There are many grim projections for the Cosmic Era, and far from being the most horrific or exotic, the Universe 26 model has a frighteningly real chance of happening.
As they travel they notice a horrible rotten smell. As they travel it gets stronger and stronger forcing them to cover their noses and mouth less they choke on the horrible smell. Stream full of dead fish. Spawning trout die in mass quantities after they spawn littering areas with dead fish dying and deteriorating in the sun.