This comment on Scott Adams' blog gives some suggestions:

World War II? Decimation of major fisheries? Genocide of North American natives? Sort of by definition, we haven't wiped out the whole human race yet, but we have endured some seriously bad disasters that people saw coming - and a certain amount of "that wasn't so bad" comes from the perspective of the survivors and the children who never knew how good things were before the disaster hit. Maybe loss of the chestnut trees isn't a big deal to people today, but if chestnuts were important to you - things certainly aren't looking so good in the North American chestnut department anymore.