the climatic record from Greenland - last glacial maximum

http://www.sciencedirect.com/science/article/pii/S1040618212031965

If we accept an earlier colonization into the Americas, the story is not so neat, because there were substantial time periods between 60 and 30 thousand when colonization was possible environmentally—though these were relatively brief. But, if we look at the climatic record from Greenland rather than Antarctica (Fig. 6b), which should be more appropriate for northern latitudes, it would seem that between 55 and 25 thousand years ago, the warmer episodes were short lived extremes in a very rapidly fluctuating climate (Bender et al., 1994)—and perhaps it was the unpredictability of climate that made it difficult to work out how to adapt to the north. By the time the first Americans crossed Beringia, they seem to have learned to deal with such unpredictability because they survived the Younger Dryas fluctuations (Haynes, 2008).

last glacial maximum