The most extreme speculations about rising sea levels have to do with the melting of all the ice trapped in Greenland and Antarctica, which could cause sea levels to rise 60 or 70 meters. Of course, that's nothing more than informed speculation: people just don't know enough about the the Antarctic ice sheet to predict how it will behave in the future. The current arrangement of the continents means that ocean currents are very different from the ancient worlds that had well-mixed oceans and ice-free poles, and as long as Antarctica is located over the South Pole, keeping the ice from floating off, a great deal of ice may be parked there.
But, speculation is fun! According to the USGS, even a 10-m rise would displace 25% of the US population. Sea Level Explorer shows height above sea level using Google Maps. Flood Maps uses Google Maps to show sea level rise from 7 meters to over 60 meters. At 13 meters, Shanghai and Tianjin in China are completely flooded, and much of Bangladesh and Calcutta in India are flooded. Not to mention Miami and New Orleans. At 60 meters, huge population centers in Bangladesh, China, coastal Europe, and the southern U.S. are under water. Johnston's Archive has some neat pictures of a 66-meter rise in sea level.
The 100-meter maps are the most fun. Maps for "An End to Global Warming" has maps of North America with a 100-meter rise (also here), released by Dave Pape under a CC-Attribution license.
New England is an island, stretching from the Hudson Straits to New Brunswick.Below its cataracts, the Ohio River flows through the tidewater directly into the huge Mississippi Sea.In the Greater Antilles, Cuba is a much-reduced archipelago, while Jamaica and Hispaniola mostly retain their shapes.
With Dubia, Chris Wayan speculates on an Earth after a thousand years of unrestrained carbon pollution and global warming:
That world heats up. Climate zones move north until the poles thaw. Greenland and Antarctica melt. Coastal nations are drowned. In the end, the sea rises some 110 meters. Global hothouse! It's happened before, of course, on this scale, but not in the last 50 million years or so.His Planetocopia has all sorts of images of speculative versions of Earth and other planets. There's also some interesting discussion at the Randi forums.
...we shouldn't be surprised to find that Florida now has no governoror voters. It's a scuba paradise rivaling Australia's Barrier Reef, but there's no dry land at all. Louisiana was doomed too, of course, but I was startled to find that the sea swallows half Alabama toosouth of Tuscaloosa, only Red and Grove Islands and the small Troy Peninsula are left. Mississippi is even worse offthe Gulf chews inland to Tupelo and Mantee, leaving only the Jackson Peninsula and Brookhaven Island, and a jungly strip up at the Tennessee border. Mississippi Bay nibbles all the way up into Illinois, though it's broken up on the west side by long Crowley Island and the Spring and Pleasant Isles. Further south, in Texas, fishermen avoid the rotting, polluted Houston Reefs. But Austin surviveswith a steamy coastal climate, flora, and culture resembling lost New Orleans.
Unfortunately, Dallas survives too.
...For, by definition, most readers will be from our world's high-population zones. Random changes will, on average, degrade them. And the lands that improve, that become the heartlands of Randomia's civilizations, are likely to be barren obscure lands in our world, mere names (if that) to non-Randomian readers. The great European cities are all flooded on Dubia (millions of European readers groan), while the green Sahara nurtures great civilizations (a handful of Saharan readers cheer). If you love civilization, Randomia will probably kill or cripple the ones you love, and plant its greatest civilizations in places you associate with backwardness.
So the grass always looks browner in a parallel world--because what you value most, what you KNOW to value, is generally lost. This principle makes it hard to see alternate worlds fairly.