It's a question many people have probably asked themselves, seeing the ever-increasing environmental degradation around the world: why aren't we doing more to protect our planet? And it's not that easy to answer, as it seems such an obvious course of action, given the parlous state the Earth is in. But Achim Steiner has an answer of sorts. He thinks things are so bad that people can't quite grasp it.
He is worth listening to, because there are not many individuals who could be said to have a truly comprehensive overview of the state of the planet. This 50-year-old Brazilian-German is the executive director of the United Nations Environment Programme (Unep), the part of the UN family that deals with planetary ills, and he has spent a long career trying to help communities across the world to develop, without trashing their surroundings and their natural resources base. In other words, without screwing up their future.
Sustainable development, it is called. For more than 20 years it has been thought of as a great idea whose time has come. So why is so much of what is happening on every continent still clearly so unsustainable? "In a sense ... reality has overtaken our cognitive capacity," Mr Steiner says. "I mean the reality of it has overtaken our capacity to understand it, to understand quite what we are causing and unleashing, almost ... I think we have not even begun to understand how serious are the underlying trends that we have brought to bear on the sustainability of this planet.
"A classic illustration is the ... luxury of this continued debate about scientific uncertainty with climate change. If even 10 per cent of what the IPCC [the UN's Intergovernment Panel on Climate Change] said were to come true, it should actually make us sit up and say immediately, 'change course!'."
But we don't say that, Mr Steiner believes, because "there is an accelerating set of trends, from the atmosphere to the biosphere, to our ability to feed ourselves in a world which will soon have nine billion people, that gives us a sense of what will happen in the next 20, 30, 50 years, that we have simply not yet begun to appreciate".
He can see the trends, quite clearly, because it is his job to, and he talks about them vividly: agriculture which is no longer "a management of that one metre of arable land on which we depend for virtually everything that grows" but a process which "very often has become a mining operation"; oceans which have been overexploited to the point where "two-thirds or more of the fish stocks are either at maximum offtake or actually depleting"; carbon dioxide pumped into the atmosphere "to the point where we are actually fundamentally changing the climate prospects of our planet".
It is the fact that we have got our hands on everything, humans are grabbing everything, and we still do not realise the extent of our grab. In his words: "That notion, of realising that actually we as human beings have moved from somewhere in the food chain, to be right on top of it in planetary terms, is something we have not yet grasped.
"And partly it is a luxury, because it is much easier not accept that reality, because then you really have to take responsibility; and we are at the moment avoiding taking responsibility. Individually as much as collectively."
Yet Mr Steiner is no misanthropist; he is not one for scolding, like a hell-fire preacher; he does not think people are the problem. Just the opposite: he is a convinced humanist and has spent his life in development, focusing on the poorest countries and how they can grow their way out of poverty. The way forward is to combine economic growth with respect for the environment, in that powerful concept of sustainability. Yet it is not happening.
A major conference in Brazil this June will try to find out why not – 20 years on from the Earth Summit in Rio de Janeiro, which set out a sustainable development agenda for the world, one that was never taken up. Mr Steiner will play a key role in Rio Plus 20, as the new summit is labelled. Last week there was a preliminary conference in London which he came to address.
He spans worlds, particularly the developed and the developing: this is a man who went to a Brazilian village school (the son of a German farmer who had emigrated to Rio Grande do Sul) and then read philosophy, politics, and economics at Oxford University. He looks like a banker and, indeed, would be perfectly equipped to be the head of JPMorgan or perhaps even more, of a giant corporation such as Volkswagen. Yet he went from Oxford not into a management training scheme but to a small village charity in Tamil Nadu, helping the poorest of India's poor, and has spent much of his career in the world's most deprived regions, from Pakistan to Zimbabwe.
Combining toughness with a genial approachability, he has run Nairobi-based Unep for six years. His tenure has indeed given him a panoramic overview of the state of the planet but it has not shaken his belief that doing things in a different way is possible. "I am always reluctant to be a sort of doomsday-scenario person," he says. "Yes, I am aware of the trends, but I am committed to using that knowledge not as a kind of 'aren't we awful and terrible' message but, rather, to ask how can we continue doing this when we actually have a choice, the means, the technology, to change course. That's the motto of our era. We are able, and we must change course."
After hearing his chilling and convincing litany of the planet's problems, I doubted if he could be an optimist, but when I asked him, he said at once: "Yes, I'm an optimist, otherwise I would not do this job now."
What, even in the face of watching the world going to hell in a handcart?
"Yes, because I think we have enough examples in history where people have changed course, and there are good reasons to believe that we can do it now. I think we are losing time, but I don't think that's a reason to lose hope."
A LIFE IN BRIEF
Born: 17 May 1961, Carazinho, Brazil.
Education: schools in Carazinho; Dover College (England); Worcester College, Oxford (BA 1983); School of Oriental and African Studies, University of London (MA 1985); German Development Institute; Harvard Business School.
Career: GTZ (German government foreign aid organisation): technical adviser 1986-88; desk officer for Asia and the Middle East, 1988-89; policy adviser, 1989-91.
International Union for the Conservation of Nature: regional office for Southern Africa 1991-94; global policy unit, Washington DC, 1994-97; Mekong River Commission; 1997-98; director-general, 2001-05.
Secretary-general of the World Commission on Dams, 1998-2001.
Current job: executive director of the United Nations Environment Programme since 2006.
RIO PLUS 20: the agenda
The key themes at this summer's Rio summit will be how to create a green global economy and how to make development sustainable.
Within these two broad topics are seven priority issues: jobs, energy supplies, food security, cities, water supplies, the health of the oceans, and recognising disaster risks.
Plans for making the global economy greener emphasise the need to reduce poverty. Other measures on the agenda include bringing down trade barriers, environmental regulations and the transfer of technology.
The summit is also expected to address concerns among many countries that green measures, such as tackling climate change, will damage national economies.