A federal appellate court has upheld a judge's decision to fine Carnival Corp $20m for violating probation tied to environmental crimes.

The Eleventh Circuit Court of Appeals has denied a 17 June challenge by four people who alleged they were harmed by Arnold Donald-led Carnival's actions, including dumping garbage into the sea.

The appeal had been filed by Fotini Tsavousis Duncombe, co-founder of Bahamas environmental group reEarth, Responsible Cruising in Alaska president Theodore Thoma, and retired Alaskan fishermen Ronn Buschmann and Eric Forrer.