With two months left before leaders assemble for the UN's Rio Summit, prospects for a radical fix of the planet's worsening environmental ills and poverty seem remote.

Around 100 heads of state and government are expected in Rio de Janeiro for the June 20-22 summit on sustainable development.

It takes place 40 years after the first big global environment meeting and 20 years after the near-legendary Earth Summit, where the United Nations set up two fora to combat climate change and biodiversity loss.

That initiative nailed environment firmly to the top of the world's political agenda.

Yet two decades later, the problems are worse than ever.

Indeed, many experts gloomily say mankind is destroying his future by the reckless drive for prosperity today.

Scientists at a pre-Rio conference in London last month said the UN's goal, enshrined less than 18 months ago, of limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) is already out of reach.

"We have to realize that we are looking at a loss of biodiversity that is unprecedented in the last 65 million years. We are clearly entering the (planet's) sixth mass extinction," said Bob Watson, former head of the UN's climate panel and chief advisor to Britain's environment ministry.

The summit faces a triple task of tackling this crisis, eradicating entrenched poverty and placing growth onto a sustainable path, with measures to stimulate the green economy.

But -- in contrast to 1992 -- no-one is expecting a horizon-sweeping master plan.

The West's financial crises, the near-fiasco of the 2009 Copenhagen climate summit and shifting grounds of geopolitics, with the emergence of China, India and Brazil, all point to a low-key event.

"In the past 20 years, the landscape has completely changed," said Laurence Tubiana, head of a French thinktank, the Institute of Sustainable Development and International Relations (IDDRI).

"Today there is a crisis of multilateralism, conflicts and absence of leadership," she said.

Summit boosters predict the big event will deliver a dose of pragmatism or at least sketch a useful -- if un-visionary -- path for future generations.

Before that, though, a text battle looms.

At an "informal informal" meeting in New York from next Monday, negotiators will start digging into a draft summit communique that in three months has ballooned from 20 pages into a sprawl of 180 pages.

"I am worried by the risk of a lowest common denominator, which can happen in any big conference demanding consensus," Brice Lalonde, executive coordinator of the summit, told AFP.

"But for the time being, everyone is talking about the need to be ambitious."

The New York meeting will have to sort out the one area of major controversy.

France, supported by what it says is a hundred or so developing countries, wants to transform the UN Environment Program (UNEP) into a super-agency, widely endowed with autonomy, that could act on the environment.

This is opposed by summit hosts Brazil and also by the United States.

Many grassroots activists warn a fudge is in sight.

Bazileu Alves Margarido of a Brazilian NGO, the Institute for Democracy and Sustainable Development, said the mood at a planned counter-meeting -- "the Summit of the Peoples" -- would be angry.

"The atmosphere will be one of protest," he told AFP. "We are expecting very little out of the official conference. We see a Rio+20 which is hopeless, without political will by countries to change things."

Around 30,000 people are expected for the June 15-23 alternative conference, including representatives from Amazonian Indians, Spain's "Indignants" youth revolt and from Arab Spring countries.

It will take place in the Aterro do Flamengo Park, at the foot of Rio's trademark Sugarloaf Mountain.

No official word has filtered through on summit names, but US President Barack Obama is unlikely to attend, according to well-informed sources.

The host country is mobilizing 20,000 personnel, including 10,000 troops, to provide security.