I had to "laff" out loud by Penny's response...but, Barbara, I have to think on this. Probably the biggest natural disaster is global human population. Man-induced climate change could become the biggest but is not yet. Maybe for the last 15,000 years, if we are limited to North America, it might be the possible comet impact at 12,900 (Kennett and others), assumimng they are correct. Katrina is a blip...but is the most costly in terms of $. In terms of lives, it is the 1900 Galveston hurricane.
Chris