An Ocean of Air (12 page)

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.

Tyndall knew all about infrared light. He decided to investigate whether the atmosphere traps infrared light on its way back out into space, and so keeps our planet warm. But what gases should he include in his artificial atmosphere? By now, 150 years after the pioneering experiments of Joseph Black, science had progressed mightily. Everybody knew that the atmosphere was made up of many different gases, but that most of them were present only as tiny whiffs. Since by far the bulk of the air consists of nitrogen and oxygen, Tyndall started with these. But try as he might, he couldn't get his air to take up infrared light. The light simply slipped through, taking its heat with it.

And then one day, without much hope that it would make any difference, Tyndall decided to try another component of the atmosphere: carbon dioxide. It seemed a long shot. After all, air contains nearly 79 percent nitrogen, 20 percent oxygen, and barely 0.04 percent carbon dioxide. Such an insignificant gas could hardly explain something so momentous.

Nonetheless, Tyndall shone his source of heat—a copper cube filled with boiling water—onto one side of his model atmosphere and watched what happened. To his amazement, the needles of his instruments immediately began to lurch. Even in such tiny amounts, carbon dioxide turned out to be a monster absorber of infrared light.

Carbon dioxide absorbs infrared light so well because each individual molecule is relatively big and complex. Molecules soak up light energy because they want to vibrate like a tuning fork or tumble like an acrobat. And complex molecules have many more ways to do this than the more simple varieties. Brilliant, imaginative Tyndall realized this long before advanced technologies showed it to be true. He said: "The
compound
molecule ... must be capable of accepting and generating motion in a far greater degree than the single atom." Oxygen (O2) and nitrogen (N2) are not single atoms—each is made up of two individual atoms of the same element. But they're still too simple to soak up infrared radiation; they don't have enough options for how to move. But carbon dioxide is a different matter. It's made up of one atom of carbon and two atoms of oxygen, and it can
vibrate and spin with abandon. That's why it's such a good absorber of radiation, and also why a little carbon dioxide goes a very long way.

Tyndall discovered that water vapor is an even better absorber of infrared radiation. In fact, our atmosphere is full of infrared absorbers, including methane, ozone, and the man-made chemicals that also bedevil the ozone layer. By far the biggest warming effect comes from water vapor, not because it's the most effective pound for pound—it isn't—but because there's so much of it in the sky. However, carbon dioxide is still a significant climate driver, because even small changes in the gas can make big differences to the temperature. Since warmer air soaks up more water vapor from the ocean, the two gases, carbon dioxide and water, work together to wrap the planet in a comfort blanket of warmth that keeps us all alive.

This insight from Tyndall was the beginning of our understanding of the impact the famous "greenhouse effect" has on Earth's climate. "Greenhouse" in this case is actually a misnomer, since greenhouses work mainly by trapping the air inside them. The glass windows allow light to enter and warm the air, but they also prevent this newly warmed air from wafting away. The gases in our atmosphere don't work quite like this. Rather than keeping warm
air
in place, they catch the infrared radiation on its way from the surface out into space. They vibrate with the energy for a brief instant and then throw the energy back out like a fielder returning the baseball he's just caught. Since, unlike most baseball fielders, they throw out their energy wildly in random directions, some of it succeeds in escaping into space. But enough is hurled back down to Earth to keep our lifeblood—water—from freezing.