A human can start off a process by their own design, but with the details implemented by phenomena not in their direct control, and still copyright the resulting work.

If I take a funnel full of paint and let it drip onto a canvas in a pattern caused by the movement of a pendulum, and incorporate random movement from wind on a windy day, how would you assign a “percentage” of human creation there? What about letting the hot desert sun melt some crayons into another canvas where I placed the crayons but didn’t control the drip pattern? What if I record some barking dogs but auto tune it into a melody? Or photograph the natural beauty of a wave crashing onto shore? These are all things that can be copyrighted, even if they’re inherently dependent on natural phenomena not in the artist’s control, because the process itself is initiated or captured or designed by a human author.