Someone who knows it's all going to shit, but still thinks it will turn out OK
"It" most commonly refers to the environment, or humanity, but could be other things as well.
Guy 1: "The envrionment is being degraded at an unparalled rate, we're literally destroying ourselves... but, it'll probably turn out OK. If we fail, the planet will likely heal itself in our abscence."
Guy 2: "Man, he's such an apocaloptimist."