"We're going to become extinct," the eminent scientist says. "Whatever we do now is too late."
Fenner is an authority on extinction. The emeritus professor in microbiology at the Australian National University played a leading role in sending one species into oblivion: the variola virus that causes smallpox.
And his work on the myxoma virus suppressed wild rabbit populations on farming land in southeastern Australia in the early 1950s.
He made the comments in an interview at his home in a leafy Canberra suburb. Now 95, he rarely gives interviews. But until recently he went into work each day at the ANU's John Curtin School of Medical Research, of which he was director from 1967 to 1973.
Decades after his official retirement from the Centre for Resource and Environmental Studies, which he set up in 1973, he continued a routine established when he was running world-class facilities while conducting research.
He'd get to work at 6.30am to spend a couple of hours writing textbooks before the rest of the staff arrived.
Fenner, a fellow of the Australian Academy of Science and of the Royal Society, has received many awards and honours. He has published hundreds of scientific papers and written or co-written 22 books.
He retrieves some of the books from his library. One of them, on smallpox, has physical as well as intellectual gravitas: it weighs 3.5kg. Another, on myxomatosis, was reprinted by Cambridge University Press last year, 44 years after the first edition came out.
Fenner is chuffed, but disappointed that he could not update it with research confirming wild rabbits have developed resistance to the biological control agent.
The study showed that myxo now had a much lower kill rate in the wild than in laboratory rabbits that had never been exposed to the virus.
"The [wild] rabbits themselves had mutated," Fenner says.
"It was an evolutionary change in the rabbits."
His deep understanding of evolution has never diminished his fascination with observing it in the field. That understanding was shaped by studies of every scale, from the molecular level to the ecosystem and planetary levels.
Fenner originally wanted to become a geologist but, on the advice of his father, studied medicine instead, graduating from the University of Adelaide in 1938.
He spent his spare time studying skulls with prehistorian Norman Tindale.
Soon after graduating, he joined the Royal Australian Army Medical Corps, serving in Egypt and Papua New Guinea. He is credited in part with Australia's victory in New Guinea because of his work to control malaria among the troops.
"That quite changed my interest from looking at skulls to microbiology and virology," he says. But his later research in virology, focusing on pox viruses, took him also into epidemiology and population dynamics, and he would soon zoom out to view species, including our own, in their ecological context.
His biological perspective is also geological.
He wrote his first papers on the environment in the early 1970s, when human impact was emerging as a big problem.
He says the Earth has entered the Anthropocene. Although it is not an official epoch on the geological timescale, the Anthropocene is entering scientific terminology. It spans the time since industrialisation, when our species started to rival ice ages and comet impacts in driving the climate on a planetary scale.
Fenner says the real trouble is the population explosion and "unbridled consumption".
The number of Homo sapiens is projected to exceed 6.9 billion this year, according to the UN. With delays in firm action on cutting greenhouse gas emissions, Fenner is pessimistic.
"We'll undergo the same fate as the people on Easter Island," he says. "Climate change is just at the very beginning. But we're seeing remarkable changes in the weather already.
"The Aborigines showed that without science and the production of carbon dioxide and global warming, they could survive for 40,000 or 50,000 years. But the world can't. The human species is likely to go the same way as many of the species that we've seen disappear.
"Homo sapiens will become extinct, perhaps within 100 years," he says. "A lot of other animals will, too. It's an irreversible situation. I think it's too late. I try not to express that because people are trying to do something, but they keep putting it off.
"Mitigation would slow things down a bit, but there are too many people here already."
It's an opinion shared by some scientists but drowned out by the row between climate change sceptics and believers.
Fenner's colleague and long-time friend Stephen Boyden, a retired professor at the ANU, says there is deep pessimism among some ecologists, but others are more optimistic.
"Frank may be right, but some of us still harbour the hope that there will come about an awareness of the situation and, as a result, the revolutionary changes necessary to achieve ecological sustainability," says Boyden, an immunologist who turned to human ecology later in his career.
"That's where Frank and I differ. We're both aware of the seriousness of the situation, but I don't accept that it's necessarily too late. While there's a glimmer of hope, it's worth working to solve the problem. We have the scientific knowledge to do it but we don't have the political will."
Fenner will open the Healthy Climate, Planet and People symposium at the Australian Academy of Science next week, as part of the AAS Fenner conference series, which is designed to bridge the gap between environmental science and policy.
In 1980, Fenner had the honour of announcing the global eradication of smallpox to the UN's World Health Assembly. The disease is the only one to have been eradicated.
Thirty years after that occasion, his outlook is vastly different as he contemplates the chaos of a species on the brink of mass extinction.
"As the population keeps growing to seven, eight or nine billion, there will be a lot more wars over food," he says.
"The grandchildren of today's generations will face a much more difficult world."
