US President Barack Obama has placed climate change at the forefront of the G20 summit in Brisbane, announcing a $US3 billion contribution to the Green Climate Fund, and calling on young Australians to pressure politicians to resist vested interests and tackle global warming.

In a wide-ranging speech he also declared the United States would never abandon its leadership in the Asia-Pacific and called on countries to tackle Ebola rather than build moats around their island nations.

Australia resisted US pressure to place climate change on the official G20 agenda. However, it will be raised in the discussion on energy.

Prime Minister Tony Abbott raised his government's repeal of Labor's carbon tax in a speech to a gathering of world leaders, which included President Obama, earlier on Saturday.

A few hours later, Mr Obama was applauded every time he mentioned climate change in his speech at the University of Queensland. He warned that no region in the globe had more at stake than the Asia-Pacific and said climate change meant more bushfires, flooding, extreme storms and rising seas in Australia.

"The incredible natural glory of the Great Barrier Reef is threatened," Mr Obama warned.

The President said he wanted to return to Australia to visit the reef with his daughters when he had more time. "And I want them to be able to bring their daughters or sons to visit and I want that there 50 years from now," he said to applause.

As foreshadowed, he pledged $US3 billion to the Green Climate Fund to help developing nations adapt to cleaner-fuelled economies.

"Let them leap frog some of the dirty industries that powered our development, go straight to a clean-energy economy that allows them to grow, create jobs and at the same time reduce their carbon pollution," he said.

And he directly pleaded with Australia's youth to "keep raising their voices" and challenge "entrenched interests".

"It is in the nature of the world that those of us who start getting grey hair are a little set in our ways. That interests are entrenched.  Not because people are bad people, it's just that's how we've been doing things and we make investments and companies start depending on certain energy sources and change is uncomfortable and difficult," he said.

"And that's why it's so important for the next generation to step in and say 'it doesn't have to be this way'," he said.

Mr Obama pointed to his "ambitious new goal" to speed up the rate of carbon reduction in his country, in a deal struck with China's President at the APEC summit in Beijing earlier this week. The Abbott government has welcomed the agreement but the Prime Minister has described the Sino-US climate deal as relating to the "far distant future".

Mr Obama promised to pursue co-operation on overlapping interests including on trade, Ebola and tourism but in a blunt message to China pledged the US would always seek to lead in the Asia-Pacific

But he praised China's achievement in lifting millions of people out of poverty: "That is a good thing," he said.