What the Weather Is

At the end of July 2019 in the English Lake District, half a mile from the spot known as the wettest place in Britain, the river was low, the waterfalls silent. A heat wave was scorching Europe and there were wildfires across the Arctic Circle. Clare Nullis, from the UN World Meteorological Organization, reported that the flow of searing air from the Sahara would reach Greenland, where the ice sheet built up over thousands of years is disappearing. “In July alone, it lost 160 billion tonnes of ice through surface melting. That’s roughly the equivalent of 64 million Olympic-sized swimming pools. Just in July. Just surface melt—it’s not including ocean melt as well.”

In the eight detailed, immensely readable essays of Waters of the World, Sarah Dry shows how over the past 150 years scientists have slowly come to see climate as a global system, and to recognize how human activity contributes to changes in the complex interactions of ice, oceans, and the atmosphere. Until the late twentieth century, climatology was chiefly a geographical subject that aimed to define the distinctive qualities of the apparently stable climates in different regions. Climate science, by contrast, developing after World War II, reconfigured the discipline as the study of change rather than continuity, drawing on oceanography, atmospheric physics, meteorology, glaciology, and the new field of computer science. “Before this interdisciplinary synthesis,” Dry writes, “the notion of climate change was an oxymoron.”

Dry is a science historian and a trustee of the Science Museum Group in London, but she is also a biographer, the author of a life of Marie Curie (2003) and of The Newton Papers (2014), the strange story of Isaac Newton’s manuscripts. In Waters of the World, too, she takes a biographical approach, grounding her study in the personal quests that lie behind leaps in understanding, which must be followed by the lengthy, meticulous testing of new theories. This makes for dramatic storytelling and also brings out a secondary theme: the tension between objective research and subjective response—the restless curiosity and passionate sense of wonder that so often drives those who study the natural world.

John Tyndall is a case in point. Dry introduces him, aged thirty-nine, “in the place he most loved to be”: climbing an Alpine glacier, the Mer de Glace, in December 1859, carrying a notebook, a flask of tea, and a hard biscuit crammed into his pocket. Tyndall was an experienced climber who found the mountains an escape from his life as a professor of natural philosophy at the Royal Institution in London. Sometimes working in a blizzard, he set a row of stakes in the glacier’s surface snow and measured their position each day, hoping to find out how fast the glacier moved, and then—a trickier question—to determine the mechanism of its movement. The Scottish physicist and geologist James David Forbes, his predecessor in the study of glaciers, had concluded that ice slid like a viscous substance, smooth as honey, but after painstaking experiments back in his London laboratory, Tyndall deduced that it moved with a juddering motion, melting under pressure and then freezing again, a process he called “regelation.”

The Forbes-Tyndall controversy raged throughout the 1850s and 1860s. But on the Mer de Glace, Tyndall had also become enthralled by the contrast of scale, by “the ages it had taken for the miniscule action of molecules upon molecules to carve out the landscape before him.” How had this happened? Geologists of the previous generation had puzzled over erratic boulders that seemed to have traveled vast distances, and over the strange nature of glacial drifts—the accumulation of clay, silt, sand, and gravel found in unexpected places. In 1840 Louis Agassiz boldly suggested that these could be explained by the existence of a vast ice sheet that had once covered most of Europe and North America. At the same time, the work of physicists, including the experiments of James and William Thomson (Lord Kelvin) on heat, pressure, and energy, was giving rise to the theory of thermodynamics—the idea that the earth was in fact slowly cooling down. How could this be reconciled with a far colder period in the past? What might cause an ice age? Had there been more than one?

The prickly Scottish autodidact James Croll tried to answer these questions. He proposed that variations in the earth’s orbit around the sun would produce warm and glacial periods. During the latter, cooling was intensified by secondary causes such as the movement of water carrying heat around the globe and the way that the increased amount of snow reflected heat and light back into the atmosphere and deflected the course of the trade winds. Tyndall was impressed by Croll’s ideas, and made connections between the smallest particle and the largest system, the molecule and the universe: “You cannot study a snowflake profoundly,” he wrote, “without being led back by it step by step to the constitution of the sun. It is thus throughout Nature. All its parts are interdependent, and the study of any one part completely would really involve the study of all.”

It was understandable, then, that he would turn from ice to gases, and study the way solar and terrestrial heat was transmitted through the atmosphere. His experiments, described in vivid detail by Dry, led to the startling conclusion, reported in his Bakerian lecture of 1861, that even small changes in the amount of certain gases could drastically alter the amount of heat trapped in the atmosphere, thus warming or cooling the planet. Air free of moisture absorbed little heat, while water vapor absorbed far more, acting “like a great blanket, swaddling the earth in heat which would otherwise be lost to space.” Although he did not single out carbon dioxide, Tyndall had pointed to the physical basis of the “greenhouse effect.”

Tyndall intrigues Dry partly because of his intense emotional response—he was “persistently moved to his very core by his own ineffable experience of wonder”—and his alertness to the “exquisite paradox” that natural forms produced by physical laws could produce such deep feeling in humans, who were themselves “mere matter organized according to the same physical laws.” The Scottish Astronomer Royal Charles Piazzi Smyth was similarly moved when he sailed into Tenerife harbor in 1856 and saw the veil of cloud suddenly part to reveal the mountain peak—a sight also noted by Humboldt and Darwin. Clouds, Dry notes, whether in the standardized nomenclature of Luke Howard in 1803 or in the paintings of Constable, “elided—glided across—the boundary between objectivity and subjectivity, between science and art, between fact and feeling.”

In the clear air of the mountaintop, Smyth could see with his telescope further into the heavens than anyone had before and was able to distinguish a mass of distant stars. But by then astronomers were no longer cartographers, mapping Newton’s “clean clockwork universe.” Instead, they were investigating a cosmos “filled with energy that bombarded the planet, bathing it in a relentlessly dynamic flux of light and magnetism.” To understand this flow, it was crucial to distinguish between the forces emanating from the solar and from the terrestrial atmosphere. Using his spectroscope—an instrument for separating light into its spectrum of colors—Smyth observed that as the sun set, a number of the strange dark lines on the prism above the colored bands grew visibly: these, then, must be indicators of “some invisible substance” that increased as the terrestrial atmosphere thickened. A later spectroscopic discovery (after an eccentric detour measuring the Great Pyramid at Giza and judging that its builders had used a unit equivalent to the British inch) was the result of Smyth’s observation, in a Parisian storm in 1875, of a dark band between the red and orange section of the prism. This “rainband,” he thought, indicated water vapor and could predict coming rain: with an army of observers, weather forecasting could be a real possibility.

“Rainband fever” soon faded, but forecasting had long been a dream of governments. Since the 1840s, meteorologists had collected data from observation posts, and by the late nineteenth century they were beginning not just to map the weather but to ask what drove it and how this information might solve practical problems. Imperial powers saw forecasting as a step to exploiting natural resources: in both England and Prussia, meteorology had close links with state statistical offices. Thus the mathematician Gilbert Walker was appointed director-general of meteorological observatories in India in 1903, despite having no meteorological background. His task was to collect data to improve monsoon forecasting. The summer monsoon had failed in three of the previous seven years, as a result of which, an article in The Lancet estimated in 1901, around 19 million people had died of hunger—though as Dry points out, the famine was due less to lack of rain than to the British imposition of a cash economy, which deterred farmers from the traditional practice of storing grain for hard times.

Walker’s data-gathering was stupendous: rainfall figures from 2,677 rain gauges; readings of pressure, temperature, and wind speed from several dozen observatories; ocean data from the logbooks of ships docking at Calcutta and Bombay; information on the middle and upper atmosphere from balloons over the Bay of Bengal and the Arabian Sea; and photographs of snowfall in the Himalayas. Data also came from correspondents across the southern hemisphere. The patterns that Walker found demolished the belief that the monsoons were related to sunspot cycles; it showed instead that they were linked to pressure and temperature variations far away in the western Pacific. Although these statistical patterns were not explained as physical processes until 1969, through his “Southern Oscillation,” Dry writes, Walker had discovered “world weather…large regions of alternating high and low pressure that spanned the globe and changed with the seasons.”

Often, as in this case, discoveries were a by-product of political or military initiatives. Joanne Gerould—later, after three marriages, Joanne Simpson—began lecturing on meteorology to aviation cadets at the University of Chicago in 1943, at the age of twenty-one. Fascinated by the sudden, dramatic transformation of tropical clouds from calm to storm, she even managed to commandeer an old navy plane and take bumpy rides into the heart of the storm clouds. With her former supervisor Herbert Riehl, Simpson then moved to investigating, on “a vastly increased scale, the entire tropical zone…tracking the movement of the sun’s energy around the planet.” Exploring this, they found an unexplained gap: How did the planet manage to transfer heat from the equator to the poles and thereby maintain average temperatures? Hot air clearly moved from the equator up to the higher troposphere and was then swept poleward, but the middle atmosphere lacked the energy to move this air upward. Simpson and Riehl made an intuitive leap, suggesting that the answer might be giant columns of buoyant air, “hot towers” that acted as escalators for heat, bypassing the middle atmosphere.

These columns, they thought, might also play a part in hurricane formation. In the early 1960s, in the “Stormfury Project” and later as its director, Simpson led experiments in seeding clouds with silver iodide, seen by the US government as a potential way of weakening hurricanes: the silver iodide would freeze the supercooled water around the eyewall, the center of the of storm, releasing latent heat, disrupting the storm’s structure, and lessening the wind force. Eventually, this hypothesis was disproved, and the project was canceled in 1983. But later, as head of NASA’s Tropical Rainfall Measuring Mission, Simpson used a new satellite “carrying a space-based rain radar that could peer deep into the heart of the clouds,” and in 2002, it measured the latent heat released by tropical systems, confirming the work she and Riehl had done fifty years earlier.

Revelations often came slowly, after years of work, using fabulously costly technology. Occasionally, however, they could happen in a flash. As a young researcher at Woods Hole Oceanographic Institution in Massachusetts and then at the University of Chicago, Henry Stommel asked why the major currents in all oceans were always stronger on the western side. Imagining a crude rectangular ocean, modifying it with wind stress on the surface and friction below, and adding the impact of the earth’s rotation, he calculated the results. Discovering that his model reproduced the currents exactly, in 1948 he wrote a five-page paper, “The Westward Intensification of Wind-Driven Ocean Currents.” “He was not yet twenty-eight years old, and he had just created a new science, dynamical oceanography,” Dry writes.

Another of Stommel’s experiments that year, made with Lewis Fry Richardson, a veteran inquirer into atmospheric turbulence, was even simpler. In Richardson’s Scottish garden they dug up parsnips, biked to the nearby loch, threw them two at a time off the pier, and with bits of wood and string measured the speed at which each pair separated. Parsnips were the perfect subject, floating almost submerged and thus unaffected by wind. Their paper, Dry tells us, “is today remembered as much for the peculiar audacity of its first line—‘We have observed the relative motion of two floating pieces of parsnip’—as for its conclusion that the atmosphere and the ocean exhibit similar forms of turbulent diffusion.” Decades of research followed, involving ships and planes, probes and pressure gauges, vast budgets and hordes of observers and modelers. The ocean, it turned out, was not slow and steady, as had been thought, but full of fast-moving eddies, “swirling vortices of water” hundreds of miles wide that increased in speed the deeper they were found. By the 1980s, huge research projects into energy and motion showed that the ocean was a central element in the planetary climate system.

One of the fascinating aspects of Dry’s account is the way that surprise results prompt new questions and new directions. On a stormy Saturday night in June 1952, Willi Dansgaard was collecting samples of rainwater in his Copenhagen garden using a funnel in a beer bottle. He planned to test the samples collected over the weekend in his laboratory, using a mass spectrometer, with the aim of finding out if isotopes of oxygen and nitrogen could be used in medicine, as those of radium had been. But in these particular samples, Dansgaard found that the numbers of the oxygen isotope 18O (heavier, by two neutrons, than the dominant 16O, and thus more likely to condense) had increased as the storm grew. “It was as if he’d put a stethoscope to the storm,” writes Dry, “and listened to the heartbeat of isotopic oxygen pulsing within it.” Clouds, it seemed, were acting like giant condensers, the changing temperature sending the lighter 16O up to evaporate and the heavier 18O down as rain. Then came the question: Would this hold true across the globe, or even through time? Could one use the ratios of oxygen isotopes in water to identify changing temperatures in earth’s distant past? This, Dansgaard said later, was “maybe the only really good idea” he ever had. His published research, based on tests of Greenland ice cores, showed “just how varied the earth’s climate had been in the past, and, just as importantly, how abruptly the climate could change.”

Over the years a picture slowly emerged of a complex, interlocking system of global climate change. This was refined by research into the movements of fluids on a planetary scale and by staggeringly complex computations that calculated the dynamics of earth systems and focused on identifying connections, feedback loops, and tipping points. But at the same time, another story was emerging, of how humans were affecting the atmosphere. Early calculations, made in 1895 and again in 1938, of the effects of human activity were rejected as implausible. But it is now nearly seventy years since research in the 1950s showed the “gradual and inexorable” rise of carbon dioxide in the atmosphere as a result of burning fossil fuels and releasing carbon deposited millennia ago, and over forty since Wally Broeker’s paper in Science in 1975, “Climate Change: Are We on the Brink of a Pronounced Global Warming?”—a headline startlingly unfamiliar at the time. Since then, “earth system science” has enabled scientists to create models against which we can measure instances of “unnatural” change, like the pattern of warming we are seeing now.

Dry’s clear scientific explanations are matched by a lyrical evocation of natural phenomena, but although she tells her linked stories with verve and wit, she never falls into the trap of presenting her subjects as lone heroes. Instead she shows how their work was bolstered by that of other researchers, by advances in different fields, by developing technologies, and by funding and institutional support. Indeed, the discipline of climate studies turns out to have its own weather, full of storms and squalls, battles for priority, and tension between theorists and modelers, restless individuals and large-scale projects. There is a benefit, Dry suggests, in recognizing the limitations of science and identifying the various values within it, “such as the importance of interestedness, commitment, emotional connection, and self-determination”:

This clears the way to recognizing that the decisions which we make as a society about how we live on the planet can be informed by scientific values without being determined by them. Our choices about how we use energy, how we dispose of our goods, how we live with and in the landscape, have always been about so much more than, for example, our understanding of the ice ages, or our ability to predict the weather.

Dry is rightly wary of presenting scientific advance as simple progress, a straight line between two points. People belong to their time. Tyndall, for example, could hardly be called “the father of climate change,” since he never mentions carbon dioxide or the human impact on the atmosphere. His study of heat, Dry notes, “was grounded in his deep appreciation of the recently revealed laws of thermodynamics, not in an appreciation of the living, green earth to which we are now so attuned.” And anyway, she adds, he was beaten to the discovery of the effect of water vapor in the atmosphere by an American scientist, Eunice Foote, three years earlier.

Foote was ignored largely because she was a woman without institutional support, and Dry’s account of Joanne Simpson also highlights the difficulties and constant scrutiny facing women in an almost entirely male profession. When Simpson left her archive to the Schlesinger Library at Radcliffe College (before it merged with Harvard) she included the journal she kept during a long, secret affair, explaining that although she was seen as a “pretty cool character…nothing could be farther from the truth. To understand how a woman, or a man, for that matter, creates original work in any field, it is necessary to penetrate the emotional masks, and my masks have intentionally been hard to penetrate.” Dry looks beneath her subjects’ masks with sympathy and curiosity. Noting their shared sense of a quest, at once playful and serious, in the end she turns back to the reader: “They each, in their own way, sought something deeply meaningful from their engagement with the planet. So should we all.”