FEATURE How a century of ammonia synthesis changed the world
On 13 October 1908, Fritz Haber fi led his patent on the “synthesis of ammonia from its elements” for which he was later awarded the 1918 Nobel Prize in Chemistry. A hundred years on we live in a world transformed by and highly dependent upon Haber–Bosch nitrogen.
Although over 78% of the atmosphere is composed of nitrogen, it exists in its chemically and biologically unusable gaseous form. Haber discovered how ammonia, a chemically reactive, highly usable form of nitrogen, could be synthesized by reacting atmospheric dinitrogen with hydrogen in the presence of iron at high pressures and temperatures. Today, this reaction is known as the Haber–Bosch process: Fritz Haber was the inventor who created the breakthrough and laid the foundations for high- pressure chemical engineering, but it was Carl Bosch who subsequently developed it on an industrial scale, for which he was awarded a Nobel Prize in 1931. Th e importance of Haber’s discovery cannot be overestimated — as a result, millions of people have died in armed confl icts over VAN DUINEN GERT the past 100 years, but, at the same time, billions of people have been fed. Agricultural production for food and fuel has increased in the past few years; for example, oilseed rape as In his Nobel lecture, Haber explained shown here. that his main motivation for synthesizing ammonia from its elements was the growing demand for food, and the and the course of modern history, has natural reservoirs of reactive nitrogen, concomitant need to replace the nitrogen literally changed the world. particularly Peruvian guano, Chilean lost from fi elds owing to the harvesting What Fritz Haber could not saltpeter and sal ammoniac extracted of crops: “it was clear that the demand foresee, however, was the cascade of from coal. Early attempts to fi x nitrogen for fi xed nitrogen, which at the beginning environmental changes, including the from the atmosphere were ineffi cient and of this century could be satisfi ed with a increase in water and air pollution, the energetically expensive. Th e Haber–Bosch few hundred thousand tons a year, must perturbation of greenhouse-gas levels and process has signifi cantly lower energy increase to millions of tons”1. We now the loss of biodiversity that was to result requirements and was therefore know that his vision was right: the current from the colossal increase in ammonia substantially cheaper, allowing it to form worldwide use of fertilizer nitrogen is production and use that was to ensue3. the basis of an alternative expanding supply about 100 Tg N per year. Here we refl ect on the infl uence that of reactive nitrogen. Haber’s nitrogen has Haber’s other motivation, not Haber’s invention has had on society since boosted the production of many mentioned in his lecture, was to provide over the last century, both the benefi ts previously expensive or rare compounds, the raw material for explosives to be used and unintended consequences. And, such as dyes and artifi cial fi bres, but it has in weapons, which requires large amounts based on diff erent scenarios of future had its greatest impact on the production of reactive nitrogen. Haber’s discovery has nitrogen fertilizer use, we discuss some of explosives and fertilizers2. therefore had a major infl uence on both of the challenges likely to be faced by our World Wars and all subsequent confl icts. ‘nitrogen economy’ in the next 100 years. EXPLOSIVES In addition, the large-scale production of ammonia has facilitated the industrial ECONOMIC AND SECURITY BENEFITS Th e central role that nitrogen has in the manufacture of a large number of manufacture of explosives is refl ected chemical compounds and many synthetic Up until the fi rst decades of the not only in the Nobel prizes awarded to products. Th us the Haber–Bosch process, twentieth century, many industrial Haber and Bosch, but in the very origin with its impacts on agriculture, industry processes were dependent on limited of the Nobel Prize itself. Alfred Nobel’s
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© 2008 Macmillan Publishers Limited. All rights reserved. FEATURE wealth was built on the development of 7,000 World population safe methods for using nitroglycerine, 50 and his patents for dynamite and gelignite World population 6,000 eventually fi nanced the Nobel Foundation. (no Haber Bosch nitrogen) As a German patriot, Haber was keen to develop explosives and other chemical % World population 40 5,000 fertilizer input (kg N ha Average weapons, which to his mind were more fed by Haber Bosch nitrogen production (kg person Meat humane, because they “would shorten the war”4. Th e need to improve munitions 4,000 Average fertilizer input population/ % World 30 supplies was in reality a central motivation for industrial ammonia production. With the blockade of Chilean saltpeter 3,000 Meat production supplies during the First World War, the 20 World population (millions) World –1
Haber–Bosch process provided Germany –1 yr
2,000 yr with a home supply of ammonia. –1 –1 ) )/ Th is was oxidized to nitric acid and 10 used to produce ammonium nitrate, 1,000 nitroglycerine, TNT (trinitrotoluene) and other nitrogen-containing explosives. Haber’s discovery therefore fuelled the 0 0 First World War, and, ironically, prevented 1900 1950 2000 what might have been a swift victory for the Allied Forces. Since then, reactive nitrogen produced by the Haber–Bosch Figure 1 Trends in human population and nitrogen use throughout the twentieth century. Of the total world process has become the central foundation population (solid line), an estimate is made of the number of people that could be sustained without reactive of the world’s ammunition supplies. As nitrogen from the Haber–Bosch process (long dashed line), also expressed as a percentage of the global such, its use can be directly linked to population (short dashed line). The recorded increase in average fertilizer use per hectare of agricultural land 100–150 million deaths in armed confl icts (blue symbols) and the increase in per capita meat production (green symbols) is also shown. throughout the twentieth century5.
FERTILIZERS to produce — this is partly refl ected in Together with the role of reactive At the same time, the Haber–Bosch the global increase in per capita meat nitrogen in ammunition supplies, these process has facilitated the production of consumption (Fig. 1). In contrast, the fi gures provide an illustration of the agricultural fertilizers on an industrial latest Food and Agriculture Organization huge importance of industrial ammonia scale, dramatically increasing global report shows that approximately 850 production for society, although, on agricultural productivity in most regions million people remain undernourished8. balance, it remains questionable to what of the world7 (Fig. 1). We estimate that the Overall, we suggest that nitrogen extent the consequences can be considered number of humans supported per hectare fertilizer has supported approximately as benefi cial. of arable land has increased from 1.9 to 27% of the world’s population over 4.3 persons between 1908 and 2008. Th is the past century, equivalent to around UNINTENDED CONSEQUENCES increase was mainly possible because of 4 billion people born (or 42% of the Haber–Bosch nitrogen. estimated total births) since 1908 (Fig. 1). Of the total nitrogen manufactured by Smil estimated that at the end of For these calculations, we assumed the Haber–Bosch process, approximately the twentieth century, about 40% of that, in the absence of additional 80% is used in the production of the world’s population depended on nitrogen, other improvements would agricultural fertilizers10. However, a large fertilizer inputs to produce food2,6. have accounted for a 20% increase in proportion of this nitrogen is lost to the It is diffi cult to quantify this number productivity between 1950 and 2000. environment: in 2005, approximately precisely because of changes in cropping Consistent with Smil6, we estimate, 100 Tg N from the Haber–Bosch methods, mechanization, plant breeding that by 2000, nitrogen fertilizers were process was used in global agriculture, and genetic modifi cation, and so on. responsible for feeding 44% of the world’s whereas only 17 Tg N was consumed However, an independent analysis, based population. Our updated estimate for by humans in crop, dairy and meat on long-term experiments and national 2008 is 48% — so the lives of around products11. Even recognizing the other statistics, concluded that about 30–50% of half of humanity are made possible by non-food benefits of livestock (for the crop yield increase was due to nitrogen Haber–Bosch nitrogen. example, transport, hides, wool and so application through mineral fertilizer7. In addition, fertilizer is required on), this highlights an extremely low It is important to note that these for bioenergy and biofuel production. nitrogen-use efficiency in agriculture estimates are based on global averages, Currently, bioenergy contributes 10% (the amount of nitrogen retrieved in which hide major regional diff erences. of the global energy requirement, food produced per unit of nitrogen In Europe and North America, increases whereas biofuels contribute 1.5%. Th ese applied). In fact, the global nitrogen- in agricultural productivity have been energy sources do not therefore have a use efficiency of cereals decreased matched by luxury levels of nitrogen large infl uence on global fertilizer use9. from ~80% in 1960 to ~30% in 200012,13. consumption owing to an increase in the However, with biofuel production set to The smaller fraction of Haber–Bosch consumption of meat and dairy products, increase, the infl uence of Haber–Bosch nitrogen used in the manufacture of which require more fertilizer nitrogen nitrogen will only grow. other chemical compounds (~20%) has nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience 637
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Efficiency additional carbon stored per kilogram 250 A1 + biofuel Diet optimization of nitrogen deposited range from 40 A2 Biofuels to 400 kilograms of carbon, although Food equity 200 B1 B2 Population growth the most recent estimates suggest that Tilman et al. 27 the largest values are unlikely18–20. In Tubiello and Fischer28 150 FAO (baseline)26 the meantime, further eff orts are being FAO (improved)26 directed to understand the overall eff ect Tg N 100 of reactive nitrogen on the greenhouse gas balance, including its interactions with nitrous oxide, methane, ozone 50 and aerosols21,22.
0 1900 1950 2000 2050 2100 A1 A2 B1 B1 THE NEXT CENTURY OF HABER–BOSCH We project global nitrogen fertilizer demand over the next century on the Figure 2 Global nitrogen fertilizer consumption scenarios (left) and the impact of individual drivers on 2100 basis of the four storylines developed for consumption (right). This resulting consumption is always the sum (denoted at the end points of the respective the Intergovernmental Panel on Climate arrows) of elements increasing as well as decreasing nitrogen consumption. Other relevant estimates26,27,28 are Change Special Report on Emission presented for comparison. The A1, B1, A2 and B2 scenarios draw from the assumptions of the IPCC emission Scenarios (SRES)23. The storylines reflect scenario23 storylines as explained in the text. varying economic, demographic and technological developments. The A1 storyline assumes a world of very rapid economic growth, a global population an uncertain fate, with its escape into the biodiversity. Similarly, the transfer that peaks mid-century, and rapid environment depending on the life-cycle of reactive nitrogen from terrestrial introduction of new and more efficient of the product. to coastal systems has doubled since technologies. B1 describes a convergent A recent study suggested that pre-industrial times16. As with terrestrial world, with the same global population approximately 40% of fertilizer nitrogen ecosystems, many of the coastal as A1, but with more rapid changes in lost to the environment is denitrifi ed back ecosystems receiving increased nitrogen economic structures towards a service to unreactive atmospheric dinitrogen14. loadings are nitrogen-limited, leading to and information economy. B2 describes In principle this loss is environmentally algal blooms and a decline in the quality a world with intermediate population benign, although it represents a waste of surface and ground waters. and economic growth, emphasizing of the energy used in the Haber–Bosch In addition to these ecosystem-level local solutions to economic, social process equivalent to at least 32 MJ kg–1 N disturbances, reactive nitrogen alters and environmental sustainability. fi xed, or about 1% of the global primary the balance of greenhouse gases, A2 describes a very heterogeneous energy supply. However, the rest enhances tropospheric ozone, decreases world with high population growth, of the excess nitrogen escapes into stratospheric ozone, increases soil slow economic development and slow environmental reservoirs, where it acidification and stimulates the technological change. We consider the cascades through atmospheric, terrestrial, formation of secondary particulate following five parameters to be the aquatic and marine pools before matter in the atmosphere, all of which main drivers of the estimated trends eventually being denitrifi ed or stored as have negative effects on people and in fertilizer use, and we apply a subset fossil reactive nitrogen. In principle, one the environment. of these parameters to the respective molecule of reactive nitrogen can have The effects of reactive nitrogen scenarios (see Fig. 2). multiple eff ects during its lifetime in the on the environment can be mitigated cascade. Understanding this cascade is through several intervention strategies, (1) Population growth is the main driver therefore essential for the development of which should focus on reducing the behind the increase in nitrogen fertilizer eff ective abatement measures15. creation of reactive nitrogen, increasing use. Th e SRES population projections The influence of Haber–Bosch the efficiency with which it is used, range between 7 (B1) and 15 billion people nitrogen on the global nitrogen cycle or converting it back to atmospheric (A2) in 2100. Recent research suggests can be seen in present-day atmospheric dinitrogen. Such strategies could include that global fertility rates will continue to and aquatic nitrogen pools. Emissions increasing the efficiency of nitrogen use decrease. As a consequence, population 24,25 of NO and NH3 to the atmosphere in food production, altering human diets growth is expected to eventually halt . have increased about fivefold since and improving the treatment of human pre-industrial times14. Atmospheric and animal waste10. (2) The potential for increasing nitrogen deposition in the absence of Another unintended, but positive, yield per hectare is large, and could human influence is ~0.5 kg N ha−1 yr−1 environmental consequence of the allow food output to keep pace with or less, but in large regions of the world, Haber–Bosch process may be an increase population increases, without requiring average atmospheric deposition rates in the amount of carbon sequestrated an increase in cropping area. At the same exceed 10 kg N ha−1 yr−1, exceeding in non-agricultural ecosystems, due to time, an increase in fertilizer efficiency natural rates by more than an order an increase in atmospheric nitrogen is expected. In agreement with Smil2, we of magnitude. Much of this reactive deposition. Recent debate has focused assume that nitrogen management can nitrogen is deposited in nitrogen-limited on the response of forests17–20, where be improved, resulting in a 50% increase ecosystems, leading to unintentional the strength of this fertilization eff ect is in nitrogen-use efficiency, thus reducing fertilization and loss of terrestrial contentious. Estimates of the amount of nitrogen application.
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3) Further demand on agriculture may THE FUTURE NITROGEN ECONOMY 3. Erisman, J. W., Bleeker, A., Galloway, J. N. & Sutton, M. A be posed by biofuel production, calling Environ. Pollut. 150, 140–149 (2007). 4. Stoltzenberg, D. Fritz Haber: Chemist, Nobel Laureate, for an expansion of crop land as well It is appropriate to mark a century of German, Jew. (Chemical Heritage Press, Philadelphia, 2004) as an increase in nitrogen demand. Haber’s invention. Given its multiple roles 5. White, M. Historical Atlas of the Twentieth Century (2003); In the technology-oriented ‘global’ in military security, food production, available at
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