It has been part-religion and part socio-political crusade, and has penetrated all corners of the globe. It has swept up young people -- and not a few of their elders -- into its embrace and on Saturday it is 50 years old. But how much of a difference has the modern environment movement really made?
We know the noble causes. They trip off our tongues, from banning whaling, halting deforestation and saving endangered species to mending the hole in the ozone layer and bringing a halt to global warming -- never mind acid rain, nuclear power and genetically modified organisms.
The modern Green movement began with the publication of Rachel Carson's 'Silent Spring', the 1962 book which awoke in millions the sense that the Earth as a whole was threatened by human actions and needed to be defended.
We can recount the history of the movement in a number of ways: through the progress of its ideas (such as that of sustainable development); through the birth and growth of its pressure groups (such as Greenpeace and Friends of the Earth, both founded in 1971); through the rise of its influence in Big Government (the US Environment Protection Agency was founded in 1970) or even through the record of its giant conferences, such as the Earth Summit in Rio de Janeiro in 1992 which has a successor meeting -- 20 years on -- taking place in Rio next week.
But perhaps the most powerful way is to recount it through its people. They are overwhelmingly the idealistic activists of the environmental pressure groups around the world who down the years have not only protested at despoliation of the planet, they have taken direct action to try to stop it -- and let this be said, have done so in the Greenpeace tradition of resolute non-violence.
Most of them are of course anonymous, until sometimes they pay the ultimate price, like Fernando Perreira, the photographer killed when French agents blew up the Greenpeace ship Rainbow Warrior in Auckland Harbour in 1985, or Chico Mendes, the rainforest activist assassinated by Brazilian ranchers in 1988.
But some personalities do emerge with key ideas or actions, at key moments, and one is a name that will surprise most people: William Anders. He was a US astronaut, one of the crew of Apollo 8, the first manned spacecraft to leave the Earth's orbit and circle the Moon. When, on Christmas Eve 1968, he and his fellow crewmen Frank Borman and Jim Lovell emerged in their craft from behind the Moon's dark side, they saw in front of them an astounding sight -- an exquisite blue sphere hanging in the blackness of space.
The photograph Anders took is known as 'Earthrise', and its taking was one of the most profound events in the history of human culture, for at this moment we truly saw ourselves from a distance for the first time; and the Earth in its surrounding dark emptiness not only seemed infinitely beautiful, it seemed infinitely fragile. This fed into a burgeoning concept -- that of Spaceship Earth.
A series of thinkers then began to explore the limits of Spaceship Earth, and in two books in particular produced what we might term the first environmental scare stories.
One was 'The Population Bomb' by a biology professor at Stanford University, Paul Ehrlich, and the other was 'The Limits to Growth' by a group of researchers from the Massachusetts Institute of Technology working under the label of The Club of Rome.
Both confronted issues which still confront us today -- the soaring rise in human numbers, and the finite nature of natural resources -- but both made claims which now appear exaggerated. However, at the time, they gave to nascent environmentalism its essential characteristic, its sense of urgency.
More lasting in its effect was a more unconventional critique: 'Small is Beautiful' by the British economist EF Schumacher, published in 1973. Subtitled 'A study of economics as if people mattered', the book was an attack on the dehumanising effect of massive companies and the continuing belief of most economists that bigger was better. For this, it is as loved by environmentalists today as it was on its appearance four decades ago.
But the profoundest of all the books to have appeared during the Green movement's 50 years is undoubtedly 'Gaia', the revolutionary look at life on Earth by the British scientist James Lovelock, which appeared in 1979.
Mr Lovelock's idea was that the planet behaved like a single giant organism -- that it possessed a planetary-scale control system which itself kept the environment fit for life. It was an entirely scientific study, but his naming the system after the Greek goddess of the Earth gave it a mystic flavour which appealed to many environmentalists, and if 'Earthrise' is the Green movement's iconic image, 'Gaia' might be said to be its sacred text.
It is surprising that there is not a single book which stands out about the most serious of all our environ-mental problems, which emerged half-way through the Green movement's 50 years: climate change.
But there is a key figure: James Hansen, the Nasa scientist who first alerted Americans to the dangers of global warming more than 25 years ago, and continues to do so today. Professor Hansen's single-minded insistence that the climatic future is truly perilous, however distracted we are by recessions or anything else, once again reinforces the fact that the people of the environment movement have been as vital as its ideas.