• Elon Musk is scared low birth rates could lead to what he calls "population collapse."

• Experts told Insider the global population is still growing, and when it shrinks it'll be gradual.

• This won't necessarily be a problem for society, they added.

Elon Musk has a thing about birth rates.

The Tesla billionaire and world's richest man frequently tweets about what he calls "population collapse," arguing we as a species need to increase our fertility rates to counter ageing populations.

Musk said at a conference in May humankind needs to: "At least maintain our numbers."

"We don't necessarily need to grow dramatically, but least let's not gradually dwindle away until civilization ends with all of us in adult diapers," he added.

In an August tweet, Musk sounded the alarm again, saying: "Population collapse due to low birth rates is a much bigger risk to civilization than global warming."

Insider spoke to a demographer and a geographer, both of whom disputed the idea that there will be a "population collapse."

"Although there are countries have declining populations as a result of below replacement fertility, overall the world's population is still growing and is likely to do so (although at slower and slower rates) until the end of the century," said Prof. Alice Reid, director of the Cambridge Group for the History of Population and Social Structure.

Reid said this is in part due to a phenomenon called "population momentum," which means that even when women are having less than two children on average populations keep growing because: "large cohorts of women already born contribute large numbers of births."

"Global population decline is therefore still some way off, and is likely to happen gradually when it does take place. This can hardly be seen as a catastrophe for civilization," she said.

Dr. Peter Matanle, an expert in East Asian social and cultural geography, agreed low birth rates "will not cause a population collapse."

"Populations will age and shrink steadily, and they may result in a number of collective benefits," he said.