An Ocean of Air

by Gabrielle Walker

  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.

  Tyndall's description of this effect was typically poetic. Without it, he said, "The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost."

  To Tyndall and his contemporaries, carbon dioxide was not the menace we see it as today—indeed, it was a lifesaver. But he also realized that because there's so little carbon dioxide in the atmosphere, even small changes in the past could account for dramatic swings in climate, such as the ice age that had left such a mark on the Alps. This, he said, might explain "all the mutations of climate which the researches of geologists reveal."

  Though Tyndall didn't realize it, this was the first hint of carbon dioxide's downside. Yes, it is the crucial source of all our food, and yes, without it we would freeze to death. But, like oxygen, carbon dioxide has the potential to deliver too much of what would otherwise be a very good thing. The hero that protects us was also to be revealed as a villain, threatening us with a potentially deadly menace: global warming.

  ***

  1896

  STOCKHOLM, SWEDEN

  Svante Arrhenius was depressed. Now age thirty-seven, he had just been through a messy divorce, and he had lost not only his wife, but custody of their young son. The pouches under his eyes and the mustache that plunged sharply downward on either side of his mouth only served to emphasize his present misery. He urgently needed something to distract him. But what?

  Arrhenius was a scientist. Mainly he worked on the chemistry of liquids that conduct electricity. In five years' time he would win a Nobel prize for his research. This would embarrass his thesis examination committee, who had labeled his work "mediocre" and barely let it scrape by. But though he was fascinated by his usual subject, he was looking for something different to dabble in for a while. What he wanted most was a change.

  That's when he happened on Tyndall's idea of the role that carbon dioxide might play in causing the ice ages. Arrhenius was intrigued by this notion and he wanted to take it a bit further. Being a theorist, he decided to calculate exactly how much carbon dioxide Earth would have to lose to trigger an ice age.

  This was going to be more complicated than it first seemed. For Arrhenius realized that he couldn't just stick to the direct cooling caused by less carbon dioxide in the atmosphere. There would be some important additional effects as well. In particular, he knew that cooler air is a less effective sponge—it soaks up less water from the oceans.

  That matters because, as Tyndall had noticed, water vapor is a very effective greenhouse gas in its own right, so that losing some of it would make the atmosphere colder still. In other words, a small change in carbon dioxide could make a big difference in climate. (This highlights an important aspect of the way that carbon dioxide levels can affect our climate. Many skeptics have pointed out that water vapor is the principle source of greenhouse warming in the atmosphere; in terms of absolute effect, carbon dioxide comes a very poor second. However, Arrhenius was right that changing the amount of carbon dioxide by only a small amount significantly changes the amount of water vapor, which boosts the overall impact. By this means, carbon dioxide punches considerably above its weight.)

  Arrhenius realized that if he wanted a plausible answer, he would have to consider both the direct and indirect effects of carbon dioxide in tandem. This would entail long, tedious calculations. Perfect. It was just the sort of distraction he had been looking for. He picked up pencil and paper and settled down for several months' hard labor.

  First, he imagined a world in which carbon dioxide was reduced by half. He then carefully calculated the amount of moisture in the air, and the amount of light energy entering and leaving Earth, for every zone of latitude. Eventually, he had an answer. It was crude, with many assumptions, but it was the first attempt to put numbers on the effect of changing carbon dioxide. Halving the CO2 levels would reduce global temperatures by about 9 degrees Fahrenheit. That, he thought, could be just enough to trigger an ice age.

  Arrhenius was a theoretical chemist, not an atmospheric scientist. He had picked the amount of carbon dioxide to try almost at random, and he had no idea whether it was realistic. So he asked a colleague's advice. Arvid Högbom had already come up with numbers for how much carbon dioxide appears naturally from volcanoes and how much disappears
into Earth's rocks and oceans. It should certainly be possible to reduce levels of carbon dioxide, he said, if some volcanoes dried up for a while, or something happened to stop the oceans from soaking it up. But while he was fiddling with the numbers, Högbom noticed something curious. Never mind lowering carbon dioxide levels; they were already being raised in a way that had nothing to with volcanoes or oceans, or indeed any other natural process. To keep the factories of the industrial revolution running, humans were now burning coal on an unprecedented scale. In the process, they were producing tons of carbon dioxide. When he compared this with the natural sources, Högbom found that humans were producing carbon dioxide at the same rate.

  Hogbom wasn't particularly alarmed by this. After all, even in 1896, at what seemed like the height of the industrial revolution, a full year's worth of coal wouldn't increase carbon dioxide in the air by very much—perhaps only one part in a thousand. He had no concept—nobody did—of how dramatically the world's population would increase, and how much industrialization would accelerate. But his results did set Arrhenius thinking.

  He realized the heating process would be almost the exact mirror image of cooling. Just as cooler air holds less water vapor, so warmer air holds more. So more carbon dioxide would warm the air in its own right, and also encourage more water to evaporate from the oceans, which would warm the air even more. Once again, Arrhenius worked through his calculations. If, say, carbon dioxide were to double from its 1896 level, even though that would still be only a tiny fraction of the air as a whole, Arrhenius predicted that it would still cause a large amount of warming, perhaps changing temperatures by as much as 9 degrees Fahrenheit. Though that might not sound like much, raising global average temperatures everywhere in the world by this amount could make a huge difference to the overall climate. (Amazingly, it is also very close to the result that many computer models produce today, using sophisticated calculations and vastly improved knowledge of how the climate works. More than a hundred years ago, Arrhenius was on the right track.)

  Arrhenius's finding caused a flicker of interest but not much concern. Assuming industry continued at more or less the same rate, doubling carbon dioxide levels would take thousands of years, and the calculation seemed like a curiosity rather than a cause for worry. And even if warming happened more quickly, this was the time when technology was almost universally considered to be a good thing. Who was to say that a warmer world wouldn't be a better one? Another scientist of the time, Walter Nernst, thought it would. He suggested deliberately heating up Earth a bit, by burning useless coal deposits.

  But even this kind of speculation died down when experiments appeared to show that Arrhenius had got his calculations completely wrong. One researcher tried passing infrared light through a tube containing the contemporary proportion of carbon dioxide. As Tyndall had found, a certain amount of the light was blocked. But then the researcher doubled the proportion of carbon dioxide, and nothing changed. The same amount of infrared light disappeared into the gas.

  How could that be? Surely adding more carbon dioxide should mean more of the infrared would be blocked. However, carbon dioxide turned out to be unexpectedly choosy about which frequencies of light to soak up, going for only a few restricted "colors" of infrared. So restricted, in fact, that the merest trace of carbon dioxide could absorb all the available light in these color ranges. After that, you could double, triple, even quadruple the amount of carbon dioxide in the tube and the rest of the infrared light would still pass through untouched.

  Soon, other objections began to emerge. The oceans contain huge quantities of carbon dioxide—fifty times as much as the atmosphere. Most of the extra gas put out by factories would surely be taken up by that vast reservoir, leaving behind only a minuscule amount to slip into the air.

  All in all, these comforting notions sat well with the prevailing picture of a world in which nature was vastly stronger than the puny forces of mankind, and where natural cycles somehow balanced everything in the end. There was nothing to worry about, or even to be particularly interested in. Extra carbon dioxide couldn't possibly warm the planet. Or so it seemed.

  In the decades that followed, a few researchers kept up their interest in the climatic effects of carbon dioxide. Some were just vaguely curious, others convinced there was something in Arrhenius's idea, and among them all they just about kept the issue alive. Meanwhile, the cities of the world began to spread. Lifestyles in many countries started to shift from the grinding slog of agrarian societies to the glories of the industrialized world. Year after year, more factory chimneys were springing up to pour carbon dioxide into the air. Then there were railways, and motorcars, and jet engines, and what had been a trickle of carbon dioxide became a flood. Between Arrhenius's time and the end of the twentieth century, Earth's population would more than quadruple, and the average use of energy by these people would also quadruple. The rate that carbon dioxide poured into the atmosphere from human activities would rise by a staggering sixteen-fold. Nobody guessed this would happen—how could they? For the time being, the growing amounts of carbon dioxide in the air went unnoticed, and unsung.

  Then, in 1952, one of the central criticisms of Arrhenius's work unexpectedly crumbled. Adding carbon dioxide was supposed to make no difference because the amount already present in our air appeared to be grabbing all the infrared radiation that it could. But new measurements and theory began to suggest that this argument was seriously flawed. Those early experiments were made in an ordinary lab, at ordinary temperatures and pressures. But aloft, where most of the infrared fielding takes place, the air is frigid and thin. That, it turns out, changes everything. At such low pressures and temperatures, carbon dioxide no longer soaks up every bit of radiation in its favored colors.

  This new finding inspired a weapons researcher at the Lockheed Aircraft Corporation. Gilbert Plass was an expert on infrared radiation—he spent his days using it to try to develop heat-seeking missiles. But in the evenings, Plass enjoyed reading about science more generally. When he came across Arrhenius's discredited theory about carbon dioxide and infrared light, he became curious about how much of a difference the new results would make. Fortunately, he had no need to resort to months of calculations with pencil and paper; thanks to his day job, he had access to one of the newly invented digital computers. Working mainly in his spare time, Plass fed in the revised figures. The result was just as he had expected: Adding more carbon dioxide to the air can make a difference after all, and the effect on climate looked to be significant.

  The next notion to fall was the idea that oceans would soak up most of the carbon dioxide. Researchers began to realize that the warm surface layer of the ocean doesn't mix very much with the colder water underneath, which means most of the carbon dioxide taken up by the ocean is quickly recycled into the atmosphere. Nobody could be exactly certain what difference this would make. What they really needed was to know whether carbon dioxide levels in the atmosphere were actually changing. And if so, by how much?

  That's when a young American researcher named Charles "Dave" Keeling entered the scene. Keeling had read Plass's work and had discussed it with him. He was fascinated by carbon dioxide and what effect it might have on Earth's climate, and he became convinced that the only way to know for sure was to measure it. To do this, he developed instruments to measure carbon dioxide levels with delicate, extraordinary accuracy. Next, he placed them on the top of Mauna Loa, a volcano on the Big Island of Hawaii, well away from the sorts of local industrial influences that could ruin his results. But he didn't want to measure for only a month, or even a year. He wanted to keep the measurements going indefinitely.

  Keeling was inspired, technically brilliant, and also—fortunately—bull-headed. Fortunately, because he discovered that there was no funding available for long-term studies like the one he had in mind. There was nothing wrong with making a few measurements once in a while, he was told repeatedly by the U.S. science-funding agencies. But keeping highly exp
ensive and very technical instruments ticking over constantly in Hawaii for years? There was simply no need.

  Keeling, however, refused to hear the word "no." He wrangled and scraped and insisted, and he somehow managed to keep his instruments in place and switched on. It wasn't long before he was proved to be right. Even between one year and the next, he could see the difference in carbon dioxide levels. And it was exactly what you'd expect if the oceans were not, after all, soaking up those human outpourings.

  Keeling made those measurements for more than forty years. When plotted out on a graph, his "Keeling curve" has become one of the most famous icons of the global warming debate. For as the years have passed, the carbon dioxide levels it shows look nothing like a flat line, or even a gentle rise. Instead, they rear up exponentially, like a malevolent tidal wave ready to crash.

  But could carbon dioxide really be warming up the world? Sophisticated new computer models suggested that it should, but they struggled to come up with a consistent answer. Some said that doubling carbon dioxide levels would increase global temperatures by one degree, others by eight or nine. Perhaps the answer was to look at exactly how much temperatures really had risen, if at all. But here, there was another problem. Temperatures fluctuated perfectly naturally from year to year, and that made it very difficult to discern any possible warming from the thicket of ordinary highs and lows.

  This is one reason that global warming researchers have always had an image problem. It's not too hard to jolt people into action if you can point to a massive oil spill, or a forest that's been devastated by acid rain. But where the effects of carbon dioxide are concerned, the long view is the only one that matters. Nobody will ever be able to say "this particular heat wave was caused by global warming" or finger it as the culprit for that individual flood. Instead, the potentially nefarious effects of carbon dioxide are all about something that's much harder to pin down: trends.