When talking about how weather has changed human history, there are actually two separate questions that need to be answered: first, how weather (individual incidents like high or low temperatures, storms, etc.) has affected human history, and second, how climate has changed human history. Climate refers not just to single events, but to longer-term trends: milder weather, harsher temperatures, etc., lasting for many years. Both are important, although the second is probably more important.
– How Weather Has Changed Human History –
Aside from the more general truth that civiliations tend to prosper when the weather is mild enough to produce a large food surplus, and tend to decline when it is not, the weather has changed human history in another and more obvious way as well: individual events, like storms or freak winters, that turn out to have tremendous historical consequences.
One of the most important in modern European history, oddly, may have caused (at least in part) the French Revolution of 1789, which alongside the American Revolution of twenty years before, forced democratic and what was then regarded as liberal thought to the centre of Western political discussion. In 1789, the French countryside had suffered from several successive years of severe winters, summer droughts, and crop failures, including a devastating hailstorm which ruined crops the previous year. French bread riots were not just a response to aristocratic hoarding: there really was less bread to go around than there had been years before.
Other events are more subtly important. Rapid rat population growth in Asia following a drought in the early medieval period might have contributed to the spread of the most infamous rat-carried disease of the era, the bubonic plague (known as the Black Plague in medieval Europe).
– How Climate Has Changed Human History –
Scientists have been able to piece together relatively accurate, if not comprehensive, records of climate going back centuries, and in some ways even for millennia. These have revealed that the Earth undergoes a regular cycle of cooling and warming, even if the current warming trend seems to be more rapid than those which came before. Even more fascinating, we can link this cycle to the rise and fall of some of the great human civilizations.
Over the relatively recent history of the Earth (on a geological timescale, not a human one), one of the warmest periods yet in Europea occurred about one thousand years ago, between 900 and 1200 A.D. This period is referred to as the Medieval Warm Period after the period of European history during which it occurred, the Middle Ages. However, it is not a stretch to conclude that the relationship also went in the opposite direction: in other words, unusually warm temperatures actually contributed to northern Europe’s relative prosperity during that part of the Middle Ages. Some places which have dense populations today, like California, experienced what would be crippling drought. However, farther north, the climate turned unusually mild. The British Isles and Scandinavia became agriculturally prosperous in a way not seen since. The Norse, or Viking, age of expansion was in large part the result of new ice-free corridors allowing easy transit to Iceland, Greenland, and the eastern coast of North America.
After the Medieval Warm Period, however, came a period of relative global cooling, known as the Little Ice Age. Where the Medieval Warm Period was marked by the expansion of human civilizations, the Little Ice Age was marked by their decline. Climate reached the low point of its trough between 1650 and 1850, resulting in unusually cold winters, unusually heavy spring floods, and waves of crop failures. (On the plus side, it led to the rise of ice skating and winter carnivals.) It is possible that this cooling period contributed to the severity of the winters in France prior to the Revolution, mentioned above.