If the sea levels of Earth were to suddenly rise by 49 meters, or 160.7 feet, what would the world map look like?
floodmap.net has a tool with quite a large range (0-800 meters). I set it to 49 meters, and got some interesting results:
Eastern United States:
- Most of the Eastern Seaboard is gone, including New York, Philadelphia, Washington, D.C, Jacksonville, and Boston. Other cities are now on the beach or very close to the sea, susceptible to flooding. This is a huge blow to the American economy and population.
- Florida is gone. Just gone. That's another population problem.
- The lower Mississippi basin is also in trouble. We've lost a significant amount of arable land, and any shipping on the Mississippi needs to be redone. The Gulf Coast is now further inland, and needs restructuring.
Western United States:
- Baja California begins to flood. I don't know how this will impact US-Mexico relations.
- The area west of the Sierra Nevada is flooded. San Francisco, Sacramento, and Anaheim are lost, and other major California population centers are endangered. Part of Los Angeles is underwater.
- The area around northwestern Washington and Oregon (not pictured) experiences problems, especially near Seattle. The area is now a bunch of small islands and peninsulas.
- Southern and eastern Britain is no more. Say goodbye to London, Southampton, Norwich, Cambridge, Cardiff, Liverpool, Glasgow, Dublin, and Belfast. Parts of Leeds, Newcastle-upon-Tyne, and Edinburgh are endangered. This destroys almost all of the government of the British Isles.
- The Low Countries, Denmark, and Northern Germany are now islands, if anything. Copenhagen, Hamburg, Berlin, Amsterdam, Antwerp, Brussels, and part of Paris have been lost. This devastates the European economy and politics.
- The area around Venice and Northern Italy is covered. Most of the rest of the Mediterranean does well, with the exception of northern Egypt.
- Most of eastern China is gone. That's a huge population loss, and a major industrial and economic problem. Too many major cities to count have been covered.
- Tokyo, Pyongyang and Seoul are gone. That hurts Japan and the Koreas.
- Southeast Asia, too (not pictured) also takes a beating. Vietnam, Cambodia, and Indonesia lose sizable fractions of their entire countries.
- As you might have predicted, the Amazon Basin is flooded.
- Most of the northeastern coast of the continent is now underwater.
- The Andes protect the western coast fairly well.
There are some overarching problems here, as you can no doubt see.
- Many cities are built near water. This allows for easy trade, which was necessary for many economies before the advent of easier transportation methods, such as efficient overland travel or aircraft. However, this also means that the cities will be some of the first to go when water levels rise.
- Rivers are also spots of trouble. One issue during Hurricane Katrina was that many riverside settlements were protected only by levees, which can only handle so much flooding. In this case, the Mississippi floods catastrophically, as does the Amazon. The Thames and the Seine rise, hurting London and Paris, respectively - on a smaller scale but hitting large population centers.
- Evacuations can only help so many people. I'm not sure what the warning time is for the sea level rise here. However, you cannot easily evacuate millions and millions and millions of people with only a short amount of notice. Those at the bottom tip of Florida, for instance, would either need to drive all the way up to Georgia or try to evacuate via boat - and the latter doesn't seem great on large scales.
There are maps which shows this, for example this map shows scenarios from 1 to 60 meters.