It's banding, due to poor color bit compression techniques, and such. I'm going to assume you're on an Nvidia GPU?. The same happens for me, in virtually all games. Skyboxes are particularly prominent with this, due to the very smooth gradiant.
I also hate it, and I've tried everything. It's due to Nvidia(so far as my own research is concerned). Others have also been complaining about it for a few years now. It was introduced when they did a big revamp of their driver systems, and it was never resolved, after countless threads / complaints, etc.
I'm not going to get into this right now, as it's a bit off topic. But, I've previously inspected the nvidia driver files with a hex editor, and I noticed they cut a lot of corners with their driver "optimizations". Even going so far, as to blatantly disable some graphical features in games, when their cards don't like them. For more FPS, for benches, etc...
I'm seriously considering going back AMD next buy, because of this, tbh. I never had banding problems on my AMD cards.