Knowledge and responsibility.
If you have the knowledge to foresee a really bad outcome — not just to foresee its possibility, but to foresee that it is reasonably likely — and if that outcome could hurt a lot of people, you probably have a duty to share that knowledge. Certainly, the people who are at risk might benefit from the knowledge. But, you'd also want to make sure that the people who might have the power to prevent the outcome had that knowledge, too, so they could prevent it.
But if you make that knowledge public, and you explain it very clearly, and you jump up and down to impress upon the people with the resources and the organization to head off catastrophe that they really need to take the risk seriously, doesn't the responsibility shift to the people with the means to do something who hem and haw and say, "Well, I don't know, it's a hypothetical risk and there are other interests we need to attend to …" ? I mean, do the scientists who see disaster looming have to get the people who can do something in a headlock and make them do the right thing?!!
Or do the people who pooh-pooh the responsible scientists as hysterical doom-sayers have to lie in the bed they've made for themselves?
What Would Krugman Say?
Either way, if you're able, do what you can to help the victims of Hurricane Katrina.
(Acknowledging Rob's chalky help with the links.)