Many of the issues the Environment Movement has faced over the last 50 years have been difficult, but none has been as formidable as the two challenges confronting it over the next half century, confronting the earth: population growth and climate change.
These two colossal problems, it is clear now, cannot be "solved"; they can only be coped with, and the coping will have to be by governments. The task of the Green Movement will be to keep the pressure on governments, and companies, and individuals, to do what is necessary, however difficult that is.
Both issues are controversial. Some believe the rise in human numbers, from a world population of 7 billion to perhaps 9.3 billion in 2050 – that's the UN's medium estimate – will not be a problem, and certainly there is no plan to bring the estimated 2050 figure down.
But finding food for an extra two billion mouths in a mere 38 years, on top of those who are hungry today, is clearly going to be a Herculean task, and if we disaggregate the world figure into the projections for individual nations, the task seems more daunting still, especially with some of the "high growth" countries in Asia and Africa.
Bangladesh, with an estimated 148 million people at 2010, goes, according to the UN medium estimate, to 194 million in 2050; Pakistan goes from 189 million to 274 million. In some of the poorer parts of sub-Saharan Africa, the projections are quite remarkable, with doublings and even treblings expected in four decades: Kenya goes from 40 million people in 2010 to 96 million in 2050, while Niger, a country in the Sahel semi-desert belt where agriculture is difficult at best, will see its population go from 15 million to 55 million.
Yet the concern for the environment movement will be, can these extra billions be not only fed, but brought out of poverty through economic growth, without the planet being trashed? Can it be done without rainforest being torn down for agriculture, without the fish stocks of the oceans being exploited, without the atmosphere being swamped with climate-changing carbon from thousands of new power stations?
The idea that grapples with this is sustainable development and next week the world community meets in Rio de Janeiro to try to shape the first global sustainable development goals, which would probably concern food, water and energy, and run from 2015.
But even if they are agreed, they can be destabilised by a changing climate. A great fear of far-sighted environmental thinkers is that global warming and its effects will combine with population growth, in interactions which will make things much worse. For example, Bangladesh will find it much harder to accommodate its extra 46 million people by 2050 if, as is expected, it begins to be affected by climate-change-induced sea level rise, with much of the nation below sea level already.
Yet climate change has slipped down the pubic agenda. There are three reasons for this, one being the recession, which affects us now, while global warming is a concern mainly of the future.
The second is that climate change sceptics, hardly any of whom are climate scientists, and many of whom are funded by the fossil fuel industry, have induced a certain amount of uncertainty in the public mind about the issue. And this has been able to take root over the last few years because – and this is the third reason – the warming process appears to have paused.
No-one really knows why. A good guess is the gigantic cloud of sulphur emissions from Chinese power stations, which doubled their output of waste gases between 1996 and 2006: the sulphur particles have the opposite effect of the carbon emissions, and reflect back the sun's heat. But unless the laws of physics are altered, those global carbon emissions, now 33 billion tons annually and rising at six per cent a year, are going to make world temperatures rise considerably in the coming decades with potentially disastrous consequences.
What can the Green Movement do about this? Quite a lot, really, as was evinced by Friends of the Earth's (FoE) "Big Ask" campaign for a climate change law, which would commit the UK Government to make legally-binding annual cuts in its carbon emissions. It succeeded, and in the Climate Change Act, 2008, Britain now has the toughest climate legislation in the world.
Tony Juniper, the FoE director who oversaw the campaign, is aware that climate change is the most difficult of all environmental problems. "It's bloody huge," he says. "It's about everything – aviation policy, transport, energy, nuclear power, agriculture, recycling. With the other issues we can get a tactical victory without all that baggage, but climate is different, and so is the timetable."
So he doesn't think the Green Movement has failed?
"No," he says. "It's work in progress."
There's going to be an awful lot of that work needed by the greens over the next half century.
Rio plus 20: Another talking shop?
The UN sustainable development conference in Rio de Janeiro next week is called "Rio Plus 20" as it marks the 20th anniversary of the celebrated Earth Summit, held in the Brazilian city in June 1992.
The earlier gathering, which brought together more than 100 heads of state and government, created two major institutions to help the world deal with its growing environmental crisis: the UN Convention on Climate Change, to tackle global warming, and the UN Convention on Biodiversity, to tackle the increasing assault on wildlife and the natural world.
The summit's success marked a huge step in integrating environmental concern into world politics. But 20 years on, this summit looks like being a pale shadow of its predecessor, and the most positive outcome is likely to be merely an agreement to talk about sustainable development goals. The lack of excitement is reflected in the guest list.
The heads of state of Brazil, Russia, India and China will be there, But President Obama and the German Chancellor Angela Merkel, will not, nor for that matter will David Cameron. He is sending the Deputy Prime Minister Nick Clegg.