Jean-Baptiste Joseph Fourier was a very busy man. The French mathematician and physicist was, at various times, a baron, imprisoned for his political activities in the French Revolution, and a scientific adviser to Napoleon during his Egypt campaign. Best remembered today for his eponymous mathematical and physical theorems about vibrations and heat transfer, Fourier also found time to play a pivotal role in our understanding of the Earth’s climate. In 1822, Fourier’s quest for a universal theory of terrestrial temperatures culminated in his magnum opus, Théorie analytique de la chaleur. The core of the book expounded the relationship between the Earth as a cooling body, the Sun as the heat source, and the atmosphere, delightfully described as the “diaphanous” intermediary, slowing the rate of heat loss from the Earth’s surface to space. Tangible evidence for the mechanism by which Fourier’s diaphanous intermediary might affect the Earth’s climate came decades later. The person traditionally credited with demonstrating the heat-trapping potential of different atmospheric gases was Irish physicist John Tyndall. A superb experimentalist, Tyndall devised a machine to measure the ability of different gases – nitrogen, oxygen, water vapour, carbon dioxide, ozone, methane – to absorb radiant heat. He published his work in 1859. Without wishing to diminish Tyndall’s great scientific breakthroughs, we now know that he was pipped to this particular post by three years. Eunice Newton Foote was a scientist, inventor, painter and women’s rights campaigner from New York. In a series of elegantly simple experiments, Foote compared how glass jars filled with air, water vapour and carbon dioxide heated up when placed in the sun. “The highest effect of the sun’s rays I have found to be in carbonic acid gas,” she wrote, using the contemporary term for carbon dioxide. With remarkable prescience, she went on to speculate that “An atmosphere of that gas would give to our earth a high temperature.” Foote’s work was presented at a conference of the American Association for the Advancement of Science in 1856. Since Foote was a mere woman and thus ineligible to be a member of that august body, a Professor Joseph Henry gave the presentation. He prefaced it with the words “The sphere of woman embraces not only the beautiful and the useful, but the true.” A nice if somewhat patronising gesture, though one can’t help but feel aggrieved that it was not until the discovery of the conference transcript in 2011 that Foote was finally given her due. Credits for the next few scientific breakthroughs in this story belong to a trio of Swedes. In 1894, geologist Arvid Högbom, seeking to understand the global carbon cycle, estimated that industrial sources of CO2 emissions (mainly from coal burning) were roughly equivalent to that which natural sources absorb and emit. Inspired by Högbom’s results, the great chemist Svante Arrhenius quantified the relationship between atmospheric CO2 and global temperatures, noting that “any doubling of the percentage of carbon dioxide in the air would raise the temperature of the earth’s surface by 4°”. But he went one step further, specifically arguing that the burning of fossil fuels by humans was substantial enough to affect the Earth’s temperature: “The enormous combustion of coal by our industrial establishments suffices to increase the percentage of carbon dioxide in the air to a perceptible degree.” Arrhenius thought all this would happen very slowly, perhaps over thousands of years, and would be a good thing. He speculated that by increasing CO2 in the atmosphere “we may hope to enjoy ages with more equable and better climates, especially as regards the colder regions of the earth, ages when the earth will bring forth much more abundant crops than at present, for the benefit of rapidly propagating mankind”. Meteorologist Nils Gustaf Ekholm, Arrhenius’s great friend and colleague, went one optimistic step further, noting that human control of the atmosphere might even prevent the arrival of the next ice age. We can also credit Ekholm for popularising the metaphor of the greenhouse. In 1901 he wrote that “the atmosphere may act like the glass of a green-house, letting through the light rays of the sun relatively easily, and absorbing a great part of the dark rays emitted from the ground, and it thereby may raise the mean temperature of the earth’s surface”. Arrhenius’s calculations were hotly disputed – sound familiar, anyone? – and over the next few decades became subsumed into a larger debate as to whether atmospheric changes caused the ice ages. But let us fast forward to the 1930s, to the work of English steam engineer, inventor and amateur climatologist Guy Callendar. Callendar painstakingly compiled measurements of temperatures around the globe from 1880 to 1935 and correlated these with measurements of atmospheric CO2 – surely one of the nerdiest hobbies of all time. He calculated that a 0.3 oC rise in global temperatures over the previous 50 years was largely due to the increased atmospheric CO2 from fossil-fuel burning. Callendar’s hand-drawn graph of the correlation, published with little fanfare in 1938 in the Quarterly Journal of the Royal Meteorological Society, looks like it was sketched on the back of the proverbial envelope, belying the meticulous work behind it. Echoing the earlier
optimism of the Swedes, Callendar concluded that the warming trends he graphed would be beneficial, delaying “the return of the deadly glaciers”. In a preview of later attitudes, the “Callendar Effect” was met with scepticism – most notably from Sir George Simpson, then director of the Meteorological Office, London, who thought the correlation between CO2 and temperature was merely a coincidence. Simpson had previously aired his view that CO2 in the atmosphere has “no appreciable effect on the climate” and made a point of describing Callendar as a “non-meteorologist”. Undeterred, Callendar doggedly continued to publish during the 1940s and 1950s. He eventually inspired US geochemist Charles Keeling to establish the Mauna Loa Observatory in Hawaii in 1958, the first scientific establishment dedicated to measuring atmospheric gases. By 1961, Keeling had already collected enough data to show that CO2 levels were rising steadily, and he produced the now-iconic “Keeling Curve”. Untrammelled optimism about the prospect of a world warmed by human activities finally started to pall a little in the 1960s. The most prominent and vocal worrier was Paul Ehrlich, whose 1968 book The Population Bomb is better remembered for its dire Malthusian warnings about the perils of too many people. But Ehrlich also wrote that “the greenhouse effect is being enhanced now by the greatly increased level of carbon dioxide … At the moment we cannot predict what the overall climatic results will be of our using the atmosphere as a garbage dump.” By the mid 1970s, the term “global warming” had begun to appear in the scientific literature (although some in the mainstream news media were expressing concern that the opposite phenomenon was occurring). And in the 1980s increasingly strong statements from international bodies such as the United Nations Environment Programme pointed to projections of significant warming over the next century. But the most newsworthy event came on a sweltering June day in 1988, when James Hansen, the director of the NASA Goddard Institute for Space Studies, was invited by Senator Al Gore to testify to the Senate Committee on Energy and Natural Resources. Hansen told the assembled lawmakers that he was 99 per cent certain that the year’s record temperatures were not the result of natural variation. Global warming, he said, “is already happening now” and “it is time to stop waffling so much and say that the evidence is pretty strong that the greenhouse effect is here”. (Sorry to be a pedant, but what he really meant was the greenhouse effect, see above. Apart from that, Jim was absolutely spot on!) Hansen’s testimony sparked headlines all over the world and is regarded by many as a pivotal moment in raising public awareness. So here we are, three decades since Hansen’s testimony, and nearly 200 years since Fourier’s first brilliant insights. The recently leaked draft of the Intergovernmental Panel on Climate Change Special Report on 1.5 oC describes “unequivocal and sustained global warming and sea level rise”, and suggests that having a two-thirds probability of holding warming to below 1.5 oC throughout this century is “already out of reach”. Callendar’s “deadly glaciers” have certainly been kept at bay – indeed many have been obliterated completely, with most of the rest receding rapidly. The concentration of CO2 in Fourier’s diaphanous intermediary recorded in July this year at Mauna Loa Observatory was more than 40 per cent higher than pre-industrial concentrations. We are closing in on 1 oC of warming, and already the benign world envisaged by Arrhenius and friends has been battered and burnt, flooded and fried. Some of us are slow learners.