Even the smallest changes in the earth's climate have always made a big difference to human societies. When a volcanic eruption in 535 AD brought about slightly cooler, drier weather, the results included migrations, hunger, the spread of plague, and changes in agriculture -possibly even the collapse of a native American civilization. Five hundred years later, when the climate was warmer, Vikings were able to maintain colonies in Greenland. As the climate turned colder again, ferocious storms drowned scores of towns and villages near the North Sea.
Today climate changes are in the offing which are far more devastating than these or any others in human history. The main cause is a rise in the amount of carbon dioxide in the atmosphere.