Brazil's President Dilma Rousseff, left, greets Sweden's King Carl Gustaf as U.N. Secretary-General Ban Ki-moon looks on.

Story highlights

More than 50,000 delegates are expected in Rio de Janeiro for the Rio+20 Summit

The conference is aimed at promoting economic growth and poverty reduction

Preserving the planet's resources also a main topic of discussion

The U.S. delegation is headed by Secretary of State Hillary Clinton

Rio de Janeiro CNN  — 

World leaders poured into Rio de Janeiro on Wednesday, 20 years after the landmark Earth Summit, to commit themselves to a new roadmap for sustainable development – with that roadmap already under fire for failing to set firm goals.

The three-day Rio+20 Summit opened with words of warning from the United Nations Secretary-General Ban Ki-moon.

“Let us match words with actions,” he told reporters. “Our scarcest resource is time, and it is running out.”

More than 50,000 delegates are expected to participate in the conference, which is aimed at promoting economic growth and poverty reduction while simultaneously preserving the planet’s resources.

Population growth can’t be ignored

But the summit has been overshadowed by the crisis in Europe and by key elections in the United States and elsewhere that have hobbled governments’ ability, or willingness, to act.

Notable no-shows included U.S. President Barack Obama, British Prime Minister David Cameron and German Chancellor Angela Merkel.

Negotiations over a final document that would be signed by leaders when they flew in dragged on until the last moment because countries could not agree on many of the more polemic issues.

Even Ban said he had higher expectations.

“I know some member states hoped to have a bolder and more ambitious outcome document,” Ban said. “I also hoped that we should have a more ambitious outcome document.”

The resulting text is an often vague commitment to sustainable development, without measurable targets or financial commitments.

Can summit solve environmental problems?

Many member states nonetheless praised the host government for getting all countries to finally agree on a document.

The U.S. delegation, represented by Secretary of State Hillary Clinton, called the summit a “historic opportunity to communicate the value of sustainable development and help galvanize real-world, collaborative action to stimulate growth, protect the environment and provide a healthy future for our citizens.”

Businesses played a much bigger role at this summit than they did 20 years ago, with many observers saying they have actually taken the lead by providing real examples of sustainable development.

Georg Kell, head of the U.N. Global Compact, said his group hopes to increase the number of companies agreeing to concrete sustainable development goals to 20,000, from 7,000.

“I hope also we will be able to inspire governments to have the courage to set the right incentives,” he told CNN.

Extinction threat ‘a call to world leaders’

On Wednesday, the first session of the high-level summit was addressed by 17-year-old activist Brittany Trilford of New Zealand, who challenged leaders: “Are you here to save face or are you here to save us?”

Ban suggested that the highly criticized final document could still be revised by world leaders before the summit ends on Friday.

When asked by reporters about that possibility, he responded: “Why do we have a summit meeting? Why have I been inviting and urging leaders, heads of state and government? They are the ones who can make political decisions, who can make a choice.”