Research brings forward two possibilities from the past:
Kill a lot of people:
Over the course of the century and a half run of the Mongol Empire, about 22 percent of the world's total land area had been conquered and an estimated 40 million people were slaughtered by the horse-driven, bow-wielding hordes. Depopulation over such a large swathe of land meant that countless numbers of cultivated fields eventually returned to forests.
Or, set off a lot of nuclear bombs:
This study suggests that the cause of the stagnation in global warming in the mid 20th century was the atmospheric nuclear explosions detonated between 1945 and 1980. The estimated GST [global surface temperature] drop due to fine dust from the actual atmospheric nuclear explosions based on the published simulation results by other researchers ….has served to explain the stagnation in global warming. Atmospheric nuclear explosions can be regarded as full-scale in situ tests for nuclear winter. The non-negligible amount of GST drop from the actual atmospheric explosions suggests that nuclear winter is not just a theory but has actually occurred, albeit on a small scale.
This site endorses no such violent techniques, no matter how well they work in the movies.
Kit Stolz 