President Barack Obama heads a list of high profile absentees for the Rio sustainable development summit this week where UN leaders say some tough decisions will have to be taken for the future of the planet.

Britain's Prime Minister David Cameron, Germany's Chancellor Angela Merkel and China's President Hu Jintao will also go to the Group of 20 rich nations summit in Mexico but then head straight home before the Rio de Janeiro event starts Wednesday.

The US presidential election, the debt crisis in Europe and China's looming leadership transition have all weighed heavily on acceptances to the Rio summit that will seek to set some ground rules for global growth that helps the poor and does not wreck the environment.

Helen Clark, who was prime minister of New Zealand from 1999 to 2008 before she took over the UN Development Programme (UNDP) in 2009, says she can understand the pressures.

"It is always great to have the G8 leaders there," Clark, who has played a key role in bringing together the summit, told AFP.

"I have been a leader in election year where you look very carefully whether you are going to travel anywhere four months out from an election. That is a very practical consideration -- where are you most needed. So I am not prepared to criticise anyone for not coming."

The summit will still be attended by presidents Vladimir Putin of Russia, Francois Hollande of France, Jacob Zuma of South Africa alongside Prime Ministers Manmohan Singh of India and China's Premier Wen Jiabao.

And the shadow of the global troubles to be debated will follow the absent leaders home, Clark said.

"The toxic combination of falling incomes, social unrest and environmental degradation. This is reality. We have got a common problem here. We need to have a shared vision of how to tackle it," she said.

"We are heading for chaos if we don't tackle these issues."

So the summit, where corporate chiefs and grass roots activists will take the total attendance to 50,000 people, has an ambitious agenda.

Tempering the voracious consumer appetite of rich nations, saving oceans and mountains, how to keep cities livable in a world of seven billion people, how to measure human development alongside economic prosperity will all be debated.

The UNDP says its message is that each policy and investment decision must take into equal consideration its economic, social and environmental impacts.

"I think there is a high level of awareness that the planet is in peril to put it bluntly," said Clark.

The absentees all insist they do take the topic seriously. The United States will still be represented by Secretary of State Hillary Clinton.

"Development is profoundly in the interests of our economy and our national security as well as obviously the interests of the people around the world," said Todd Stern, the US Special Envoy for Climate Change.

"It makes business sense for the United States, and it makes national security sense," he added.

Britain's Cameron is a co-chairman of a high-level UN panel that wll make recommendations on Sustainable Development Goals -- a broad set of global targets to protect the environment and reduce poverty.

And the industrial world's agenda and concerns will be strongly put by the leaders who do go.

According to French diplomats, President Hollande will call for major steps toward a new vision for development to counter the "extremely alarming" degradation of resources and the environment.

The so-called rich leaders can expect to hear many complaints however from their counterparts in struggling nations who say that past promises to help with money and technology have not been kept.