This the "cold winters theory", popularized by the academic fraudster Richard Lynn (of "IQ and the Wealth of Nations" fame). It's a just-so story. You could tell the same story about the selective pressure for intelligence and problem-solving in hot climates; try to eat in the Kalahari sometime.
Really, the more you think about that theory, the dumber it gets.
Really, the more you think about that theory, the dumber it gets.