A volcanic winter is a reduction in global temperatures caused by volcanic ash and droplets of sulfuric acid and water obscuring the Sun and raising Earth's albedo (increasing the reflection of solar radiation) after a large, particularly explosive volcanic eruption. Long-term cooling effects are primarily dependent upon injection of sulfur gasses into the stratosphere where they undergo a series of reactions to create sulfuric acid which can nucleate and form aerosols. Volcanic stratospheric aerosols cool the surface by reflecting solar radiation and warm the stratosphere by absorbing terrestrial radiation. The variations in atmospheric warming and cooling results in changes in tropospheric and stratospheric circulation.
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 1887–1888 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. However Franklin's proposal has been questioned.
In 1600, the Huaynaputina in Peru erupted. Tree ring studies show that 1601 was cold. Russia had its worst famine in 1601–1603. 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 1257 Samalas eruption in Indonesia. The eruption left behind a large caldera next to Rinjani, with Lake Segara Anak inside it. This eruption probably had a Volcanic Explosivity Index of 7, making it one of the largest eruptions of the current Holocene epoch. An examination of ice cores showed a large spike in sulfate deposition around 1257. This was strong evidence of a large eruption having occurred somewhere in the world. In 2013, scientists proved that the eruption occurred at Mount Samalas. This eruption had four distinct phases, alternately creating eruption columns reaching tens of kilometres into the atmosphere and pyroclastic flows burying large parts of Lombok Island. The flows destroyed human habitations, including the city of Pamatan. Ash from the eruption fell as far away as Java Island. The volcano deposited more than 10 cubic kilometres (2.4 cu mi) of material. The eruption was witnessed by people who recorded it on palm leaves, the Babad Lombok. Later volcanic activity created additional volcanic centres in the caldera, including the Barujari cone that remains active. The aerosols injected into the atmosphere reduced the solar radiation reaching the Earth's surface, which cooled the atmosphere for several years and led to famines and crop failures in Europe and elsewhere, although the exact scale of the temperature anomalies and their consequences is still debated. It is possible that the eruption helped trigger the Little Ice Age.
The extreme weather events of 535–536 are most likely linked to a volcanic eruption. The latest theorised explanation is the Tierra Blanca Joven (TBJ) eruption of the Ilopango caldera in central El Salvador.
A proposed volcanic winter occurred 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 that the eruption caused an immediate return to a glacial climate by accelerating an ongoing continental glaciation, causing massive population reduction among animals and human beings. 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 abrupt occurrence of most human differentiations in 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. However, archaeologists who in 2013 found a microscopic layer of glassy volcanic ash in sediments of Lake Malawi, and definitively linked the ash to the 75,000-year-old Toba super-eruption, went on to note a complete absence of the change in fossil type close to the ash layer that would be expected following a severe volcanic winter. This result led the archaeologists to conclude that the largest known volcanic eruption in the history of the human species did not significantly alter the climate of East Africa.
Effects on life
The causes of the population bottleneck – 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. Such events may diminish populations to "levels low enough for evolutionary changes, which occur much faster in small populations, to produce rapid population differentiation". With the Lake Toba bottleneck, many species showed massive effects of narrowing of the gene pool, and Toba reduced the human population to between 15,000 and 40,000 or even fewer.
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