Elon Musk has stated:

Population collapse due to low birth rates is a much bigger risk to civilization than global warming

and also:

We should be much more worried about population collapse

Resolves YES if global annual population growth becomes negative and Elon is still alive.

Resolves NO if Elon dies while global annual population growth is still positive.

For the purposes of this bet, "human population" includes sentient genetically-modified humans, but does not include AIs, uploaded minds, uplifted animals, etc. However, if Elon's consciousness is uploaded (in a form that is capable of witnessing this event), that still counts as him being "alive".