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What about building canals to move water from the ocean to lands that are below the surface of the sea, for instance a canal from Los Angeles to Death Valley, or from the Mediterranean Sea to the Dead Sea?
Would it be too costly? Is there enough low-elevation land to absorb increased water levels from the oceans? Would the flow of water be too slow to fight rising waters? Would it make sense to dig very big holes near the ocean, and fill them with sea water, to decrease sea levels?
Other solutions consist in building floating cities, or cities under the sea in shallow waters. Less costly, much more realistic than sending people to Mars. Could be a great topic for an Hollywood movie too.
An example of man-made sea (accidental though) is the Salton Lake in California. It took several years to form. It's still well below sea level, but not rising anymore.
Given water shortages in many places for agriculture, household and industrial use, desalinization might be a viable option as well. Water tables currently depleted could be restored as well perhaps.
Here are two crazy ideas - there is no chance they could ever work, but maybe a good idea for an Hollywood movie: