You are what you eat. The atoms in your body come from the food and drink you consume – and, to some extent, from the air you breathe. That is not terribly surprising. What few people realise, however, is that about half the nitrogen atoms in your body have passed through something called a Haber-Bosch reaction. This chemical process, invented just before the first world war, did as much to change the world during the 20th century as the atom bomb or the microchip. Its story deserves to be more widely known, because it offers hope today for a fight whose front line is fast approaching: the battle against climate change.
The tale begins with a dispute between two German chemists, which erupted at a conference in Hamburg in 1907. At the time, solidified bird excrement from South America, known as guano, was used around the world as fertiliser. Compared with manure, it contains 30 times more nitrogen, the key ingredient. Why not extract that element from the air, which is 78% nitrogen? Alas, nitrogen molecules are so stable and unreactive that chemists were having great difficulty getting them to combine with other elements. When Fritz Haber, a German scientist, reacted nitrogen with hydrogen to make ammonia, for example, only 0.0048% of the mixture combined.
Walther Nernst, another German chemist, took issue with Haber’s results. The proportion of gas that combined, he calculated, ought to have been 0.0045%. Most people would have thought Haber’s figure was close enough, but not Nernst, who demanded that Haber withdraw his results. Greatly distressed at this rebuke, Haber concluded that repeating the experiment was the only way to restore his reputation. But when he did so, he discovered that performing the reaction at a higher pressure vastly increased the amount of ammonia produced: 10% of the mixture combined. This suggested that, rather than waiting for birds to do their business, fertiliser could be made directly from the atmosphere.
But the promise of “Brot aus Luft” (bread from the air) could be realised only if the process could be scaled up. BASF, the chemicals firm that was funding Haber’s work, gave that job to Carl Bosch. He built a series of converters of increasing size, and by 1914 the firm could produce 20 tons of ammonia a day. The outbreak of the first world war meant that much of this ammonia was soon being used to make explosives, rather than fertiliser.
While Bosch focused on ammonia production, Haber had turned his attention to using ammonia in munitions. He oversaw the first large-scale use of chemical weapons in April 1915, when Germany deployed chlorine gas against the French and Canadians at Ypres, causing 5,000 deaths. Haber argued that killing people with chemicals was no worse than using any other weapon. His wife Clara, also a chemist, violently disagreed, and shot herself with his gun in May 1915.
Haber’s work on chemical weapons meant that many scientists protested when he was awarded the Nobel prize in chemistry in 1918 for developing “an important means of improving the standards of agriculture and the well-being of mankind”. After the war BASF was forced to share details of the Haber-Bosch process with other countries, and ammonia plants were built in Britain, France and America. Global consumption of fertiliser tripled between 1910 and 1938: Haber-Bosch ammonia overtook guano to become the primary source.
This set the stage for a dramatic expansion in agricultural output after the second world war, as artificial fertiliser was combined with new strains of wheat, maize and rice. Asia’s cereal production doubled between 1960 and 1995, and its population increased by 60%. By 2008, artificial fertiliser was responsible for sustaining half the world’s population.
The story of the Haber-Bosch process offers hope as mankind faces the challenge of climate change. If efforts to reduce emissions of carbon dioxide fall short of current targets, as seems likely, other approaches will be needed. One option is “direct air capture” – extracting carbon dioxide directly from the atmosphere using giant industrial plants. At the moment, this is technically possible, but is an inefficient, costly and energy-intensive process. It seems hopeless. But that was how people saw the prospect of making fertiliser from the atmosphere at the start of the 20th century, before Fritz Haber found a way to do it. If a modern-day Haber can make a similar breakthrough, humanity will, once again, have discovered a chemical process that delivers salvation from the air.