"In just two months, the world must unite around a strong global agreement," Obama said in an address to a United Nations development summit.
"All our countries will be affected by a changing climate, but the world's poorest people will bear the heaviest burden - rising seas, more intense droughts," Obama said.
"We'll be seeing climate change refugees," he said.
The UN-led conference in Paris aims to seal a wide-ranging agreement to limit the worst effects of climate change.
Obama, addressing a summit on development, said that anti-poverty goals would be at risk without action on climate.
"We need to establish the tools and financing to help developing nations embrace clean energy, adapt to climate change and ensure that there is not a false choice between economic development and the best practices that can save our planet," he said.
He cited Pope Francis, who has won a warm reception on a visit to the United States and has heartened Obama by taking up the cause of climate change.
"As His Holiness Pope Francis has rightly implored the world: this is a moral calling," Obama said.
French President Francois Hollande, the host of the talks, said Sunday that more work needs to be done to reach a serious agreement.
Obama faces strong domestic opposition, with much of the rival Republican Party supportive of the fossil fuel industry.