The Little Ice Age was a period of cooling that occurred between 1350-1850 AD. It began after a medieval warming period and saw global temperatures drop by up to 4 degrees Celsius. Several factors may have contributed to the cooling, including decreased ocean currents and increased volcanic activity. The Little Ice Age had widespread effects on human civilizations. Crop failures led to famine and plague outbreaks. Societal changes included the rise in popularity of potato crops and hard liquors in Northern Europe. The freezing of the Thames River in London and cold weather were factors in the French Revolution. Napoleon's 1812 invasion of Russia was devastated when his army was trapped by the frigid temperatures during their retreat from Moscow.