US president Barack Obama has formally announced a $US3 billion contribution to an international fund to help poor countries cope with the effects of climate change.

He outlined his pledge to the UN Green Climate Fund while speaking to university students in Brisbane ahead of the official opening of the G20 Leaders' Summit.

"Today, I'm announcing that the United States will take another important step," he said.

"We will contribute $3 billion to the Green Climate Fund to help developing nations deal with climate change."

Mr Obama's speech made reference to Queensland's Great Barrier Reef, which the Intergovernmental Panel on Climate Change (IPCC) has warned could be at risk if more is not done to reduce carbon emissions.

"I have not had time to go to the Great Barrier Reef and I want to come back," Mr Obama said.

"I want my daughters to be able to come back and I want them to be able to bring their daughters or sons to visit. And I want that there 50 years from now."

The US contribution to the climate fund doubles what other countries had previously pledged ahead of a November 20 deadline.

Mr Obama told the audience that every nation has a responsibility to act on climate change, including Australia.

"The United States and Australia have a lot in common and one of the things we have in common is we produce a lot of carbon," he said.

"Historically we have not been the most energy efficient of nations, which means we've got to step up."

In his speech, Mr Obama also spoke about the situation in Ukraine, describing Russia's actions as aggressive.

"We're leading in dealing with Ebola in West Africa and in opposing Russia's aggression against Ukraine, which is a threat to the world, as we saw in the appalling shoot-down of MH17," Mr Obama said.

Speaking at an earlier news conference in Brisbane, UN secretary-general Ban Ki-moon said he hoped other G20 leaders would contribute to the fund.

"The transition towards a low-carbon, climate-resilient future is accelerating," he told reporters.

"I urge other leaders and major economies, especially at the G20, to come forward with contributions that will sustain this momentum."

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