“There are not enough people,” Musk said. “I can’t emphasize this enough, there are not enough people.” 

He went on to say that low birth rates could contribute to a societal collapse. 

“So many people, including smart people, think that there are too many people in the world and think that the population is growing out of control. It’s completely the opposite,” he said. “Please look at the numbers—if people don’t have more children, civilization is going to crumble, mark my words.”

It appears that more and more Americans are reluctant to have children. The U.S. birth rate declined by 4% in 2020, but fertility rates have been at a record low since before the pandemic, and a growing number of adults have recently expressed they don’t expect to ever have children. 

Many people have said their decision to not have children is partly due to the climate crisis. A survey conducted by tech company Morning Consult last year found that one in four childless adults cited climate change as a factor in their reproductive decisions. In July, analysts at Morgan Stanley wrote in a note to investors that the “movement to not have children owing to fears over climate change is growing and impacting fertility rates quicker than any preceding trend in the field of fertility decline.”

But some experts say that reducing the global population is not necessarily the priority for addressing climate change, which requires immediate action. 

“It is true that more people will consume more resources and cause more greenhouse gas emissions,” Kimberly Nicholas, a Swedish professor of sustainability science, told Vox earlier this year. “But that’s not really the relevant time frame for actually stabilizing the climate, given that we have this decade to cut emissions in half.” 

This isn’t the first time that Musk has expressed concerns about the declining population.

“Population collapse is potentially the greatest risk to the future of civilization,” he tweeted in July.