Tens of thousands of delegates are converging on Rio de Janeiro in a global jamboree of networking and talks on Earth's growing population and ways of sustaining it.

Opening the Rio+20 summit, Brazilian President Dilma Rousseff said participating countries need to agree on actions that can reduce threats to the environment and works against poverty and social deprivation worldwide.

Cynicism abounds amid high-minded rhetoric that has filled thousands of pages of background materials, committee proceedings, speeches and transcripts.

Amid warnings that the planet cannot sustain human population growth at current rates, critics say some of the worst problems are still down to poor governance, human rights abuses and unequal distribution of food and wealth rather than net scarcity of resources.

Poor government is blamed for unequal distribution of natural resources, including food and water.

Modern living has led to a culture of waste, with thousands of tons of food discarded daily at the same time as thousands of people die elsewhere of starvation, say critics.

The summit itself threatens to become a glaring example of needless waste as millions of dollars are lavished on hospitality, hotels, transportation and telecommunications.

Host Brazil is a frequent target of criticism for paying lip-service to the inequality rhetoric while hundreds of thousands of its own citizens live in squalid poverty, say critics.

The conference marks the 20th anniversary of the Earth Summit that declared the environment a priority but the ongoing conference is the largest ever organized, with 50,000 delegates, U.N. sources said.

About 115 international leaders have confirmed attendance but U.S. President Barack Obama and Germany Chancellor Angela Merkel won't attend -- Obama because of the looming presidential election and Merkel because of the eurozone crisis.

Talks in committees and panels began last Wednesday after Rousseff's highly publicized speech but the summit isn't scheduled to officially begin before next Wednesday.

On the sidelines of environmental and political talks are numerous meetings in which manufacturers of renewable energy industrial equipment and technologies, environmental cleanup and other related industries hope to secure new deals.

Global energy consumption is forecast to increase 45 percent by 2030. Food demand is also expected to soar to double current volumes but critics say the rise of emerging markets, changing lifestyles and urbanization is generating more food waste.

Despite those conflicting versions of the problem, China and the G77 developing countries want to create a sustainable development fund with an initial investment of $30 billion. Analysts say the proposal aims also at creating new consumer markets and to help revive global economic performance since the 2008 financial crisis.

In addition to the government officials, lobbyists and business promoters, the conference is being attended by protesters for a vast range of causes.

The Greenpeace ship Rainbow Warrior arrived in Rio last Wednesday to push its demand for banning deforestation of the Brazilian Amazon. Analysts said the campaign has little chance of success, however, because the cutting of trees is backed by powerful vested interests who are set to thwart any move by environmentalist groups.