A (N)ice Bridge

QUESTION: How could climate have influenced early human migration to the Americas?

ANSWER: A recent study published in the Proceedings of the National Academy of Sciences identified two time periods—22,000–24,500 years ago and 14,800–16,400 years ago—when sea ice was present during winters in the northeast Pacific Ocean, which would have created a bridge for the first migration of humans from Asia to North America. Sediment cores taken from the Gulf of Alaska contained traces of the remains of algae that grew around sea ice along the shoreline, and a high-resolution ocean model showed that ice retreating from the Cordilleran ice sheet—which periodically covered much of North America during glacial periods—drained fresh water into the Pacific, accelerating northward currents and making boat travel southward along the coast difficult. The findings suggest that contrary to some previous theories, the first Americans may have arrived from Asia by land rather than water. “Our research indicates that during the last ice age, the ice along the west coast of North America, from Seattle to Alaska, moved back and forth quite a bit,” says coauthor Alan Mix of Oregon State University. “Surprisingly, there were times when ice didn’t block the way for those early people. In fact, some ice might have made migration easier.” Since seasonally the sea ice would have been attached to the shoreline, it would have been strong enough for humans to walk over and would have provided “a more traversable surface than the hazardous pathway of crevassed glaciers or paddling against strong ocean currents,” says lead author Summer Praetorius of the U.S. Geological Survey. [Source: Oregon State University]

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