Volcanic activity is another trigger of ice ages. The major volcanic gases released into the atmosphere during an eruption are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). Gases released in smaller amounts can include hydrogen sulfide (H2S), hydrogen (H2), carbon monoxide (CO), hydrogen chloride (HCl), hydrogen fluoride (HF), and helium (He). During a volcanic eruption, ash can reach great heights in the atmosphere and spread around Earth. This cloud of ash acts as a blanket to block out the incoming solar radiation. If the eruption is massive enough, the ash cloud can effectively block incoming solar radiation for a couple of years, causing a worldwide cooling.
According to the National Aeronautics and Space Administration (NASA), volcanic gases are thought to be responsible for the "global cooling" that is experienced after major volcanic eruptions. The degree and intensity of the cooling effect varies based on the specific characteristics of the eruption. The factors that control it include the force of the eruption, the amounts of particular gases emitted, and the geographical location of the volcano with regard to major atmospheric circulation patterns. If the eruption material also contains sulfur dioxide gas, when it reaches the stratosphere, it turns into sulfuric acid particles (aerosols), which serve to reflect the Sun's rays, causing an even greater reduction in the amount of sunlight reaching Earth's surface. This process can cause a significant cooling effect.
According to scientists at Michigan Technological University, two indices are used to measure the effects of volcanic eruptions on climate: the dust veil index (DVI) and the volcanic explosivity index (VEI). The DVI uses estimations of the amount of material dispersed into the atmosphere in addition to the temperatures at Earth's surface and the amount of sunlight reaching Earth's surface. The VEI ranks eruptions using specific criteria to determine the magnitude, intensity, dispersion, and destructiveness of a volcanic eruption. Eruptions are ranked from 1
to 8, with 8 being the most explosive. Eruptions that inject material into the stratosphere have a VEI of 4 or higher.
One of the most well-known examples of this was the eruption of Mount Pinatubo in June 1991. The eruption was so violent and ejected so much gas and particulates into the stratosphere that it temporarily offset the predicted greenhouse warming effect for a few years. During major eruptions, millions of tons of sulfur dioxide gas can reach the stratosphere. According to NASA, observations of the effects of Mount Pinatubo aerosols on global climate have been used to validate scientists' understanding of climate change and the ability to predict future climate change. In fact, researchers at NASA's Goddard Institute for Space Studies (GISS) in New York City have applied the general circulation model (GCM) of Earth's climate to it and have been successful in predicting the effects of the sulfate aerosols from Mount Pinatubo's eruption and its part in cooling global temperatures.
Short-term "global cooling" has been linked with other volcanic eruptions throughout history. Another notable occurrence was an eruption in 1816 that caused "the year without a summer." When the Tambora volcano in Indonesia erupted, it emitted 200 million tons (181 million metric tons) of sulfuric acid aerosol into the stratosphere, causing significant weather disruption worldwide. According to researchers at Michigan Technological University, the average decrease in temperature in the Northern Hemisphere was between 0.7° and 1.2°F (0.4°-0.7°C). Western Europe experienced weather disruptions, and the United States and Canada suffered killing summer frosts. The sulfur dioxide aerosol layer in the stratosphere caused brilliant sunsets around the world for several years afterward.
About 71,000 years ago a far more significant volcanic eruption occurred in Sumatra. Mount Toba erupted and affected the climate to the point where the next 1,000 years saw the death of many species throughout the world. In 1783, the Laki Fissure eruption occurred in Iceland and lasted for eight months. During the eruption, more than 100 million tons (91 million metric tons) of sulfur dioxide was ejected into the atmosphere. As a result, Iceland suffered great losses. Three-quarters of its livestock died, and all the agricultural crops failed. The famine that it caused led to the starvation of 25 percent of the Icelandic population.
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