Cosmic catastrophe - 3

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Overactive vulcanism

Violent volcanic activity might be capable of modifying the earth's climate for years, or decades. Volcanoes can inject dust into the stratosphere; this leads to a larger fraction of the sunlight incident on the earth being reflected; this leads to less energy being absorbed by the Earth; this, in turn, leads to lower temperatures.

The Mt Pinatubo eruption forced dust and sulphur dioxied into the stratosphere, which was distributed around the earth. Its effects were evident in increased night-sky brightness (more scattered light at night), spectacular red sunsets and a decrease of ~0.3 F in the average temperature over a 2 year period. See this link and Volcano World for more information
A combination of several similar-strength eruptions might have more significant, long-term effects

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