Twenty years ago, a burst of sunny optimism radiated from Rio de Janeiro as world leaders staged a meeting that would prove pivotal.

Amid post-Cold War euphoria and a desire to tackle the problems of the looming millennium, the UN's 1992 Earth Summit inscribed protection of the planet on the world's priority list.

It set down a blueprint, Agenda 21, for sustaining nature rather than destroying it, and created UN mechanisms designed to brake the oncoming juggernauts of climate change, desertification and species loss.

Leaders gather once more in Rio from June 20-22 for the 20-year followup to that great event.

But how very different the world is today, and how much darker the mood.

By almost every yardstick, as the UN Environment Programme (UNEP) reported in a landmark assessment last week, our planet is sicker than ever.

Despite the rising prosperity in China, India and other emerging giants, billions remain in the rut of poverty.

And as the world's financial calamity nears its fourth anniversary, the ability -- and will -- of countries to embrace green growth is badly constrained.

"Governments are mired in crisis and their eyes are fixed on the present, whereas Rio+20 requires them to calmly draw up a future for the planet," Brice Lalonde, a former French environment minister who is co-coordinator of the summit, told AFP.

"It's hard to do the two things at the same time. But that, in principle, is what heads of state are there for."

Around 115 leaders are expected for the summit, which will cap more than a week of meetings gathering as many as 50,000 activists, business executives and policymakers.

This frenzy of contacts and deal-making could well be more fruitful than the UN process itself, say some. The nation-state system remains traumatised by the failures of the 2009 climate summit in Copenhagen.

There is "a risk of division between developed countries, emerging countries, poor countries, the risk of failure because there may be other pressing matters," France's new president, Francois Hollande, said on friday.

"The world is today turned towards the economic crisis, the financial crisis, and is worried about a certain number of conflicts, such as Syria... might easily turn away from what is however the top priority, the environment."

Already, many in the green movement fear that Rio+20 will fall dismally short of guiding the planet towards better health and a brighter future 20 years from now.

Behind the scenes, there is incipient panic over the draft summit communique.

The charter is supposed to sum up the challenges and spell out pledges to nurture the oceans, roll back climate change, promote clean growth and provide decent water, sanitation and electricity for all.

There have so far been three rounds of "informal informal" negotiations on the document, the last of which -- an emergency session -- ran in New York from May 29 to June 2.

Out of 329 paragraphs, only 70, or 21 percent, have been settled.

The rest of the text is lost in a sea of brackets, denoting discord, as countries squabble over the level of ambition.

The biggest divergences lie in four areas, according to sources close to the negotiations.

They include action on climate change, protecting the oceans and achieving food security, and whether "Sustainable Development Goals" should replace the Millennium Development Goals when these objectives expire in 2015.

The drafting panel meets in Rio for three days from Wednesday in a new bid to end the deadlock.

"As things currently stand, we are facing two likely scenarios -- an agreement so weak it is meaningless or complete collapse," said WWF's director general, Jim Leape.

For radicals, a parallel "People's Summit" in Rio will be the chance to ram home their message that the world's economic model is broken and tinkering with it is pointless.

In their view, it has neither protected the environment nor ended poverty, and now its failure has engulfed many rich countries too.

"The tone of the People's Summit will be one of protest," said Bazileu Alves Margarido of a Brazilian NGO, the Institute for Democracy and Sustainable Development.

"We see Rio+20 as offering no hope, no political will by countries to change things."