The 1990 Galveston Hurricane was the deadliest natural disaster to ever strike the US. It came on shore September 8, 1900 and killed somewhere between 6,000 and 12,000 people.

http://www.photolib.noaa.gov/bigs/wea00589.jpg

The storm surge was 15 feet high, which flooded the entire island. They have since built a seawall.

Adjusted to 2005 US Dollars, it was the second costliest Atlantic Hurricane.

http://en.wikipedia.org/wiki/1900_Galveston_hurricane

Atmospheric CO2 was 296 ppm at the time.