Today it's raining outside, for the first time since I am here in Rio de Janeiro. The heat has broken but the fog remains. Helicopters are circling above us at the UN Conference on Sustainable Development (Rio+20) and security is very tight with police and military everywhere as the world's heads of state begin to arrive.

Delegates and participants at Rio+20 now number above 30,000. The atmosphere has picked up, the food courts are full and there is no spare space to sit, charge a laptop or take a break. The media are here with their cameras up and the place is buzzing. But here is the thing: There is little to report about. Journalists keep asking what the story is. There is a story but it's a very dark one and in our hearts everybody is looking for a ray of sunshine from Rio+20. But there are worrying signs of the very real and severe failure of the negotiations.

Stripped of ambition and substance

The Brazil government and its negotiating team have railroaded the negotiations to finalise an outcome text last night for heads of state to sign-off. But there have been complaints from many countries that the Brazilians have pushed the process too far so it has been stripped of any ambition and substance.

A senior negotiator from the UK delegation team stated that "there is almost nothing left now for the heads of state to negotiate and it's almost a done deal. But the real problem is this isn't a deal that anyway near addresses what we need." From analysing the text it's clear that the deal, as it stands right now, is a black hole of low ambition and little urgency. And we are all worrying that the black hole is gathering pace.

No new sources of finance

It's not just the Brazilians who bear responsibility. Leaders of the world's major economies came to Rio empty-handed with nothing to offer; no (financial) commitments and a dire lack of leadership. The current outcome text provides no clear targets for reducing climate emissions or reversing environmental degradation, there are no legally binding commitments and - more worrying - no new sources of finance. Without these elements as a foundation the Rio+20 outcome will be an epic failure on a planetary scale.

The science is clear: we can't continue to grow our economies by gobbling up and depleting our stocks of natural capital, be it for example fish, carbon or water. We are undermining the very foundation of our planetary survival and its natural capital. Increasingly the impacts of climate change and resource degradation are severely impacting the world's poorest and most vulnerable people. If we don't urgently tackle climate change as well as other environmental issues we will reverse development gains and lock out future generations from the development choices they so urgently need in order to escape from poverty.

Only two days left

But there is a slim chance to make a huge difference, if just a few world leaders could demonstrate bold political leadership and state that they are not happy to commit the planet to an unsustainable future and many millions more people to a future of grinding poverty. Without tackling climate change and poverty reduction there will be no sustainable future. Whether Rio+20 will be game over for the planet remains to be seen.

There is just one day left until the conference closes Friday evening. One day for leaders to act and deliver a roadmap for a sustainable future, and I just hope they have the courage and determination to deliver the future we want rather than the future we can't live with.