For those of you still arrogant enough to think man can ruin this planet, consider the year 1816, otherwise known as "The Year without a Summer". The 1815 volcanic eruption of Mount Tambora in Indonesia is generally considered the cause of this (from Wikipedia):
"The unusual climate aberrations of 1816 had the greatest effect on the American northeast, the Canadian Maritimes, Newfoundland and northern Europe. Typically, the late spring and summer of the northeastern U.S. are relatively stable: temperatures average about 68–77°F (20–25°C), and rarely fall below 41°F (5°C). Summer snow is an extreme rarity, though May flurries sometimes occur.
In May of 1816, however, frost killed off most of the crops that had been planted, and in June two large snowstorms in eastern Canada and New England resulted in many human deaths. Nearly a foot of snow was observed in Quebec City in early June. In July and August, lake and river ice were observed as far south as Pennsylvania. Rapid, dramatic temperature swings were common, with temperatures sometimes reverting from normal or above-normal summer temperatures as high as 95°F (35°C) to near-freezing within hours. Even though farmers south of New England did succeed in bringing some crops to maturity, maize (corn) and other grain prices rose dramatically. Oats, for example, rose from 12¢ a bushel the previous year to 92¢ a bushel."
Somehow, the Earth managed to return to normal. But our little CO2 emissions are ruining the planet?
"Go sell crazy somewhere else, we're all stocked up here." - Jack Nicholson, from As Good as It Gets