Several hundred scientists from across the globe will gather in Ludwigshafen, Germany, next week to discuss a simple topic: "A hundred years of the synthesis of ammonia." As titles go, it is scarcely a grabber. Yet the subject could hardly be of greater importance, for the gathering on 11 November will focus on the centenary of an industrial process that has transformed our planet and threatens to bring even greater, more dramatic changes over the next 100 years.

The ammonia process – which uses nitrogen from the atmosphere as its key ingredient – was invented by German chemist Fritz Haber to solve a problem that faced farmers across the globe. By the early 20th century they were running out of natural fertilisers for their crops. The Haber plant at Ludwigshafen, run by the chemical giant BASF, transformed that grim picture exactly 100 years ago – by churning out ammonia in industrial quantities for the first time, triggering a green revolution. Several billion people are alive today only because Haber found a way to turn atmospheric nitrogen into ammonia fertiliser. "Bread from air," ran the slogan that advertised his work at the time.

But there is another, far darker side to the history of the Haber process. By providing Germany with an industrial source of ammonia, the country was able to extend its fight in the first world war by more than a year, it is estimated. Britain's sea blockade would have ensured Germany quickly ran out of natural fertilisers for its crops. In addition, Germany would also have run out of nitrogen compounds, such as saltpetre, for its explosives. The Haber process met both demands. Trains, bursting with Haber-based explosives and scrawled with "Death to the French", were soon chugging to the front, lengthening the war and Europe's suffering.

"If you look at the impact of the Haber process on the planet, you can see that it has been greater than any other discovery or industrial process over the past 100 years," said Professor Mark Sutton, of Edinburgh University. "On the positive side, there are the billions of people who are alive today thanks to it. Without it, there would have been no food for them. On the other hand, there are all the environmental impacts that a soaring world population, sustained by Haber fertilisers, have had. In addition, there is the pollution triggered by the release of ammonia fertilisers into water supplies across the globe and into the atmosphere.

"And, for good measure, there have been all the deaths caused by explosives created from Haber-manufactured ingredients. These have reached more than 100m since Haber invented the process, according to one estimate. So we can see Haber's work has been a mixed blessing."

Bald and absurdly Teutonic in demeanour, Haber was an ardent German nationalist. He was happy his invention was used to make explosives and was a fervent advocate of gas weapons. As a result, on 22 April 1915 at Ypres, 400 tons of chlorine gas were released under his direction and sent sweeping in clouds over Allied troops. It was the world's first major chemical weapons attack. Around 6,000 men died. Haber later claimed asphyxiation was no worse than blowing a soldier's leg off and letting him bleed to death, but many others disagreed, including his wife, Clara, herself a chemist. A week after the Ypres attack, she took Haber's service revolver and shot herself, dying in the arms of Hermann, their only son.

In 1918 Haber was awarded the Nobel prize for chemistry, a decision greeted with widespread indignation. Many British, French and US diplomats and scientists refused to attend his award ceremony in Stockholm. After the rise of Hitler, Haber – who had become a rich industrialist – was expelled from Germany because he came from a Jewish family, and died in Switzerland in 1934.

The ironies that afflicted Haber's life continued in death. One of the most effective insecticides he had helped to develop was Zyklon B, which was subsequently used by the Nazis to murder more than a million people, including members of Haber's extended family, including children of his sisters and cousins.

Since then, the use of Haber's process – or more properly the Haber-Bosch process in acknowledgement of Carl Bosch's work in turning Haber's ideas into a practical industrial process – has expanded dramatically. Today more than 100m tonnes of nitrogen are taken from the atmosphere every year and converted into ammonia compounds, in Haber-Bosch plants. These are then spread over the surface of the Earth, turning arid land into fields of plenty. As a result, our planet has been able to feed and sustain an unprecedented number of people. In 1900 there were 1.6 billion people on Earth. There are now more than 7 billion. Most of the extra mouths have been fed on food sustained by the Haber-Bosch process.

It has been calculated that half the nitrogen atoms in our bodies come from a Haber factory, via its fertilisers and the food nourished by them. As the Canadian scientist Vaclav Smil has put it in his book Enriching the Earth, the Haber-Bosch process "has been of greater fundamental importance to the modern world than the airplane, nuclear energy, spaceflight or television".

This has come at a price, however. There is the sheer strain placed on the natural environment by the number of human beings now sustained by artificial fertilisers. In addition, there are problems caused by our ever increasing appetite for ammonium chemicals. Our bodies may accumulate nitrogen atoms from fertiliser plants, but far more of these atoms fail to make it into the food chain and are instead released into the environment. The result, in many areas, has been calamitous. Nitrogen fertilisers get washed into streams, rivers, lakes and coastal areas where they feed algae that spread in thick carpets over the waters, suffocating life below.

Then there is the atmospheric release of all the excess ammonia, says Sutton. "Ammonia is released into the air from fertilisers on farms and can then be deposited on natural habitats with very unwelcome consequences," he said. "Consider the sundew … It can grow in very harsh environments in this country because its sticky leaves allow it to catch insects, which provide it with nitrogen and other important compounds. But when ammonia from artificial fertilisers is dumped nearby other less hardy plants grow and crowd out the sundew."

Sutton believes that while the dangers of fossil fuels and greenhouse gases are well known today, those of the nitrogen cycle, which affects drinking water, contributes to air pollution and affects the health of large parts of the population, have gone unrecognised. "We need nitrogen compounds to sustain our food supply but we need to be much more careful how we use them. That is the real lesson of the Haber process centenary."