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FEATURE
On 13 October 1908, Fritz Haber filed 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.
How a century of ammonia synthesis
changed the world
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. e
importance of Haber’s discovery cannot
be overestimated — as a result, millions of
people have died in armed conicts over
the past 100 years, but, at the same time,
billions of people have been fed.
In his Nobel lecture, Haber explained
that his main motivation for synthesizing
ammonia from its elements was the
growing demand for food, and the
concomitant need to replace the nitrogen
lost from elds owing to the harvesting
of crops: “it was clear that the demand
for xed nitrogen, which at the beginning
of this century could be satised with a
few hundred thousand tons a year, must
increase to millions of tons”
1
. We now
know that his vision was right: the current
worldwide use of fertilizer nitrogen is
about 100 Tg N per year.
Haber’s other motivation, not
mentioned in his lecture, was to provide
the raw material for explosives to be used
in weapons, which requires large amounts
of reactive nitrogen. Haber’s discovery has
therefore had a major inuence on both
World Wars and all subsequent conicts.
In addition, the large-scale production
of ammonia has facilitated the industrial
manufacture of a large number of
chemical compounds and many synthetic
products. us the Haber–Bosch process,
with its impacts on agriculture, industry
and the course of modern history, has
literally changed the world.
What Fritz Haber could not
foresee, however, was the cascade of
environmental changes, including the
increase in water and air pollution, the
perturbation of greenhouse-gas levels and
the loss of biodiversity that was to result
from the colossal increase in ammonia
production and use that was to ensue
3
.
Here we reect on the inuence that
Haber’s invention has had on society
over the last century, both the benets
and unintended consequences. And,
based on dierent scenarios of future
nitrogen fertilizer use, we discuss some
of the challenges likely to be faced by our
‘nitrogen economy’ in the next 100 years.
ECONOMIC AND SECURITY BENEFITS
Up until the rst decades of the
twentieth century, many industrial
processes were dependent on limited
natural reservoirs of reactive nitrogen,
particularly Peruvian guano, Chilean
saltpeter and sal ammoniac extracted
from coal. Early attempts to x nitrogen
from the atmosphere were inecient and
energetically expensive. e Haber–Bosch
process has signicantly lower energy
requirements and was therefore
substantially cheaper, allowing it to form
the basis of an alternative expanding supply
of reactive nitrogen. Haber’s nitrogen has
since boosted the production of many
previously expensive or rare compounds,
such as dyes and articial bres, but it has
had its greatest impact on the production
of explosives and fertilizers
2
.
EXPLOSIVES
e central role that nitrogen has in the
manufacture of explosives is reected
not only in the Nobel prizes awarded to
Haber and Bosch, but in the very origin
of the Nobel Prize itself. Alfred Nobel’s
Agricultural production for food and fuel has increased in the past few years; for example, oilseed rape as
shown here.
Gert van duinen
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FEATURE
wealth was built on the development of
safe methods for using nitroglycerine,
and his patents for dynamite and gelignite
eventually nanced the Nobel Foundation.
As a German patriot, Haber was keen to
develop explosives and other chemical
weapons, which to his mind were more
humane, because they “would shorten the
war”
4
. e need to improve munitions
supplies was in reality a central motivation
for industrial ammonia production.
With the blockade of Chilean saltpeter
supplies during the First World War, the
Haber–Bosch process provided Germany
with a home supply of ammonia.
is was oxidized to nitric acid and
used to produce ammonium nitrate,
nitroglycerine, TNT (trinitrotoluene) and
other nitrogen-containing explosives.
Haber’s discovery therefore fuelled the
First World War, and, ironically, prevented
what might have been a swi victory for
the Allied Forces. Since then, reactive
nitrogen produced by the Haber–Bosch
process has become the central foundation
of the world’s ammunition supplies. As
such, its use can be directly linked to
100–150 million deaths in armed conicts
throughout the twentieth century
5
.
FERTILIZERS
At the same time, the Haber–Bosch
process has facilitated the production of
agricultural fertilizers on an industrial
scale, dramatically increasing global
agricultural productivity in most regions
of the world
7
(Fig. 1). We estimate that the
number of humans supported per hectare
of arable land has increased from 1.9 to
4.3 persons between 1908 and 2008. is
increase was mainly possible because of
Haber–Bosch nitrogen.
Smil estimated that at the end of
the twentieth century, about 40% of
the world’s population depended on
fertilizer inputs to produce food
2,6
.
It is dicult to quantify this number
precisely because of changes in cropping
methods, mechanization, plant breeding
and genetic modication, and so on.
However, an independent analysis, based
on long-term experiments and national
statistics, concluded that about 30–50% of
the crop yield increase was due to nitrogen
application through mineral fertilizer
7
.
It is important to note that these
estimates are based on global averages,
which hide major regional dierences.
In Europe and North America, increases
in agricultural productivity have been
matched by luxury levels of nitrogen
consumption owing to an increase in the
consumption of meat and dairy products,
which require more fertilizer nitrogen
to produce — this is partly reected in
the global increase in per capita meat
consumption (Fig. 1). In contrast, the
latest Food and Agriculture Organization
report shows that approximately 850
million people remain undernourished
8
.
Overall, we suggest that nitrogen
fertilizer has supported approximately
27% of the world’s population over
the past century, equivalent to around
4 billion people born (or 42% of the
estimated total births) since 1908 (Fig. 1).
For these calculations, we assumed
that, in the absence of additional
nitrogen, other improvements would
have accounted for a 20% increase in
productivity between 1950 and 2000.
Consistent with Smil
6
, we estimate,
that by 2000, nitrogen fertilizers were
responsible for feeding 44% of the world’s
population. Our updated estimate for
2008 is 48% — so the lives of around
half of humanity are made possible by
Haber–Bosch nitrogen.
In addition, fertilizer is required
for bioenergy and biofuel production.
Currently, bioenergy contributes 10%
of the global energy requirement,
whereas biofuels contribute 1.5%. ese
energy sources do not therefore have a
large inuence on global fertilizer use
9
.
However, with biofuel production set to
increase, the inuence of Haber–Bosch
nitrogen will only grow.
Together with the role of reactive
nitrogen in ammunition supplies, these
gures provide an illustration of the
huge importance of industrial ammonia
production for society, although, on
balance, it remains questionable to what
extent the consequences can be considered
as benecial.
UNINTENDED CONSEQUENCES
Of the total nitrogen manufactured by
the Haber–Bosch process, approximately
80% is used in the production of
agricultural fertilizers
10
. However, a large
proportion of this nitrogen is lost to the
environment: in 2005, approximately
100 Tg N from the Haber–Bosch
process was used in global agriculture,
whereas only 17 Tg N was consumed
by humans in crop, dairy and meat
products
11
. Even recognizing the other
non-food benefits of livestock (for
example, transport, hides, wool and so
on), this highlights an extremely low
nitrogen-use efficiency in agriculture
(the amount of nitrogen retrieved in
food produced per unit of nitrogen
applied). In fact, the global nitrogen-
use efficiency of cereals decreased
from ~80% in 1960 to ~30% in 2000
12,13
.
The smaller fraction of Haber–Bosch
nitrogen used in the manufacture of
other chemical compounds (~20%) has
1900 1950 2000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
World population (millions)
50
40
30
20
10
0
% World population/
Average fertilizer input (kg N ha
–1
yr
–1
)/
Meat production (kg person
–1
yr
–1
)
World population
World population
(no Haber Bosch nitrogen)
% World population
fed by Haber Bosch nitrogen
Average fertilizer input
Meat production
Figure 1 Trends in human population and nitrogen use throughout the twentieth century. Of the total world
population (solid line), an estimate is made of the number of people that could be sustained without reactive
nitrogen from the Haber–Bosch process (long dashed line), also expressed as a percentage of the global
population (short dashed line). The recorded increase in average fertilizer use per hectare of agricultural land
(blue symbols) and the increase in per capita meat production (green symbols) is also shown.
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FEATURE
an uncertain fate, with its escape into the
environment depending on the life-cycle
of the product.
A recent study suggested that
approximately 40% of fertilizer nitrogen
lost to the environment is denitried back
to unreactive atmospheric dinitrogen
14
.
In principle this loss is environmentally
benign, although it represents a waste
of the energy used in the Haber–Bosch
process equivalent to at least 32 MJ kg
–1
N
xed, or about 1% of the global primary
energy supply. However, the rest
of the excess nitrogen escapes into
environmental reservoirs, where it
cascades through atmospheric, terrestrial,
aquatic and marine pools before
eventually being denitried or stored as
fossil reactive nitrogen. In principle, one
molecule of reactive nitrogen can have
multiple eects during its lifetime in the
cascade. Understanding this cascade is
therefore essential for the development of
eective abatement measures
15
.
The influence of Haber–Bosch
nitrogen on the global nitrogen cycle
can be seen in present-day atmospheric
and aquatic nitrogen pools. Emissions
of NO and NH
3
to the atmosphere
have increased about fivefold since
pre-industrial times
14
. Atmospheric
nitrogen deposition in the absence of
human influence is ~0.5 kg N ha
−1
yr
−1
or less, but in large regions of the world,
average atmospheric deposition rates
exceed 10 kg N ha
−1
yr
−1
, exceeding
natural rates by more than an order
of magnitude. Much of this reactive
nitrogen is deposited in nitrogen-limited
ecosystems, leading to unintentional
fertilization and loss of terrestrial
biodiversity. Similarly, the transfer
of reactive nitrogen from terrestrial
to coastal systems has doubled since
pre-industrial times
16
. As with terrestrial
ecosystems, many of the coastal
ecosystems receiving increased nitrogen
loadings are nitrogen-limited, leading to
algal blooms and a decline in the quality
of surface and ground waters.
In addition to these ecosystem-level
disturbances, reactive nitrogen alters
the balance of greenhouse gases,
enhances tropospheric ozone, decreases
stratospheric ozone, increases soil
acidification and stimulates the
formation of secondary particulate
matter in the atmosphere, all of which
have negative effects on people and
the environment.
The effects of reactive nitrogen
on the environment can be mitigated
through several intervention strategies,
which should focus on reducing the
creation of reactive nitrogen, increasing
the efficiency with which it is used,
or converting it back to atmospheric
dinitrogen. Such strategies could include
increasing the efficiency of nitrogen use
in food production, altering human diets
and improving the treatment of human
and animal waste
10
.
Another unintended, but positive,
environmental consequence of the
Haber–Bosch process may be an increase
in the amount of carbon sequestrated
in non-agricultural ecosystems, due to
an increase in atmospheric nitrogen
deposition. Recent debate has focused
on the response of forests
17–20
, where
the strength of this fertilization eect is
contentious. Estimates of the amount of
additional carbon stored per kilogram
of nitrogen deposited range from 40
to 400 kilograms of carbon, although
the most recent estimates suggest that
the largest values are unlikely
18–20
. In
the meantime, further eorts are being
directed to understand the overall eect
of reactive nitrogen on the greenhouse
gas balance, including its interactions
with nitrous oxide, methane, ozone
and aerosols
21,22
.
THE NEXT CENTURY OF HABER–BOSCH
We project global nitrogen fertilizer
demand over the next century on the
basis of the four storylines developed for
the Intergovernmental Panel on Climate
Change Special Report on Emission
Scenarios (SRES)
23
. The storylines reflect
varying economic, demographic and
technological developments. The A1
storyline assumes a world of very rapid
economic growth, a global population
that peaks mid-century, and rapid
introduction of new and more efficient
technologies. B1 describes a convergent
world, with the same global population
as A1, but with more rapid changes in
economic structures towards a service
and information economy. B2 describes
a world with intermediate population
and economic growth, emphasizing
local solutions to economic, social
and environmental sustainability.
A2 describes a very heterogeneous
world with high population growth,
slow economic development and slow
technological change. We consider the
following five parameters to be the
main drivers of the estimated trends
in fertilizer use, and we apply a subset
of these parameters to the respective
scenarios (see Fig. 2).
(1) Population growth is the main driver
behind the increase in nitrogen fertilizer
use. e SRES population projections
range between 7 (B1) and 15 billion people
(A2) in 2100. Recent research suggests
that global fertility rates will continue to
decrease. As a consequence, population
growth is expected to eventually halt
24,25
.
(2) The potential for increasing
yield per hectare is large, and could
allow food output to keep pace with
population increases, without requiring
an increase in cropping area. At the same
time, an increase in fertilizer efficiency
is expected. In agreement with Smil
2
, we
assume that nitrogen management can
be improved, resulting in a 50% increase
in nitrogen-use efficiency, thus reducing
nitrogen application.
1900 1950 2000 2050 2100 A1 A2 B1 B1
250
200
150
100
50
0
Tg N
Efficiency
Diet optimization
Biofuels
Food equity
Population growth
A1 + biofuel
A2
B2
B1
Tilman et al.
27
Tubiello and Fischer
28
FAO (baseline)
26
FAO (improved)
26
Figure 2 Global nitrogen fertilizer consumption scenarios (left) and the impact of individual drivers on 2100
consumption (right). This resulting consumption is always the sum (denoted at the end points of the respective
arrows) of elements increasing as well as decreasing nitrogen consumption. Other relevant estimates
26,27,28
are
presented for comparison. The A1, B1, A2 and B2 scenarios draw from the assumptions of the IPCC emission
scenario
23
storylines as explained in the text.
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FEATURE
3) Further demand on agriculture may
be posed by biofuel production, calling
for an expansion of crop land as well
as an increase in nitrogen demand.
In the technology-oriented ‘global’
scenario (A1), we use the Organisation
for Economic Co-operation and
Development estimate of the maximum
potential land available for bioenergy
(0.74 Gha), which represents an
extension equal to half the current
cropland area
9
.
(4) Large parts of the world population
are deprived of valuable animal protein.
We assume that food equity will increase
worldwide meat consumption to the
level observed in developed countries.
Increased meat production will increase
nitrogen usage because of the additional
nitrogen required to produce animal
feed, and the inefficiency of nitrogen
use in meat-based diets relative to
plant- based diets.
(5) We assume that human diets will be
optimized to improve nitrogen-conversion
eciency in the production cycle.
Specically, we assume that the ratio of
meat protein to milk protein (currently
about 2:1) will be reversed (1:2), as the
nitrogen-to-protein conversion ratio is
higher in milk than meat
.
Our projections are well within the
low estimates provided by the Food and
Agriculture Organization
26
, and some
higher estimates in scientic literature
27,28
,
which suggest a two- to threefold increase
in nitrogen fertilizer use by the second
half of the twenty-rst century, assuming
continuation of past practices. In all our
scenarios, anticipated improvement in
eciency will compensate for much of the
increase in fertilizer demand. Furthermore,
we do not expect global protein supply
to improve towards ‘food equity’ in the
scenarios predicting high population
growth (A2 and B2 projections). us the
drivers towards high nitrogen use will not
occur simultaneously, leading to smaller
dierences in annual nitrogen demand
(100–150 Tg N) than would be expected
from population projection alone. Only
when bioenergy calls for a large increase
in crop production is fertilizer nitrogen
demand expected to double to nearly
200 Tg N per year. Despite the uncertainty
and the many important drivers not
included, all scenarios point towards an
increase in future production of reactive
nitrogen. is will further increase the
nitrogen pressures on the environment,
with uneven distribution only exacerbating
the problem regionally.
THE FUTURE NITROGEN ECONOMY
It is appropriate to mark a century of
Haber’s invention. Given its multiple roles
in military security, food production,
biofuels and a host of adverse environmental
impacts, we argue that today’s society can be
considered dependent on a nitrogen-based
economy. It is important to note, however,
that the benets of Haber–Bosch nitrogen
are not available to all people of the world,
owing to nancial, geographical and
political constraints. It is therefore essential
to provide the infrastructure to supply
nitrogen where it is needed, and to use it in a
sustainable way.
Fritz Haber aimed to inuence the
course of history by providing strategic
advantages for his country in terms of food
and military security. His invention for
synthesizing ammonia exceeded
expectations, substantially altering the
course of the planet throughout the
twentieth century. But, as illustrated in our
future scenarios, there is a high probability
that the unintended environmental
consequences will not be reduced over
the coming decades. Each of the main
human drivers, such as growth in the
population, improvement of the world’s
food supply and the use of biomass
to provide energy, will lead to further
increases in the demand for nitrogen, and
will more than compensate for expected
improvements in eciency. In the
worst-case scenario, we will move towards
a nitrogen-saturated planet, with polluted
air, reduced biodiversity, increased human
health risks and an even more perturbed
greenhouse-gas balance.
Food and military security were the
key objectives for Haber. For us, global
environmental sustainability must surely
be the main driver for future innovation.
Examples of key advances to aim for include
improving nitrogen-use eciency and
reducing dependency on nitrogen-intensive
biofuels, as well as developing a
comprehensive supply of protein and
amino acids with greatly improved
eciency compared with traditional
agricultural systems.
It will be interesting to look back a
century from now: will another patent have
changed the world to the same extent as the
one Fritz Haber led a hundred years ago?
Published online: 28 September 2008.
References
1. Haber, F. The Synthesis of Ammonia from its
Elements Nobel Lecture (1920); available at
www.nobelprize.org/nobel_prizes/chemistry/laureates/
1918/haber-lecture.pdf.
2. Smil, V. Enriching the Earth: Fritz Haber, Carl Bosch
and the Transformation of World Food Production
(MIT Press, Cambridge, Massachusetts, 2001).
3. Erisman, J. W., Bleeker, A., Galloway, J. N. & Sutton, M. A
Environ. Pollut. 150, 140–149 (2007).
4. Stoltzenberg, D. Fritz Haber: Chemist, Nobel Laureate,
German, Jew. (Chemical Heritage Press, Philadelphia, 2004)
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available at <http://users.erols.com/mwhite28/warstat1.
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Agron. J. 97, 1–6 (2005).
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a Good ing (UNEP, WHRC, Paris, 2007).
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Acknowledgements
We acknowledge nancing from the European Commission
for the NitroEurope Integrated Project, the European Science
Foundation for the NinE programme and the COST programme
(European Cooperation in the eld of Scientic and Technical
Research) for COST 729. is article was prepared as a
contribution to the International Nitrogen Initiative and the
Task Force on Reactive Nitrogen of the United Nations Economic
Commission for Europe.
Jan Willem Erisman
1
*, Mark A. Sutton
2
,
James Galloway
3
, Zbigniew Klimont
4
and Wilfried Winiwarter
4,
5
1
Energy Research Center of the Netherlands,
ECN, PO Box 1, 1755 ZG Petten,
the Netherlands;
2
Centre for Ecology and Hydrology,
Edinburgh Research Station, Bush Estate,
Penicuik, Midlothian, EH26 0QB, UK;
3
Environmental Sciences, University
of Virginia, PO Box 400123,
291 McCormick Rd, Charlottesville,
Virginia 22904, USA;
4
International Institute for Applied Systems
Analysis (IIASA), Schlossplatz 1,
A-2361 Laxenburg, Austria;
5
Austrian Research Centers, Donau-City Str. 1,
A-1220 Vienna, Austria.
*e-mail: erisman@ecn.nl
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