Volcanic dust blasted into the atmosphere causes temporary cooling. The amount of cooling depends on the amount of dust put into the air, and the duration of the cooling depends on the size of the dust particles. Particles the size of sand grains fall out of the air in a matter of a few minutes and stay close to the volcano. These particles have little effect on the climate. Tiny dust-size ash particles thrown into the lower atmosphere will float around for hours or days, causing darkness and cooling directly beneath the ash cloud, but these particles are quickly washed out of the air by the abundant water and rain present in the lower atmosphere. However, dust tossed into the dry upper atmosphere, the stratosphere, can remain for weeks to months before they finally settle. These particles block sunlight and cause some cooling over large areas of the earth.

GOES Image: Courtesy of GSFC/NASA.

Volcanoes that release large amounts of sulfur compounds like sulfur oxide or sulfur dioxide affect the climate more strongly than those that eject just dust. The sulfur compounds are gases that rise easily into the stratosphere. Once there, they combine with the (limited) water available to form a haze of tiny droplets of sulfuric acid. These tiny droplets are very light in color and reflect a great deal of sunlight for their size. Although the droplets eventually grow large enough to fall to the earth, the stratosphere is so dry that it takes time, months or even years to happen. Consequently, reflective hazes of sulfur droplets can cause significant cooling of the earth for as long as two years after a major sulfur-bearing eruption. Sulfur hazes are believed to have been the primary cause of the global cooling that occurred after the Pinatubo and Tambora eruptions. For many months a satellite tracked the sulfur cloud produced by Pinatubo. The image shows the cloud about three months after the eruption. It is already a continuous band of haze encircling the entire globe. You can learn more about the cooling effects of sulfur hazes by through the sulfur dioxide plume from the Llaima Volcano, which erupted on New Year's Day in 2008.

Volcanoes also release large amounts of water and carbon dioxide. When these two compounds are in the form of gases in the atmosphere, they absorb heat radiation (infrared) emitted by the ground and hold it in the atmosphere. This causes the air below to get warmer. Therefore, you might think that a major eruption would cause a temporary warming of the atmosphere rather than a cooling. However, there are very large amounts of water and carbon dioxide in the atmosphere already, and even a large eruption doesn't change the global amounts very much. In addition, the water generally condenses out of the atmosphere as rain in a few hours to a few days, and the carbon dioxide quickly dissolves in the ocean or is absorbed by plants. Consequently, the sulfur compounds have a greater short-term effect, and cooling dominates. However, over long periods of time (thousands or millions of years), multiple eruptions of giant volcanoes, such as the flood basalt volcanoes, can raise the carbon dioxide levels enough to cause significant global warming.

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