Up to 90% of all species may go extinct in the next thirty years

The statement in the title is not true. Actually, more than 90% of all species may go extinct in the next thirty years. In fact, any fraction of all species may go extinct at any time.

Any fraction of all species may go extinct at any time.

That would be a true statement. Up to 100% of all species will go extinct in the next thirty years (or in the next five minutes, for that matter) is also a true statement. However, the false statement Up to 90% of all species may go extinct in the next thirty years conveys a more "effective" message. Whoever makes this sort of nonsensical statement is more interested in being "effective" than in being truthful.