Since the earth really doesn't care about environmentalists, it has been changing its own weather for millions of years without any contribution from humans. Volcanoes are a prime example of that:
"The sun was dark and its darkness lasted for eighteen months; each day it shone for about four hours; and still this light was only a feeble shadow; the fruits did not ripen and the wine tasted like sour grapes." Michael the Syrian, 536 A.D.
"During this year a most dread portent took place. For the sun gave forth its light without brightness. and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear. Procopius of Caesarea, 536 A.D.Documents for the period of 535 - 536 A.D. speak of the sun being obscured by a "dry fog", summer frosts, widespread crop failures in Europe, drought and famine in China. Tree rings reflect poor growth, and ice cores from Greenland and Antarctica contain highly elevated levels of sulfuric acid dust. A single massive eruptive event, and the climate was drastically affected.
1816 was "The Year Without a Summer", the "Poverty Year", and "Eighteen Hundred and Froze to Death", created by the eruption of Mt. Tambora in Indonesia in 1815. Killing frosts and snow storms in May and June in Eastern Canada and New England caused wide-spread crop failures, and Pennsylvania lakes and rivers were still iced in July and August. Populations plummeted, with farm loss forcing the first major migration to the American mid-west from the north-east. Artist J.M.W. Turner captured the atmospheric affects in his paintings:
And incessant rainfall during a nasty summer trapped a vacationing group inside during their Swiss holiday, leading to a contest to write the scariest story. The winner, by a young Mary Shelley, was "Frankenstein, or the Modern Prometheus".
At least 6 volcanic eruptions are known to have caused summer anomalies in the last 2,000 years:
Kuwae - 1452
Huaynaputine - 1600
Krakatoa - 1883
Novarupta - 1912
Pinatubo - 1991
In reality, these were small eruptions compared to others that predated written history. Roughly 75,000 years ago, the Toba Caldera in Sumatra erupted with a force 3500 times greater than that of Tambora. An eruption of that magnitude can cool the globe 5 to 9 degrees F for years. The sulphur aerosol is more than the atmosphere can absorb. An earth that was already in a cooling cycle can be pushed into an Ice Age, or, at minimum, a long cold cycle, devastating for agriculture.
It wasn't the first time Toba changed the climate. 788,000 years ago a Toba eruption coincided with the advent of a warm period. And a Toba-level eruption would create major temperature anomalies today:
Volcanoes capable of these sorts of eruptions are very much still around and rumbling - Yellowstone National Park sits in the caldera of a still active mega-volcano. Thousands of years are but a blink in the history of the earth. And all "The sky is falling!" scurrying around of the "We must stop global climate change!" folks will not and never will matter a whit in the face of the processes that truly affect our climate.