I have also noticed that CO2 levels goes up, even if my sub is on the surface. When that happens to me, I save the game, and load the saved game again. If I'm on the surface, the CO2 levels go down. The best way to save for me is to go to the Control Room (F2), stop the engines, and pause the game. In the past, I have saved a game this way that is close to a land mass, ships that are also close by, and under the surface of the water without having a corrupt save. Once in a while I do have a saved game that won't load. Happy Hunting!
