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This article is from the In-Depth Report A Guide to Volcanoes

How do volcanoes affect world climate?

Karen Harpp, an assistant professor of geology at Colgate University, provides this explanation:
volcano
Image: ¿PHOTODISC
VOLCANIC ERUPTIONS inject ash and aerosol clouds into the atmosphere and produce more than 100 million tons of carbon dioxide each year.

In 1784, Benjamin Franklin made what may have been the first connection between volcanoes and global climate while stationed in Paris as the first diplomatic representative of the United States of America. He observed that during the summer of 1783, the climate was abnormally cold, both in Europe and back in the U.S. The ground froze early, the first snow stayed on the ground without melting, the winter was more severe than usual, and there seemed to be "a constant fog over all Europe, and [a] great part of North America."

What Benjamin Franklin observed was indeed the result of volcanic activity. An enormous eruption of the Laki fissure system (a chain of volcanoes in which the lava erupts through a crack in the ground instead of from a single point) in Iceland caused the disruptions. The Laki eruptions produced about 14 cubic kilometers of basalt (thin, black, fluid lava) during more than eight months of activity. More importantly in terms of global climate, however, the Laki event also produced an ash cloud that may have reached up into the stratosphere. This cloud caused a dense haze across Europe that dimmed the sun, perhaps as far west as Siberia. In addition to ash, the eruptive cloud consisted primarily of vast quantities of sulfur dioxide (SO2), hydrogen chloride (HCl), and hydrogen fluoride gases (HF). The gases combined with water in the atmosphere to produce acid rain, destroying crops and killing livestock. The effects, of course, were most severe in Iceland; ultimately, more than 75 percent of Iceland¿s livestock and 25 percent of its human population died from famine or the toxic impact of the Laki eruption clouds. Consequences were also felt far beyond Iceland. Temperature data from the U.S. indicate that record lows occurred during the winter of 1783-1784. In fact, the temperature decreased about one degree Celsius in the Northern Hemisphere overall. That may not sound like much, but it had enormous effects in terms of food supplies and the survival of people across the Northern Hemisphere. For comparison, the global temperature of the most recent Ice Age was only about five degrees C below the current average.

There are many reasons that large volcanic eruptions have such far-reaching effects on global climate. First, volcanic eruptions produce major quantities of carbon dioxide (CO2), a gas known to contribute to the greenhouse effect. Such greenhouse gases trap heat radiated off of the surface of the earth forming a type of insulation around the planet. The greenhouse effect is essential for our survival because it maintains the temperature of our planet within a habitable range. Nevertheless, there is growing concern that our production of gases such as CO2 from the burning of fossil fuels may be pushing the system a little too far, resulting in excessive warming on a global scale. There is no doubt that volcanic eruptions add CO2 to the atmosphere, but compared to the quantity produced by human activities, their impact is virtually trivial: volcanic eruptions produce about 110 million tons of CO2 each year, whereas human activities contribute almost 10,000 times that quantity.

By far the more substantive climatic effect from volcanoes results from the production of atmospheric haze. Large eruption columns inject ash particles and sulfur-rich gases into the troposphere and stratosphere and these clouds can circle the globe within weeks of the volcanic activity. The small ash particles decrease the amount of sunlight reaching the surface of the earth and lower average global temperatures. The sulfurous gases combine with water in the atmosphere to form acidic aerosols that also absorb incoming solar radiation and scatter it back out into space.

The ash and aerosol clouds from large volcanic eruptions spread quickly through the atmosphere. On August 26 and 27, 1883, the volcano Krakatau erupted in a catastrophic event that ejected about 20 cubic kilometers of material in an eruption column almost 40 kilometers high. Darkness immediately enveloped the neighboring Indonesian islands of Java and Sumatra. Fine particles, however, rode atmospheric currents westward. By the afternoon of August 28th, haze from the Krakatau eruption had reached South Africa and by September 9th it had circled the globe, only to do so several more times before settling out of the atmosphere.

Initially, scientists believed that it was volcanoes' stratospheric ash clouds that had the dominant effect on global temperatures. The 1982 eruption of El Chich¿n in Mexico, however, altered that view. Only two years earlier, the major Mt. St. Helens eruption had lowered global temperatures by about 0.1 degree C. The much smaller eruption of El Chich¿n, in contrast, had three to five times the global cooling effect worldwide. Despite its smaller ash cloud, El Chich¿n emitted more than 40 times the volume of sulfur-rich gases produced by Mt. St. Helens, which revealed that the formation of atmospheric sulfur aerosols has a more substantial effect on global temperatures than simply the volume of ash produced during an eruption. Sulfate aerosols appear to take several years to settle out of the atmosphere, which is one of the reasons their effects are so widespread and long lasting.

The atmospheric effects of volcanic eruptions were confirmed by the 1991 eruption of Mount Pinatubo, in the Philippines. Pinatubo¿s eruption cloud reached over 40 kilometers into the atmosphere and ejected about 17 million tons of SO2, just over two times that of El Chich¿n in 1982. The sulfur-rich aerosols circled the globe within three weeks and produced a global cooling effect approximately twice that of El Chich¿n. The Northern Hemisphere cooled by up to 0.6 degrees C during 1992 and 1993. Moreover, the aerosol particles may have contributed to an accelerated rate of ozone depletion during that same period. Interestingly, some scientists argue that without the cooling effect of major volcanic eruptions such as El Chich¿n and Mount Pinatubo, global warming effects caused by human activities would have been far more substantial.

Major volcanic eruptions have additional climatic effects beyond global temperature decreases and acid rain. Ash and aerosol particles suspended in the atmosphere scatter light of red wavelengths, often resulting in brilliantly colored sunsets and sunrises around the world. The spectacular optical effects of the 1883 Krakatau eruption cloud were observed across the globe, and may have inspired numerous artists and writers in their work. The luminous, vibrant renderings of the fiery late day skyline above the Thames River in London by the British painter William Ascroft, for instance, may be the result of the distant Krakatau eruption.

In 1815, the Indonesian volcano Tambora propelled more ash and volcanic gases into the atmosphere than any other eruption in history and resulted in significant atmospheric cooling on a global scale, much like Krakatau a few decades later. New England and Europe were particularly hard hit, with snowfalls as late as August and massive crop failures. The cold, wet, and unpleasant climatic effects of the eruption led 1816 to be known as "the year without a summer," and inspired Lord Byron to write:

"The bright Sun was extinguish¿d, and the stars
Did wander darkling in the eternal space
Rayless and pathless, and the icy earth
Swung blind and blackening in the moonless air;
Morn came and went¿and came,
And brought no day¿"

--Lord Byron, "Darkness"

There is a story that Byron invited some of his friends to his home in Switzerland that summer to relax by the shores of Lake Geneva. The lack of sun and warm summer weather led the group to hold a competition writing ghost stories to keep themselves entertained. One of the guests, Mary Shelley, wrote the famous novel Frankenstein for this contest, revealing that in addition to major climatic effects, volcanic eruptions can have some unexpectedly far-reaching results.

Answer originally published April 15, 2002.

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