Distinguished Emeritus Laureate Professor John Aitken, of the University of Newcastle in NSW's Hunter region, said the decline in fertility rates is impossible to stop.

"It keeps on going down. The factors that are driving it are so powerful and prevailing that there is no relief from it," said Professor Aitken, the world's leading authority on sperm and fertilisation.

Professor Aitken gave the Clarke Memorial Lecture on this issue at the University of Newcastle Conservatorium in Cooks Hill on Friday.

The lecture, hosted by the Royal Society of NSW's Hunter branch, was named "The Infertility Trap" - the same title as Professor Aitken's new book.

In an interview with ACM, he said every woman must have 2.1 children to ensure a stable population.

"If you get below 2.1, then your natural fertility is below replacement level. So your population will go backwards. At the moment, all modern industrialised societies have had their fertility rates pushed down below replacement level," he said.

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The fall in fertility rates is particularly evident in the Tiger economies of South-East Asia, where countries like Singapore, Hong Kong, Taiwan and South Korea have some of the lowest fertility rates in the world.

In Australia, the fertility rate is about 1.7.

"We went below replacement level in about 1970 and have been going backwards ever since," Professor Aitken said. "Our population has been growing because Australia pulled the immigration trigger. The peak was in 2009 when about 300,000 immigrants were given visas to come to Australia.

"Last year because of COVID, we actually went into negative territory and had the lowest net migration rate since World War I."

Nevertheless, Professor Aitken said migration was "probably not a long-term solution to the infertility problem".

The world is facing depopulation, meaning "ultimately our population will go backwards".

"We will probably peak at a world population of around 10 billion people in the next 10 or 20 years. And then the population will start to reverse. The discussion is how rapid and severe this reversal is going to be."

However, academics studying this problem have difficulties getting people in power to pay attention to the issue.

"Every minute of every day, there's evidence of overpopulation - global pandemics, pollution and the various manifestations of climate change," he said.

Overpopulation affects the world every day.

"Another reason it's difficult to get the point across is the time scale over which the changes are occurring. It's not a nice, convenient three- to five-year cycle. It'll take decades.

"It's very much like climate change. By the time we start to feel the effect of depopulation, it's probably already too late."

Professor Aitken said the purpose of his book and upcoming lecture was to "forewarn people that this change will come".

"It would be a good idea to be prepared for it, so that the decline in human populations is not a chaotic affair."

The looming crisis, though, has positives and negatives.

"Many people would applaud the population going backwards," he said.

"We would like to see less people on the planet. But of course if your population goes backwards, there are ramifications to that.

"We have economic systems which are essentially addicted to continuous growth. Australia's economy has grown in parallel with the number of people inhabiting the country."

Another negative is that the age structure of the population is changing.

"Traditionally we would have a large number of young people coming into the population, with the age profile gradually peaking in the form of a triangle. What's happening now is the population triangle is being turned on its head. So you've got fewer people coming in at the bottom and a large crown of elderly people at the top. We're already beginning to see our economy and others struggling to meet the needs of the aged-care population."

The working population is getting smaller and working harder to "maintain the elderly population in the way they have a right to expect".

The fertility problem is a deep dilemma.

"It's very difficult to see how we get out of this in terms of age structure. We could raise the pension age and encourage people to work and be productive for longer, but socially that doesn't go down very well.

"Or we can encourage more and more women to enter the workforce, which we are doing. But then we make a bad situation worse. It's a catch-22. That's why it's a trap. We have to be very clever in the way we think our way out of this."

The other elements to the trap are environmental.

"Over the last 50 years, sperm counts in men have halved. Fifty years ago the average sperm count was 100 million per millilitre, now it's 50 million per millilitre. There's no sign of that abating."

Professor's Aitken's colleague Shanna Swan wrote a book called Count Down.

"She's saying sperm numbers will eventually get so low that it'll have an impact on our fertility," he said.

In the lecture, Professor Aitken will hypothesise that the decline in sperm is due to high levels of estrogen.

"That's estrogen levels that men are generating themselves because we have a worldwide pandemic of overweightness and obesity. When men become overweight they start to generate estrogen. Secondly, there's large amounts of estrogen in the food we eat. Many meat producers still give their animals steroids to promote muscle growth. So it's in the meat and dairy products and some of the plant products we eat. It's in the water we drink. It's also in chemical pollutants that enter the environment."

These pollutants include plastics, pesticides, food toxicants and cigarette smoke.

The human race is becoming less and less fertile.

"The way we address this is we adopt IVF more and more frequently. In some countries now 10 per cent of the population is generated by IVF. In Australia it's about 7 per cent. It'll be 15 per cent soon, then 20 per cent. The good side of IVF is it's brought happiness to millions of people. The negative side is when you have IVF operating at scale, you make sure bad fertility genes stay in the population.

"So the net result is the more we use IVF in one generation, the more we'll need it in the next. I can see us becoming more and more dependent on assisted reproduction."

This has enormous implications. IVF is costly.

"You can't have a society where only the rich are able to reproduce, as they can afford the IVF. There are major issues here that we have to start to grapple with. The sooner we start to think about these things, the better."