When Prinstein arrived at the small classroom the school had assigned him in the center
of campus, he was greeted with a crowd
outside the
lecture hall - one so large that he figured there'd been a fire - drill - mandated building evacuation.
Simple: If we were planning a mission
outside the solar system we'd want lots
of CO2 to grow healthy plants for our journey to the stars — e.g., growers keep CO2 levels at 1,000 to 2,000 ppm in Earthly greenhouses, which is about the level you'd find in a
lecture hall full
of students and pretty much what has been normal over most
of Earth's 550 million year history.