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 ca n'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 . ''
20,000 delegates are expected in U.S. 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 Rio de Janeiro delegation is headed by Secretary of United Nations Rio de Janeiro .