Austin, Texas - In classic Elon Musk form, the billionaire dropped a single sentence that sent Twitter followers into a tizzy, this time over what the billionaire sees as the world's most pressing problem.

"Population collapse is the biggest threat to civilization", Musk tweeted Tuesday, linking to a video of him and fellow billionaire Jack Ma talking about the future of humanity and Artificial Intelligence.

They agreed that if AI is nice and doesn't go full Skynet on us, then not having enough people in roughly 20 years will be the problem.

Musk tweeted earlier in the day with a chart from the Wall Street Journal, pointing to the slowing growth in the US, focusing on how women would have to have 2.1 kids on average to replace the previous generation.

First off, yes, the US population isn't growing very fast, and the trend is about to reverse so that each generation is smaller than the one before, according to Our World in Data.

The global trend is also slowing down, but we're still on track to increase to reach 10 billion people by the end of this century.

There would be more people over the age of 65, and fewer kids, but depending on how the US tackles issues like affordable health insurance and social security, there would be more to go around for fewer people, per Business Insider.

And even if fewer babies are born each year, it is not a clear threat to civilization. Plus, unlike other threats, like climate change, the effects of having fewer people on the planet are not clearly known.

Despite Musk's clout on Twitter, actual experts on world threats and the United Nations say that climate change is by far the biggest threat to civilization.