San Francisco SCUBA Map, circa 2050
Climate Change Skeptic: "I don't see what the big deal is. The Earth's atmosphere has had CO2 levels this high in the past."
Climate Change Believer: "Yes, but the last time that happened, sea level was 300 feet higher than it is today."
I'm not sure where I heard that exchange, and I'm not sure if it's true, but it inspired me to perform an interesting thought experiment: what would be underwater if the sea level rose 300 feet?
All of Florida and much of the southeast United States would become a shallow sea. The Mississippi would empty into the Gulf of Mexico somewhere around the confluence with the Ohio River at the southern tip of Illinois.
And San Francisco? It would look something like the map I made. To produce this Map of San Francisco Under 300 Feet of Water, I downloaded a 7.5 minute topographic map from the U.S. Geologic Survey, then traced around the 300 foot contour in Photoshop. I left the water slightly transparent so you can see the layout of the submerged streets and other features.
Here in Minnesota we would have nothing to worry about--except for all the people moving in to enjoy our newly moderate climate, thanks to the improved proximity to the Gulf of Mexico (hurricanes might become an issue, too). All of Minnesota is well above 300 feet in elevation.
As much fun as it was to make the Scuba Map of San Francisco, 300 feet of sea level rise is close to an impossible scenario. Even if all the ice caps (Antarctica and Greenland) completely melted, sea level would only rise around 200 feet--and no scientists I know of think that complete melting the Antarctic ice sheet is anything close to a realistic possibility.
A more likely outcome of global warming is a sea level rise of a foot or two over the next century--devastating enough, to be sure, but not nearly as much fun to put on a map of San Francisco.