For more than 500 years until the middle of the 19th century, much of the Northern Hemisphere experienced the “Little Ice Age,” the most extended period of anomalous cold—winter and summer—in 8000 years. Picturesque aspects of the LIA are familiar from paintings of winter scenes in northern Europe. But more somber manifestations include numerous famines in Europe and Asia and the extinction of the Norse settlements in southern Greenland.

The LIA’s start and finish dates, as well as its cause, have long been subjects of debate and puzzlement. Variations in solar irradiation and volcanic eruptions have been invoked as possible causes. But the one seems too weak and the other too ephemeral.

Since the end of the last full-fledged ice age 10 000 years ago, Earth’s 26 000-year axial-precession cycle has gradually been reducing solar irradiation of the Northern Hemisphere in summer as the June solstice migrates toward the orbit’s...

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