Fewer natural spectacles are more awesome or more terrifying that the eruption of a volcano. A volcano can shatter mountains, destroy islands and kill thousands of people. But a volcano can have a more subtle effect on it’s surroundings. Since before the dawn of recorded history, these elevated time bombs have been spewing gases and ash into the atmosphere, drastically altering global weather patterns and affecting the lives of millions of humans.

The mechanics of volcanic climate change

No matter what their height, one thing volcanoes have in common is the different materials they spew into the earth’s surface and atmosphere. These substances include, ash, magma and noxious gases. Volcanic gases in particular have a direct effect on the earth’s atmosphere and climate. Gases like sulfur dioxide are expelled by a volcano and, depending on the size of the eruption, can significantly alter weather. When sulfur dioxide ejected by an eruption enters the coolness of earth’s stratosphere, it reacts with water vapor there to form sulfuric acid. The tiny particles of sulfuric acid crowd together in the air and reflect excess sunlight. The larger the amount of sulfur dioxide released into the atmosphere, the more sunlight is reflected away from the earth. The decrease in sunlight reaching the earth’s surface causes a drop in surface temperature, often negatively affecting flora and fauna. Since these particles easily spread by means of stratospheric winds, they often cover a large atmospheric area and can remain in the stratosphere for up to two years.

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The history of volcanic climate change

The earliest proposed cooling due to volcanic gases is the aftermath of the Toba eruption around 74,000 BC. Some scientists believe that the Toba eruption caused a volcanic winter which resulted in a huge reduction in early human population. Volcano influenced cooling is also blamed for the widespread crop failures of 535 AD. Scientists postulate that the eruption of the Ilopango volcano in present-day El Salvador filled the skies with sulfuric acid, blocking out the sun’s rays. The eruption likely caused summer snowstorms in China and famine in Europe.

Volcanic winter next reared its head in 1258 AD. Consistent with volcanic winters, the summer was marked by cool temperatures that retarded the growth of new plants. The sulfur dioxide emissions were probably released by the Mexican volcano El Chichón. Climate disaster happened once again in 1452 when a volcano on the Pacific island of Kuwae erupted, splitting the island in two. The resulting cloud of gases drastically altered the global climate, causing grain yields in Europe and Asia to crash. Some historians partially credit the temperature decline for allowing the Ottomans to capture Constantinople in 1453 after a prolonged siege.

The Russian famine of 1601 was a by-product of the first well-documented global cooling caused by volcanic emissions. The cooling was the result of a massive 16-day eruption of the Peruvian volcano Huaynaputina. Samples of ice show that the volcano’s ash spread from the South Pole to the North Pole. The famine in Russia resulted from freezing weather in the spring of 1601. An estimated 2 million Russians died of cold and starvation during the ensuing famine.

A similarly devastating eruption occurred in 1783 in Iceland. The Laki fissure spewed lava for eight months, killing more than half of the cattle on the island. When it was over, Laki’s poisonous gases had killed one-fourth of Icelanders and caused a weird haze to descend over much of Europe. Iceland’s population didn’t recover until the 1850s.

One of the most famous weather shifts caused by volcanoes is the almost mythical Year Without Summer. The 1815 eruption of the volcano on Mount Tambora on Sumbawa island caused winter in July. Heavy frosts destroyed American bean crops, while in Europe and China livestock and crops died simultaneously. Volcanic particles mixed with snow and colored the flurries a dingy brown.

The loudest eruption from a volcano in recorded history is the eruption of Krakatoa in 1883. Although the resulting volcanic winter is not the coldest on record, it became the first cooling to be widely accepted as a result of volcanic activity. An opportunity to study the phenomena of volcanic climate change didn’t arise again until 1991 when scientists were able to carefully monitor the eruption of the volcano on Mount Pinatubo.