LONDON — One dam would stretch some 300 miles from the coast of Scotland to Norway. The other, roughly 100 miles, would rise in the waters between northern France and Southeastern England.

Together, the mammoth structures proposed by scientists would completely enclose the North Sea and offer protection for tens of millions of Europeans threatened by rising sea levels caused by climate change.

The scientists behind the proposal, outlined in a paper published on Thursday in the American Journal of Meteorology, said that the scale of the project — which exists only in the broadest outlines at this point — reflected the urgency of the crisis.

“See this as a warning,” said one of the authors, Sjoerd Groeskamp of the Royal Netherlands Institute for Sea Research. “What we’re saying is: Here’s a plan, a plan we don’t want. But if we end up needing it, then it’s technically and financially feasible.”