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How Volcanoes Affect Climate
Climate Effects of Historic Volcanoes
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The Volcano Helps Solve a Mystery

Climate Effects of Historic Volcanoes

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Historic Volcanoes

Why does a NASA scientist spend most of his time poring over ancient Greek and Latin history books? It's not a hobby-it's part of his research. Richard Stothers, with NASA's Goddard Institute for Space Studies, has spent decades reading ancient manuscripts in search of evidence linking volcanoes to climate changes. By comparing the dates of past volcanoes with historical accounts of atmospheric phenomena, he has found compelling evidence that volcanoes can significantly change the weather and can even cause crop failures and disease pandemics.

Stothers didn't start out researching volcanoes. Originally he was searching the historical record for signs of the solar cycle, peaks of solar turbulence that produce an increase in sunspots and displays of the aurora borealis, or northern lights, at regular intervals. He pored through thousands of pages of ancient history, writing down every reference to the aurora or sunspots. Piecing together the evidence from various historians, he found regular peaks in the solar cycle as far back as the 2nd century B.C.

During his research, Stothers also noticed references to other strange things happening in the sky. He read about the sun and moon turning different colors, rings around the sun, triple suns, and strange "dry fogs" that produced neither moisture nor rain. Stothers mentioned these curiosities to a colleague, Mike Rampino, who said they could have been caused by volcanoes.

The Year Without Summer
The 1815 eruption of Tambora, a volcano in Indonesia, is widely credited with causing the famous "year without a summer" the following summer. In 1816, frosts occurred every month of the year in the U.S., and crop failures occurred worldwide. The sun appeared green in Indonesia; the twilight sky turned pink, orange, red, and purple in England; and sunspots were visible to the naked eye in New York. Mary Shelley spent that dark, rainy summer writing a story that later grew into the novel "Frankenstein," while her friend Lord Byron wrote a gloomy poem called "Darkness." Most likely, that dark, rainy summer was caused by volcanic aerosols.

Stothers reasoned that if other ancient volcanoes had caused similar phenomena, observers at the time would have described strange skies and climate events in the years following major volcanoes. To find out whether history supported his theory, he read the entire historical record of the Mediterranean region from antiquity to the Middle Ages - the equivalent of a quarter-million pages of text.

Stothers found numerous descriptions of dark and colored skies, cold weather, and crop failures following volcanoes. The Greek historian Plutarch wrote that, following the death of Julius Caesar, the sun "rose pale and without radiance," and "the fruits, imperfect and half ripe, withered away and shriveled up on account of the coldness of the atmosphere." Pliny the Elder wrote that "portentous and protracted eclipses of the sun caused almost a whole year's continuous gloom."

Although such phenomena must have seemed like supernatural omens when they occurred, Stothers believes they were caused by a volcano. The culprit may have been Mount Etna. "After the death of Caesar," the Roman poet Virgil wrote, "how often we saw Etna flooding out from her burst furnaces veiling the sky in smoke." Some of these historical quotations had been found already; Stothers turned up many more that were previously unknown.

Tree Rings and Ice Layers
To distinguish fact from legend, Stothers wanted to correlate his findings with other types of evidence. Fortunately, volcanoes leave many clues for researchers. In addition to their atmospheric and climate effects, ancient volcanoes have left evidence of their eruptions in the rings of old trees and in the layers of polar ice.

Each year, a tree adds another layer of growth. When the tree is cut, its age can be determined by counting the number of growth rings visible in its trunk. The thickness of individual rings varies depending on the amount of growth from year to year. By examining and comparing the rings of very old or fossilized trees, a scientist determine the relative growth rates of trees dating back thousands of years. If an ancient volcano created a vast aerosol cloud that darkened the sky for a year or more-and limited tree growth-that harsh weather would be reflected in tree rings from that time.

A similar record is buried in the ice layers of Greenland and Antarctica. Each year in the polar regions, a new layer of snow is added on top of the last. As more snow piles up, it is compacted into layers of ice, a layer for each year. If a lot of volcanic dust falls during one year, it will stay trapped in the ice layer-and the age of that layer can be determined by counting the number of layers above it. Likewise, if one year a volcano releases a large amount of sulfuric acid aerosol particles, the snow that falls that year will form layers of highly acidic ice.

"The Sun Became Dark"
Stothers found that tree-ring and ice-core data frequently supported the connections he'd made between historical events and volcanoes. Ancient historians described dry fogs, dark skies, poor harvests and heavy snowfall in the year 536. "The sun became dark and its darkness lasted for 18 months," wrote Michael the Syrian. "Each day it shone for about 4 hours, and still this light was only a feeble shadow. Everyone declared that the sun would never recover its full light. The fruits did not ripen and the wine tasted like sour grapes." An examination of tree rings from the year 536 revealed extremely narrow rings in trees from around the world. The combined evidence, Stothers concludes, points to a volcanic aerosol cloud that persisted more than a year. The cloud spread over the entire northern half of the planet, but because no eruption was observed or recorded at the time, Stothers cannot say exactly where it occurred.

Stothers found signs of another unknown volcano that erupted 90 years later. "In the year A.D. 626," wrote Michael the Syrian, "the light of half the sphere of the sun disappeared, and there was darkness from October to June. As a result people said that the sphere of the sun would never be restored to its original state." Greenland ice-core samples dating from the year 626 are very acidic-so much so that the volcano that led to them may have been one of the greatest producers of volcanic aerosols in recorded history. Furthermore, bristlecone pine trees in California show evidence of a summer frost in 628, which indicates that the massive aerosol cloud produced by the 626 volcano spanned the entire globe for at least two years. Scientists are still looking for evidence that would identify which volcano was responsible; it may have been one in Iceland.

Prior to Stothers's research, the idea that volcanoes could alter the climate was considered an untested theory. But in a 1999 paper, Stothers showed that volcanoes have been a major player in human history in ways previously unsuspected. Stothers found that the seven largest volcanoes of the last 2,000 years were all followed by outbreaks of plague (except for one volcano that erupted while a plague pandemic was already underway). Plague pandemics and gigantic volcanic eruptions are both so rare that the odds of them occurring together by chance are about 100 million to 1, Stothers calculated. Clearly, history suggests that there has been some connection between the two phenomena.

The connection, Stothers believes, is the vast aerosol haze that major volcanoes produce. The resulting global cooling generates more clouds and rainfall, which in turn can cause crop failures and famines. In addition to weakening the population and making people more susceptible to illness, crop failures also drive hungry rats from barren fields into human food supplies. These rats can bring diseases, including the plague, with them. Plague bacteria do not thrive in hot, sunny weather; but the cool, moist weather caused by a major volcano creates ideal conditions for plague germs to breed and spread.

Huge Prehistoric Volcanoes
Farther back in time, before humans existed, volcanoes appear to have exerted tremendous influence on the global climate. The two biggest mass extinctions in history both occurred after enormous volcanic eruptions that produced vast amounts of basaltic lava and reshaped Earth's surface. One mass extinction occurred about 65 million years ago, when half of all existing marine species became extinct. Another huge eruption in Siberia 250 million years ago may have caused the extinction of more than 90 percent of all marine species living at the time. These mass extinctions may also have been caused by a comet or asteroid striking the Earth, which would have darkened the sky with smoke and debris.

Volcanoes Provide Air and Water
Volcanoes are not always destructive. In fact, life on Earth would be impossible without them. Most of Earth's early atmosphere came from gases freed from the depths of the Earth by volcanic eruptions. Volcanoes are also thought to have provided some or possibly all of the water on Earth's surface. (Some scientists theorize that prehistoric comets also may have brought water to Earth.) Some lava contains as much as 4 percent water, which is liberated as steam during volcanic eruptions. So volcanoes may have brought us not only the air we breathe but also the water that spawned life and that makes Earth habitable to this day.