Volcanic winter
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A volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun, usually after a volcanic eruption (hence the name).
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Effects on life
The causes of the bottleneck phenomenon, i.e. a sharp decrease in a species' population immediately followed by a period of great genetic divergence (differentiation) among survivors - might be attributed to volcanic winters. According to anthropologist Stanley Ambrose, such events diminish the population size to "levels low enough for evolutionary changes, which occur much faster in small populations, to produce rapid population differentiation"
Ancient case of volcanic winters
A terrific case of volcanic winter happened around 71,000-73,000 years ago following the super-eruption of Lake Toba on Sumatra island (Indonesia). In the following 6 years there was the highest amount of volcanic sulphur deposited in the last 110,000 years, possibly causing complete deforestation in Southeast Asia and the cooling of sea temperatures by 3-3.5°C. Remarkably, the eruption almost caused an instant Ice Age on Earth by accelerating the glacial shift that already was going on, therefore causing massive population reduction among animals and human beings on Earth.
This, combined with the fact that most human differentiations abruptly occurred at that same period, is a probable case of bottleneck linked to volcanic winters (see Toba catastrophe theory). On average, such super-eruptions and subsequent volcanic winters occur on our planet every 50,000 years.
Recent cases of volcanic winter
The scales of recent winters are much more modest:
The eruption of Mount Tambora, a stratovolcano, in 1815 occasioned mid-summer frosts in New York State and June snowfalls in New England in the "Year Without a Summer" of 1816. A paper written by Ben Franklin in 1783 blamed the unusually cool summer of 1783 on volcanic dust coming from Iceland. Following the eruption, a 6 km wide caldera was formed.
In 1883, the explosion of Krakatoa (Krakatau) also created volcanic winter-like conditions. The next four years after the explosion were unusually cold, and the winter of 1888 was the first time snow fell in the area. Snowfalls that year broke all previous records worldwide. A series of tsunamis was also created by the main explosion.
Most recently, the 1991 explosion of Mount Pinatubo, another stratovolcano, in the Philippines cooled global temperatures for about 2-3 years.