Twenty years after the first Earth Summit, a renewed bid to rally the world behind a common environmental blueprint opened Wednesday in Rio de Janeiro against a backdrop of discord and economic gloom.

Kicking off the so-called Rio+20 summit, Dilma Rousseff, president of host nation Brazil, called on "all countries of the world to commit" to reaching an accord that addresses the most pressing environmental and social woes.

The UN conference, which marks the 20th anniversary of the Earth Summit -- a landmark 1992 gathering that opened the debate on the future of the planet and its resources -- is the largest ever organized, with 50,000 delegates.

Around 115 leaders are expected to attend the main event itself on June 20-22 but a series of conferences grouping businesses, environmental groups and non-governmental organizations are being held in advance.

This frenzy of contacts and deal-making could well be more fruitful than the UN Conference on Sustainable Development itself, analysts say, mindful of the failures of the 2009 climate summit in Copenhagen.

Behind the scenes, there is incipient panic over the draft summit communique after three rounds of preliminary informal negotiations left more than 75 percent of the paragraphs still to be agreed.

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.

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 UN has not ruled out the possibility of intense negotiations continuing right up to the leaders' summit that will be attended by French President Francois Hollande and Chinese Premier Wen Jiabao, among others.

Nations all agree that the summit comes at a turning point, and its outcome is crucial.

But privately delegates expressed doubt that a consensus on how to tackle these problems will be reached while many governments remain focused on the economic crisis.

The European Union will fight to the last for credible commitments in Rio but it will be "very, very difficult", the bloc's environment commissioner Janez Potocnik said in Brussels on Wednesday.

"After tough pre-negotiations in New York, unfortunately not enough progress has been made so we have some intense days ahead of us in Rio," the commissioner said.

Privately, EU negotiators were more forthright. "It will be very, very difficult to draw up concrete measures and fix dates," one told AFP.

"No promises were made during lead-up negotiations," said another. "There was nothing concrete, just a lot of blah-blah-blah and statements of intention."

French dreams of creating a World Environment Organization, for instance, are not expected to see the light of day. "At best there'll be a bigger role for the United Nations Environment Programme (UNEP)," said a senior European official.

On the eve of the summit, the world's science academies warned that Earth faced a dangerous double whammy posed by voracious consumption and a population explosion.

According to UN figures, global food demand will double by 2030 and energy consumption will soar by as much as 45 percent, putting mounting pressure on finite resources amid growing social inequality, water shortages and global warming.

"Rapid population growth can be an obstacle to improving standards of living in poor countries, to eliminating poverty and to reducing gender inequality," said a joint statement from the science academies.

It emphasized the need to help millions rise out of poverty, brake trends of reckless consumption and address population growth through voluntary means such as education for women and access to contraception.

"If the right conditions are in place, measures that reduce fertility rates while respecting human rights can stimulate and facilitate economic development, improve health and living standards, and increase political and social stability and security," it said.

Notably absent from Rio will be US President Barack Obama, who is facing a tough re-election race at home, and German Chancellor Angela Merkel. The United States will be represented by Secretary of State Hillary Clinton.

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