An Ocean of Air

by Gabrielle Walker

  The "inconveniences," it turns out, are that without it we and most other living things on Earth would starve to death.

  Black never knew the vital role that his discovery, carbon dioxide, plays in our lives, but those who came after him quickly began to recognize its importance. In Lavoisier's experiments with respiration, he realized that the more oxygen a person or animal consumed by breathing, the more "fixed air" they produced. He deduced that we burn our carbon-based food in much the same way that a candle burns its carbon-based wax, and for the same reason: to release energy. And burning carbon-based substances in oxygen produces—what else but carbon dioxide.

  Priestley, meanwhile, spotted that the interplay between fixed air and oxygen was somehow related to plants. He knew that a mouse in an enclosed chamber would eventually be unable to breathe, but he discovered that placing a plant in the same chamber kept the air from getting noxious indefinitely. The plant and mouse seemed to work in contented cooperation to keep the air fresh.

  This is not merely a curiosity. Subsequent scientists have discovered that it's the fundamental basis for life as we know it on Earth. For the existence of carbon dioxide, and its relationship with oxygen, is the foundation for a pact between plants and animals the world over.

  We animals take in oxygen to burn our food and throw out carbon dioxide as a waste product. Plants work the other way around. They take in carbon dioxide to make food and produce oxygen as their waste product. (Plants also need to breathe, to release energy from the food they make. They use up about a quarter of the oxygen they produce, but the rest they leave for us.) So we have a deal that keeps us all alive—plants soak up our leavings and we soak up theirs. Air is the living, breathing medium for this eternal interchange.

  The plant's side of this bargain is the basis for all food production on Earth. The first hint that this might be so had come in the mid-seventeenth century, when a Dutch alchemist named Jan Baptista van Helmont performed a curious experiment. He had begun to wonder what plants are made of, or more particularly where the stuff that makes a plant comes from. So he took a large pot, and in it put two hundred pounds of earth that he had carefully dried in a furnace. In this pot he planted a young willow sapling, weighing five pounds. And over the top of the pot, so that no extra dust could enter from the air, he fitted a metal plate full of holes around the sapling's trunk. Van Helmont was a persistent fellow. He pursued his experiment for a full five years, watering, watching, and waiting. In the end, he had a towering willow tree that weighed "169 pounds and about three ounces."

  So where had the tree come from? The first thing to test was the earth in the pot. Van Helmont removed the earth, dried it, and weighed it. It had lost a mere two ounces.

  This might not seem so surprising. After all, anyone who has ever owned a house plant knows that it will grow happily without your adding new soil to the pot. But in that case, what had made the willow tree's branches, trunk, and leaves?

  Van Helmont guessed wrongly. The only thing he had added to the pot was water, so he blithely declared that water had to be the source. (He wasn't brilliantly logical in his deductions in other matters, either. Among other odd beliefs, he was convinced that living things could arise spontaneously out of the strangest ingredients. He even published a recipe for making mice out of dirty underwear and wheat: "For if you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment coming out of the underwear and penetrating through the husks of the wheat, changes the wheat into mice.")

  The problem in this case was that he hadn't even noticed that the tree was surrounded by something else that was a superb source of raw material for making plants: thin air. The source of every ounce of the solid roots, trunk, branches, and leaves of Van Helmont's willow tree was the carbon dioxide in the air around it. When plants soak up carbon dioxide, they take air and turn it into the food that eventually finds its way into our stomachs.

  Plants do this in a complex series of internal reactions, but the overall result is a simple one. They use the sun's energy to break apart carbon dioxide and turn it into the carbon-based molecules that make up our food. The scale of this activity is staggering. Every year, green plants convert carbon dioxide into 100,000 million tons of plant material. To do this, plants use up 300 trillion calories of energy from the sun, which is thirty times the energy consumption of all the machines on Earth. Even the animals we eat gain their protein and fat from plant food. Carbon dioxide in our atmosphere is the fundamental foodstuff for every plant, animal, and human on the planet.

  Trees and plants take their nutrients from our ocean of air in the same way that waving fronds of seaweed do from seawater. And when we breathe, we simply recombine the food they made with the oxygen they produced to start the process all over again. The balance isn't perfect, and that turns out to be a good thing. The only reason we have oxygen to breathe in the atmosphere today is that plants keep hold of a certain percentage of the stuff they make and prevent us animals from eating it, breathing it, and turning it back into carbon dioxide. The fraction is small, just 0.01 percent of the stuff that plants make, but that also means the same percentage of the oxygen they make also remains free to float up into the sky. Over billions of years, this has built up into the atmosphere we need to live.

  Some researchers even see the pact between plants and animals as being more like a battle. At certain times in the past, plants have had the upper hand. For instance, a little over 400 million years ago, plants discovered how to make lignin, the hard stuff that turns into the woody parts of trees. Nothing in the animal kingdom knew how to digest this strange new material, so it remained untouched and unrespired—and a little less carbon dioxide made it into the atmosphere.

  Then came the two champions of the animal kingdom: termites and dinosaurs (the vegetarian sort). Both learned how to digest lignin, and carbon dioxide levels rose again. Until, that is, the extinction of the dinosaurs, when plants learned how to make vast grasslands and the balance swung again.

  This mattered for much more than plant pride. It turns out that interfering with the amount of carbon dioxide in the atmosphere can have serious consequences. As well as providing our food, carbon dioxide plays another role, which is every bit as crucial in shaping our planet for life.

  ***

  The man who discovered this was John Tyndall, an exuberant Irish physicist who was a professor at London's ultrafashionable Royal Institution in the mid-nineteenth century.

  The Royal Institution was the perfect place for someone like Tyndall; he could perform his research in the basement laboratories and then talk about science in the famous lecture theater aboveground. Science had become one of the hottest entertainments in town. Lectures at the institution were so popular that, to cope with the crush of carriages, Albemarle Street became Britain's first one-way street. And it wasn't only scientists who were crowding onto the Royal Institution's uncomfortable wooden benches. There were poets and politicians, intellectuals and aristocrats, in fact most of London's beau monde.

  Tyndall loved lecturing. Perhaps because he had come to research late, beginning his higher education only in his late twenties, he couldn't wait to pass his findings on. He was less concerned about education than about sharing his own wonder. He choreographed his lectures as for a Broadway show and worried endlessly about how to ensure their success. One day when he was preparing a lecture, Tyndall knocked an instrument off the table but managed to vault over and catch it before it reached the ground. He was so delighted with the effect that he practiced it for hours. When he "accidentally" repeated the feat that evening, he brought the house down.

  These efforts paid off. When word went out that Tyndall was lecturing, the house was always packed. And not just at the Royal Institution. Tyndall's lectures to illiterate working men at the Royal School of Mines attracted audiences of six hundred or more. One contemporary commentator wrote: "Professor Tyndall has never for an
instant looked upon the masses as entitled to only second rate knowledge. They have had it of the highest and purest which it was in his means to supply." And during a lecture tour in America, the New York Daily Tribune said of him:

  There is no such thing as doing justice by description to Professor Tyndall's manner. It is so pleasant, so colloquial, so free of arrogance, so full of personal enthusiasm as if the wonders he displayed were as new to him as to the rest of us. He makes science easy, coaxing his audience over the hard places by promises of untold beauties to come. In short he is the very beau-ideal of a scientific lecturer.

  Tyndall was impulsive, passionate, and sincere. He had a large nose that jutted out to a point, with two deeply grooved lines running from either side in a graceful arc down to the edges of his mouth. In later years he sported an impressive white beard, in true Victorian style, which sprouted around his chin and neck, though he kept his face clean-shaven. He could be intense and sometimes self-righteous but also had his playful side, and children loved him. Practical jokes were more his style than verbal witticisms, though, and he was apt to greet any wordplay with what a friend, evolutionist Thomas Huxley, once described as "blank, if benevolent, perplexity."

  Along with Huxley and seven other friends of a scientific bent, Tyndall was a founding member of a discussion group that became famous as the "X Club," so-called because, even after many hours of disputation, nobody could agree on a better name. The founders also expended much time on discussing the possible addition of new members, until this grew so tiresome that they agreed that no proposition of that kind should be entertained unless the name of the new member suggested contained all the consonants absent from the names of the old ones. "In the lack of Slavonic friends," Huxley said later, "this decision put an end to the possibility of increase." Tyndall's membership in this club, coupled with his often obsessive leanings, earned him the nickname "Xccentric."

  Some of Tyndall's poet friends complained that learning science could deaden one's appreciation of nature, but Tyndall himself was exasperated by this attitude. For him, the better he understood the world, the more wonderful he found it, and his skill at explaining carried many others along with him. He said that science required imagination. (In fact a phrase he coined, "the scientific use of the imagination," was later quoted by Sherlock Holmes in The Hound of the Baskervilles)

  In particular, Tyndall was fascinated by the happenings of the invisible world of atoms and molecules. At the time there were no microscopes capable of capturing the motions of these minuscule entities in action; the only way to study them was to combine logical thought with a vivid imagination. Tyndall had both talents in abundance. Huxley said of him: "In dealing with physical problems, I really think that he, in a manner, saw the atoms and molecules, and felt their pushes and pulls." Tyndall thought so, too. At the end of a lecture about radiation, he said: "It is thought by some that natural science has a deadening influence on the imagination ... But the ... study of natural science goes hand in hand with the culture of the imagination. Throughout the greater part of this discourse ... we have been picturing atoms and molecules and vibrations and waves which eye has never seen nor ear heard, and which can only be discerned by the exercise of imagination."

  This capacity to picture and understand the invisible was the perfect background for studying the behavior of air. But at first Tyndall paid little attention to the atmosphere. He was more interested in the studies of magnetism and the compression of crystals. However, this led to an interest in the movement of glaciers, and it was during field trips to the Alps to study these phenomena that Tyndall's interest in the atmosphere was first kindled.

  Tyndall loved the mountains. He was sure-footed, a strong and daring climber. To follow his scientific nose, Tyndall would cheerfully hack his way up ice cliffs, dodging falling rocks, or plough his way through fields of crevasses. Once, making his way in the name of science through the seracs of the Glacier du Géant, he felt truly terrified. But afterward he described the scene with relish:

  Wherever we turned, peril stared us in the face ... Once or twice, while standing on the summit of a peak of ice, and looking at the pits and chasms beneath me ... I experienced an incipient flush of terror. But this was immediately drowned in action. Indeed the case was so bad, the necessity for exertion so paramount, that the will acquired an energy almost desperate, and crushed all terrors in the bud.

  During his trips to Switzerland, Tyndall became entranced by the alpine skies. "The shiftings of the atmosphere were wonderful," he wrote after one day out on the mountains, and "half the interest of the Alps depends on the caprices of the air," after another. He even began to feel connected to the air in a way that he had never experienced before. "In effect," he said, "we live in the sky rather than under it."

  Once his attention was caught by air, Tyndall was immediately gripped by the urge to understand it. Trips to the mountains were always undertaken for scientific purposes. After all, how can you appreciate the landscape if you don't try to make sense of it? This view was not always shared by the less scientifically minded members of the Alpine Club. One year, at the club's winter dinner, the speaker gave a sarcastic sideswipe at Tyndall's scientific obsessiveness. He was describing a mock ascent of a mountain, and finished by saying:

  "And what philosophical observations did you make?" will be the enquiry of one of those fanatics who, by a reasoning process for me utterly inscrutable, have somehow irrevocably associated Alpine travel with science. To them I answer, that the temperature was approximately (I had no thermometer) 212 degrees Fahrenheit below freezing point. As for ozone, if any existed in the atmosphere, it was a greater fool than I take it for.

  Tyndall never took his science lightly. Deeply offended, he instantly resigned from the club.

  Tyndall hoped that studying the atmosphere might help him explain a conundrum furnished by the mountains themselves. His beloved Alps were full of evidence that at some point in history there had been an "ice age." Valleys had been scoured out by glaciers that had long since vanished, rocks had been transported by ancient ice far beyond their places of origin, and jumbled piles of rubble, moraines, delineated where existing glaciers had once dramatically extended their reach. How could the world have once been so cold, and what made it warm up again? Tyndall wondered if slight changes in the atmosphere might be the answer.

  In particular, Tyndall suspected that the atmosphere might act as a blanket around the world, sometimes warming and sometimes cooling as the components slightly shifted their relative proportions. He thought this because of an effect spotted a few decades earlier by French scientist Joseph Fourier. Fourier had noticed that Earth should, by rights, be much colder than it actually is. We tend to think that Earth lies in the perfect position for habitability. Of our two nearest neighbors, Venus is too close to the sun and too hot to sustain life, and Mars is too far from the sun and too cold. Yet Earth is "just right," the perfect distance for running water, balmy breezes, and a comfortable, temperate planet. However, Fourier realized that we're actually a little too far from the sun to survive without help.

  When sunlight arrives to warm Earth, the energy it provides doesn't simply stay put. Like a central heating radiator, the warm planet starts pouring heat energy back out into space. The balance between these two effects sets the planet's thermostat. And when he calculated the difference between the heat energy arriving and leaving in this way, Fourier was perturbed by his findings. Earth should be perpetually frozen.

  Fourier had guessed that something in the air might help to trap extra heat on the planet's surface, and explain why we are so comfortably off, but he didn't know what. Thinking about Fourier's earlier work, Tyndall decided that he agreed. And if he could find this mysterious warming component, he might begin to understand how our climate could have been different in the past.

  So in the summer of 1859, Tyndall set about constructing an artificial sky in the basement of the Royal Institution. It was a splendid piece of
Victorian scientific equipment, a long tube filled with gases and surrounded by sources of heat and light, and pipes that looked like the flailing tentacles of an octopus.

  Tyndall enjoyed playing with his mini-atmosphere. He shone white light through it and discovered that tiny particles in the air scattered blue light much more than all the other colors of the rainbow. This, he surmised, could explain why the sky is blue. A similar effect happens in the oceans, with scattering from tiny bits of mud. Illustrating this point in a lecture, Tyndall said, "And thus the blue eyes so admired among the ladies of my audience owe their charm essentially to muddiness." You can see this "Tyndall effect" for yourself if you're ever out in a car on a foggy night. Scattering from the particles of water in the fog will turn the light from your headlamps a fetching shade of blue.

  But what Tyndall really wanted to know was how the atmosphere retains more heat than by rights it should. He considered both sides of the heating equation. First, the ordinary visible sunlight that comes in to heat Earth. Obviously this must slip through the sky unimpeded or it couldn't arrive at the surface; the sky would be permanently dark and we wouldn't see the sun, moon, or stars. However, perhaps the answer lay on the other side of the heating balance, the part where Earth radiates energy back out into space.

  Everything that's warmer than its surroundings radiates heat. You do it, I do it, and so does every warm-blooded animal. But we don't see each other permanently glowing, because the light we give off is invisible. There's much more to light than the ordinary visible rainbow. Just as there are sounds too high- and low-pitched for us to hear them, so some "pitches" of light evade our eyes. In this case, the invisible light is called infrared. It lies just over the edge of the red part of the rainbow, its frequency too low for us to see. Infrared light is the means by which remote controls communicate with televisions and stereos, and how "night-vision" goggles can show people moving around with ghostly glows even in the pitch black. It's also how our planet pours its heat back into space.