Sustainable development world's top issue: Moon

United Nations chief Ban Ki-moon singled out sustainable development as the top issue facing the planet with the world's seven billionth person expected to be born next month.

Key to this was climate change, and he said time was running out with the population set to explode this century.

"Next month, the seven billionth citizen of our world will be born," the UN secretary general said during a speech at Sydney University on Thursday.

"For that child, and for all of us, we must keep working to fight poverty, create decent jobs, and provide a dignified life while preserving the planet that sustains us.

"That is why the sustainable development agenda is the agenda for the 21st century.

"Above all, that means connecting the dots between challenges such as climate change and water scarcity, energy shortages, global health issues, food insecurity and the empowerment of the world's women."

Ban is in Australia after visiting the Solomon Islands and Kiribati in the Pacific, two small nations he described as "on the front line" of the climate change issue.

"I know, once again, there are the sceptics. Those who say climate change is not real," he said.

"But the facts are clear: global greenhouse gas emissions continue to rise, millions of people are suffering today from climate impacts. Climate change is very real."

He suggested the doubters take a trip to Kiribati.

"Look into the eyes of the young boy who told me: 'I am afraid to sleep at night' because of the rising water," he said.

"Talk with the parents who told me how they stood guard fearing that their children might drown in their own homes when the tide came in."

The UN holds climate talks in South Africa in November, which are seen as the last chance to renew the Kyoto Protocol, the only binding global deal to cut greenhouse gases.

The Protocol's initial five-year commitment period, covering 37 industrialized countries, expires at the end of 2012.

Ban said the summit needs to keep building on what has been achieved.

"We need ambitious mitigation targets that ensure that any increase in global average temperature remains below two degrees Centigrade," he said.

"Moreover, given that the first commitment period of the Kyoto Protocol expires next year, a political formula must be found to ensure that a robust, post-2012 climate regime is agreed upon, and is not delayed by negotiating gamesmanship."

The Kyoto Protocol's future is uncertain because China and the United States, the world's top two polluters, are not subject to its constraints.