From Dalrymple, re apocalyptic pessimists:
He is so earnest that he could almost be an optimist. He believes that the end of the world is nigh, and secretly is rather pleased about it. If he is of a scientific bent, he does the following: he takes an undesirable trend and projects it indefinitely into the future until whatever is the object of the trend destroys the world. For example, he might take the fact that Staphylococci reproduce exponentially on a Petri dish to mean that, within the week, the entire biosphere will consist of Staphylococci and nothing else. Man will be crushed under the weight of bacteria.