REYKJAVIK, Iceland — It is just one scenario among many, but by far the most cataclysmic.

Sometime over the next 100 years, human-driven warming could disrupt a vital ocean current that carries heat northward from the tropics. After this breach, most of the world would keep getting hotter — but northern Europe would cool substantially, with Iceland at the center of a deep freeze. Climate modeling shows Icelandic winter extremes plunging to an unprecedented minus-50 degrees Fahrenheit. Sea ice could surround the country for the first time since it was settled by Vikings.