The most pressing issue facing Australia is climate change, according to ethicist Peter Singer.
Addressing the Global Atheist Convention in Melbourne, Professor Singer said billions of people around the world are likely to be displaced over the next century because of climate change.
He said Australians are yet to realise the extent of the problem, and reducing greenhouse gas emissions must be made a national priority.
"What we are doing is simply turning on the air conditioning, driving gas guzzling four-wheel drives, flying too much and eating meat," he told the 2500 strong crowd on Sunday.
"These things seem quite innocent to us and yet they're devastating in their long-term consequence."
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Climate change is "the most important challenge" facing Australia in the coming decades, he said. Veteran ABC science broadcaster Robyn Williams also spoke about the need to acknowledge climate change.
"Climate change denying seems to be one of the greatest attacks on science that we've seen in decades," he told the conference.
He also defended the public broadcaster's coverage of the issue, which last week came under fire from ABC chairman Maurice Newman who said the media was too willing to accept the science of climate change.
Climate change was one of many topics discussed at the three-day talkfest, which featured speakers including popular science writer and atheist Richard Dawkins.
During a Q and A session with the audience Professor Dawkins was asked how to be critical of Islam without generating a fear of retaliation.
He advised that it would not be a good idea to "recklessly or foolishly" insult the religion.
"Because it doesn't do any good to get your head cut off," he said. "What we should say is something like this: I may give way to you, I may refrain from publishing a cartoon of your prophet but it's because I fear you, don't for one moment think it's because I respect you," he said referencing the Danish cartoons depicting the Prophet Mohammed which sparked deadly riots.
He was also asked for his views on Mary MacKilllop, who is set to become Australia's first saint. "The whole idea of creating saints is pure Monty Python," he said, saying there was no scientific evidence that praying could cure cancer.
