated by direct sunlight or by sunlight heating the glass which in turn radiated infrared inside the jar, and also heated the contents by conduction and convection. It was probably some combination of various mechanisms. The glass itself would be opaque to infrared.

Foote’s work was presented at the 1856 meeting of the American Association for the Advancement of Science, after a rather longer and much less interesting paper by her husband. But though she may have been present, the paper was read by the leading American physicist Joseph Henry. Neither Foote’s nor her husband’s papers was published in the proceedings, and they were not even mentioned as having been given, unlike many others. But hers was noticed. Scientific American published an enthusiastic article, there were reports in several newspapers, and both papers by the Footes were published in the American Journal of Science and Arts. Across the Atlantic it was rather different. Two short 8-line summaries of Eunice’s paper appeared in a Scottish and a German journal, although neither mentioned the climate suggestion. Elisha’s paper, but not Eunice’s, (which is actually much more interesting in hindsight) was republished in the Philosophical Magazine in Britain.

So the question arises, did Tyndall know of her work? Some people have claimed that he did, and further that he stole it without acknowledgement.

My view is that on balance he didn't. No reference to Foote’s work has been found in any of the papers or correspondence of the British and other European physicists of the period working in this field. Later direct correspondence between Tyndall and Joseph Henry makes no retrospective mention of her. Tyndall even performed his experiment in a way that indicated he did not know of her conclusions. If he was aware that water vapour and carbon dioxide would absorb heat, it is likely he would have tested those first. Instead, he started with gases like oxygen and hydrogen, which Foote explicitly mentioned did not display any significant heat-absorbing properties.

Perhaps Henry bears some of the blame. He is quoted in one newspaper report as saying: 'that although the experiments were interesting and valuable, there were [many] [difficulties] encompassing [any] attempt to interpret their significance.' Why did he say that? And why did the British and Europeans overlook the findings? It is possible that readers saw Foote’s name and skipped over her paper because of her gender. But I don’t think it is just because she was a woman or an amateur. For example, her second paper, on electrical excitation, was published in the Philosophical Magazine the following year. Although it must be said that the American David Wells mentioned Foote’s research in his Annual of Scientific Discovery without citation, despite citing other scientists whose work he referenced, all of whom were male. But if you look at the Scientific American article, the author recognized that the work, by a woman, was interesting, and still nobody picked up on it. Perhaps European bias came into play, and the lack of the sort of regular scientific communication, especially across the Atlantic, that is taken for granted today. But it is still curious.

Tyndall’s paper was hailed as a scientific breakthrough and became part of the climate science cannon. William Thomson himself, later Lord Kelvin, remarked on reviewing the paper that the findings were novel, suggesting that he too did not know of Foote’s work. He was not the only prominent physicist who was unaware. For example mathematical physicist Peter Guthrie Tait, an absolute sworn enemy of Tyndall, never brought up Foote’s work. If he had thought that Tyndall had essentially stolen Foote’s work I think he would have gone to town. Here the trail goes murky, and the big question for me is why Foote’s work was apparently forgotten or ignored by everyone until rediscovered by the geologist Raymond Sørenson only just ten years ago.

Foote only published one more paper, in 1857, and her two published papers are the only ones in physics by any American women before 1889. She did take out several patents for inventions and probably assisted her husband in some of his, but that was the end of her scientific research. She deserves much more recognition, and she illustrates, if we needed any more illustration, the invisibility of so many capable women. So, she may not have formally discovered nor explained the greenhouse effect, but what she did was a major achievement, and in my view she should be seen as one of the founder contributors to understanding climate science.

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