Must an "academic reference letter" be from a university-employed researcher, or will other researchers do too?
Better safe than sorry: check with the institution you're applying to, they may have formal requirements about this.

However, I don't think it's likely to be very narrow. The important points are that the recommendation letters come from people with a good track record, i.e. people who clearly perform high-quality research, and that they have worked closely with you in the past. Whether they're affiliated with a university, a meteorological agency, the NASA, a museum, or even (may God have mercy on your soul) The Industry… it doesn't matter much.