It is hard to find anyone with a bad word to say about Sara Parkin, except in some sections of the Green Party that are noisier than they are numerous. Mind you, she has always been a gift to journalists: decently objective in her account of green politics, articulate, moderate. These qualities propelled her to stardom. It is hard to see what future the party has without her, if not at its helm, at least as an attractive figurehead.
The Green Party has always depended on its stars, however much it claims that the idea of leadership is unsound. Jonathon Porritt was by far its greatest asset before he (like Ms Parkin after him) grew fed up with its wrangling and became the director of Friends of the Earth.
His defection, and perhaps the messianism of David Icke, must have contributed to the halving of the party's membership since it peaked in 1990, the year after its successes in the elections to the European Parliament, when it won 15 per cent of the UK vote.
The Green Party faces several difficulties, any one of which looks enough to sink it. The electoral system is against it; the public is deeply sceptical about its claim to have redefined the economics and politics of running industrial societies; it refuses to operate coherently. But its greatest problem is one faced by all green organisations in the modern world: they are now surrounded by voters and politicians who want to be some shade of green, are increasingly going to some pains to work out what that means - and are even acting on it.
The greens, therefore, are being challenged where they live: they are no longer unique, only uniquely committed. They also have to defend their claims in what is an increasingly sophisticated debate. Green politics, like green campaigning, has always depended on the media finding the cause compelling, and often allowing people working for green issues a softer ride than they accord people with opinions on, say, education or health.
This is beginning to change, even for the most glamorous green organisation of all. As Peter Melchett, director of Greenpeace's UK office said yesterday: 'Environmentalists now operate in a much more sophisticated and knowledgeable environment. There are more NGOs (non-governmental organisations), there are more expert environmental journalists, and more expert environmental consultants.' He believes that Greenpeace has had a lot to learn in recent years, as it appointed and then listened to scientists, lawyers and increasingly wised-up campaigners.
Partly, Greenpeace has learnt to wear a pin-stripe as well as a wetsuit. Direct action has always been crucial to Greenpeace's charisma, but it is just as successful when it attends a meeting of the International Whaling Commission, or threatens or undertakes legal proceedings against companies or - very much the American model - the regulators.
What is less clear is how long campaigners can stick to fundamentalist language and purity of ambition. Greenpeace, always simplistic, is currently campaigning against the way pollution is contained within a legal framework. The campaigners claim this is licensed planet-wrecking. Industry responds: you wanted pollution reduced and it has been, by law; how would you have done it?
Well, Greenpeace habitually wants outright bans, including - at the moment - bans on incineration (which many argue is crucial to clearing toxic wastes) and all chlorine chemistry (which makes many plastics and cleans our water, among much else).
Greenpeace faces a dilemma. If it does not insist on being very tough, it becomes little different from Friends of the Earth, which has struggled for years to find a line between realism and excitement. Lord Melchett - the great- grandson of Sir Alfred Mond, whose family firm joined with others to form Greenpeace's great enemy ICI - believes that the group's fundamentalism does not look odd in Sweden, Germany or the Netherlands.
'In Britain,' he says, 'our audience is well behind its equivalents abroad. Opinion formers, industry, government and the general public all seem to answer green questions differently to those abroad.'
Lord Melchett defends Greenpeace's use of strong language to describe the effect that ICI's pollution has on rivers and the North Sea. Press releases say these are being destroyed by effluent. Yet the worst of the damage was historic, and industry is not lying when it argues that it is imposing a rapidly reducing load on the aquatic system around it.
Increasingly the media are capable of reflecting this debate: earlier this year BBC 2's Nature programme broadcast a very Greenpeace-ish account of the damage the chlorine industry does; within weeks it ran a film criticising Greenpeace for being unrealistic in its claims and demands.
The public still strongly supports green campaigns: both Friends of the Earth and Greenpeace maintain strong UK memberships and incomes, despite the recession. There remains a following wind for faster improvement in environmental performance. Yet some serious greens see real problems even here.
Richard Sandbrook, director of the International Institute for Environment and Development, believes that some campaigners have failed to respond properly to the post-Earth Summit world: 'Governments are beginning to shift, and some corporations are shifting quite radically. The question now is how you deliver on the commitment which is coming through. There was a green kneejerk reaction that Rio was a failure. But actually it was a remarkable first shot in laying out the terms for a North-South debate.' Greens refuse to admit such progress, and look churlish.
These perspectives are sharpened by an increasing realisation that industry and science hold the key to feeding and nurturing the billions of people who are now on the planet, and will inevitably quickly reproduce. Increasingly, greens are having to lose their romantic anti-industrial stance.
Even in super-green Germany, the Greens suffered severe reversals in the first post-unification election, when they were seen to be at odds with the emerging realisation that West Germany had cleaned itself up (in so far as it had) because it was a rich and successful industrial state, whereas East Germany's dirtiness flowed from being the reverse.
Perhaps the problem for greens is simply that having had to shout for attention for so long, they now find they have lost the capacity to hold conversations. There are signs that Mr Porritt, for one, is adjusting the volume and style, now that he is a commentator rather than a campaigner - he gave up the leadership of Friends of the Earth in 1990. His recent writings, according to one green-watcher, Keith Pike, the public relations manager of ICI Seeds, are increasingly 'addressing the social, scientific and economic complexities of the real world'.
As Sara Parkin frees herself of the Green Party's constraints, will she lose some if its fundamentalism too?
The author is writing a book on the dilemmas faced by a future world population of 10 billion, to be published next spring by Fourth Estate.