A volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun and raising Earth's albedo (increasing the Earth's reflectivity of solar radiation) after a large particularly explosive type of volcanic eruption. Long-term cooling effects are primarily dependent upon injection of sulfide compounds in aerosol forms into the upper atmosphere—the stratosphere—the highest, least active levels of the lower atmosphere where little precipitation occurs, requiring a lengthy time to wash the aerosols out of the region. Stratospheric aerosols cool the surface and troposphere by reflecting solar radiation, warm the stratosphere by absorbing terrestrial radiation, and when combined with anthropogenic chlorine in the stratosphere, destroy ozone which moderates the effect of lower stratospheric warming. The variations in atmospheric warming and cooling results in changes in tropospheric and stratospheric circulation.
Historic cases 
The effects of volcanic eruptions on recent winters are modest in scale, but historically have been significant.
In 1883, the explosion of Krakatoa (Krakatau) created volcanic winter-like conditions. The four years following the explosion were unusually cold, and the winter of 1887 to 1888 included powerful blizzards. Record snowfalls were recorded worldwide.
The 1815 eruption of Mount Tambora, a stratovolcano in Indonesia, occasioned mid-summer frosts in New York State and June snowfalls in New England and Newfoundland and Labrador in what came to be known as the "Year Without a Summer" of 1816.
A paper written by Benjamin Franklin in 1783 blamed the unusually cool summer of 1783 on volcanic dust coming from Iceland, where the eruption of Laki volcano had released enormous amounts of sulfur dioxide, resulting in the death of much of the island's livestock and a catastrophic famine which killed a quarter of the Icelandic population. Northern hemisphere temperatures dropped by about 1 °C in the year following the Laki eruption.
In 1600, the Huaynaputina in Peru erupted. Tree ring studies show that 1601 was cold. Russia had its worst famine in 1601 to 1603. From 1600 to 1602, Switzerland, Latvia and Estonia had exceptionally cold winters. The wine harvest was late in 1601 in France, and in Peru and Germany, wine production collapsed. Peach trees bloomed late in China, and Lake Suwa in Japan froze early.
The extreme weather events of 535–536 are most likely linked to a volcanic eruption.
One proposed volcanic winter happened around 71,000–73,000 years ago following the supereruption of Lake Toba on Sumatra island in Indonesia. In the following 6 years there was the highest amount of volcanic sulphur deposited in the last 110,000 years, possibly causing significant deforestation in Southeast Asia and the cooling of global temperatures by 1 °C. Some scientists hypothesize the eruption caused an immediate return to a glacial climate regime by accelerating an ongoing continental glaciation, causing massive population reduction among animals and human beings on Earth. Others argue that the climatic effects of the eruption were too weak and brief to impact early human populations to the degree proposed.
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, super-eruptions with total eruptive masses of at least 1015 kg (Toba eruptive mass = 6.9 × 1015 kg) occur every 1 million years.
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—is attributed to volcanic winters by some researchers. According to anthropologist Stanley Ambrose, such events diminish populations to "levels low enough for evolutionary changes, which occur much faster in small populations, to produce rapid population differentiation". With the Toba bottleneck, many species show massive effects of narrowing of the gene pool, and it is believed Toba nearly exterminated humankind.
See also 
- Impact winter
- Nuclear winter
- Year Without a Summer
- Timetable of major worldwide volcanic eruptions
- Global dimming
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Further reading 
- MR Rampino, S Self & RB Stothers (1988). "Volcanic winters". Annual Review of Earth and Planetary Science 16: 73–99. Bibcode:1988AREPS..16...73R. doi:10.1146/annurev.ea.16.050188.000445.