Every year, schoolchildren hear the tale of George Washington and his band of barefoot soldiers in ragged clothes huddled in the bloodstained snow near the Delaware River during the bitter winter of 1777. Today, Chester County in Pennsylvania, where Valley Forge is located, is known for its mild winters, averaging only about 3 feet of snow over the course of a year.

The drastic difference isn’t evidence of global warming. Rather, it shows a historical climate shift that helped shape the course of American history. The years between 1300 and about 1850 are known as “the Little Ice Age,” years in which temperatures were, on average, much colder than today.

Crops failed across Europe, contributing to political and social strains that led to the French Revolution and the potato famine in Ireland. In the Alps, advancing glaciers swallowed up entire villages, including a famous chapel that was encased in ice despite the frantic prayers of clergy.

In the North Atlantic, the Vikings abandoned centuries of expansion into Greenland, Iceland, Atlantic Canada and possibly even New England when colder weather brought sea ice into their shipping lanes and made their journeys increasingly perilous.

In the United States, the onslaught of cold and drought was likely a factor in “lost” 16th and 17th century British colonies at Popham Beach in Maine and Roanoke Island in North Carolina.

Ice cores and historical accounts reveal that temperatures throughout the temperate regions dropped significantly within just a few stormy years. In 1315 alone, thousands died when wild storms heralded the start of the Little Ice Age, bringing widespread famine to Europe.

As the most recent example of drastic climate shifts, the Little Ice Age has attracted extensive study.

“It happened within historic time, the data sets are good, [and] it was severe for humans,” said Hal Borns of the University of Maine Climate Change Institute.

Climatologists now know that this shift to colder, often less predictable weather affected at least the entire Northern Hemisphere and perhaps the whole world. But many questions remain about the cause.

Some point to a dearth of sunspots as the primary cause of the Little Ice Age. Others theorize that different natural climate cycles overlapped to cause the deep freeze. Some even argue that the Little Ice Age might have continued to the present day, if not for the climate-changing impact of the Industrial Revolution.