It was a marriage of brilliant scientific minds. Clara Immerwahr had just become the first woman in Germany to receive a doctorate in chemistry. That took perseverance. Women couldn’t study at the University of Breslau, so she asked each lecturer, individually, for permission to observe their lessons as a guest. Then she pestered to be allowed to sit the exam. The dean, awarding her doctorate in 1900, said, “Science welcomes each person, irrespective of sex”; he then undermined this noble sentiment by observing that a woman’s duty was family, and he hoped this wasn’t the dawn of a new era.1
Clara Immerwahr saw no reason why getting married should interfere with her career. She was disappointed. Her husband turned out to be more interested in a dinner-party hostess than a professional equal. She gave some lectures but soon became discouraged when she learned that everyone assumed her husband had written them for her. He worked, networked, traveled, and philandered; she was left holding the baby. Reluctantly, resentfully, she let her professional ambitions slide.
We’ll never know what she might have achieved, had attitudes to gender been different in early-twentieth-century Germany. But we can guess what she wouldn’t have done. She would not—as her husband did—have pioneered chemical weapons. To help Germany win World War I, he enthusiastically advocated gassing Allied troops with chlorine. She accused him of barbarity. He accused her of treason. After the first, devastatingly effective use of chlorine gas—at Ypres, in 1915—he was made an army captain. She took his gun and killed herself.2
Clara and Fritz Haber had been married for fourteen years. Eight years into that time, Haber made a breakthrough that some now consider to be the most significant invention of the twentieth century. Without it, close to half the world’s population would not be alive today.3
The Haber-Bosch process uses nitrogen from the air to make ammonia, which can then be used to make fertilizers. Plants need nitrogen: it’s one of their basic requirements, along with potassium, phosphorus, water, and sunlight. In a state of nature, plants grow, they die, the nitrogen they contain returns to the soil, and new plants use it to grow. Agriculture disrupts that cycle: we harvest the plants and eat them.
From the earliest days of agriculture, farmers discovered various ways to prevent yields from declining over time—as it happened, by restoring nitrogen to their fields. Manure has nitrogen. So does compost. The roots of legumes host bacteria that replenish the soil’s nitrogen; that’s why it helps to include peas or beans in crop rotation.4 But these techniques struggle to fully satisfy a plant’s appetite for nitrogen; add more, and the plant grows better.
It was only in the nineteenth century that chemists discovered this—and the irony that 78 percent of the air is nitrogen, but not in a form plants can use. In the air, nitrogen consists of two atoms locked tightly together. Plants need those atoms “fixed,” or compounded with some other element: ammonium oxalate, for example, as found in guano, also known as bird poo, or potassium nitrate, also known as saltpeter and a main ingredient of gunpowder. Reserves of both guano and saltpeter were found in South America, mined, shipped around the world, and dug into soil. But by the century’s end, experts were fretting about what would happen when these reserves ran out.
If only it were possible to convert nitrogen from the air into a form plants could use.
That’s exactly what Fritz Haber worked out how to do. He was driven partly by curiosity, partly by the patriotism that was later to lead him down the path to chemical warfare, and partly by the promise of a lucrative contract from the chemical company BASF. That company’s engineer, Carl Bosch, then managed to replicate Haber’s process on an industrial scale. Both men later won Nobel Prizes—controversially, in Haber’s case, as many by then considered him a war criminal.
The Haber-Bosch process is perhaps the most significant example of what economists call “technological substitution”: when we seem to have reached some basic physical limit, then find a workaround. For most of human history, if you wanted more food to support more people, then you needed more land. But the thing about land is, as Mark Twain once joked, that they’re not making it anymore. The Haber-Bosch process provided a substitute: instead of more land, make nitrogen fertilizer. It was like alchemy: Brot aus Luft, as Germans put it. “Bread from air.”
Well: bread from air, and quite a lot of fossil fuels. First of all, you need natural gas as a source of hydrogen, the element to which nitrogen binds to form ammonia. Then you need energy to generate extreme heat and pressure; Haber discovered that was necessary, with a catalyst, to break the bonds between air’s nitrogen atoms and persuade them to bond with hydrogen instead. Imagine the heat of a wood-fired pizza oven, with the pressure you’d experience more than a mile under the sea. To create those conditions on a scale sufficient to produce 160 million tons of ammonia a year—the majority of which is used for fertilizer—the Haber-Bosch process today consumes more than 1 percent of all the world’s energy.5
That’s a lot of carbon emissions, and it’s far from the only ecological concern. Only some of the nitrogen in fertilizer makes its way via crops into human stomachs—perhaps as little as 15 percent.6 Most of it ends up in the air or water. This is a problem for several reasons. Compounds such as nitrous oxide are powerful greenhouse gases. They pollute drinking water. They create acid rain, which makes soils more acidic, which puts ecosystems out of kilter and biodiversity under threat. When nitrogen compounds run off into rivers, they likewise promote the growth of some organisms more than others; the results include ocean “dead zones,” where blooms of algae near the surface block out sunlight and kill the fish below.7
The Haber-Bosch process isn’t the only cause of these problems, but it’s a major one, and it’s not going away: demand for fertilizer is projected to double in the coming century.8 In truth, scientists still don’t fully understand the long-term impact on the environment of converting so much stable, inert nitrogen from the air into various other, highly reactive chemical compounds. We’re in the middle of a global experiment.9
One result of that experiment is already clear: plenty of food for lots more people. If you look at a graph of global population, you’ll see it shoot upward just as Haber-Bosch fertilizers start being widely applied. Again, Haber-Bosch wasn’t the only reason for the spike in food yields; new varieties of crops such as wheat and rice also played their part. Still, suppose we farmed with the best techniques available in Fritz Haber’s time—the Earth would support about 4 billion people.10 Its current population is about 7.5 billion, and although the growth rate has slowed, global population continues to grow.
Back in 1909, as Fritz triumphantly demonstrated his ammonia process, Clara wondered whether the fruits of her husband’s genius had been worth the sacrifice of her own. “What Fritz has achieved in these eight years,” she wrote plaintively to a friend, “I have lost.”11 She could hardly have imagined how transformative his work would be: on one side of the ledger, food to feed billions more human souls; on the other, a sustainability crisis that will need more genius to solve.
For Haber himself, the consequences of his work were not what he expected. As a young man, Haber had converted from Judaism to Christianity; he ached to be accepted as the German patriot he felt himself to be. Beyond his work on weaponizing chlorine, the Haber-Bosch process also helped Germany in World War I. Ammonia can make explosives, as well as fertilizer; not just bread from air, but bombs.
When the Nazis took power in the 1930s, however, none of these contributions outweighed his Jewish roots. Stripped of his job and kicked out of the country, Haber died, in a Swiss hotel, a broken man.