Volcanoes: Climatic Effects

Studies of volcanic activity confirm the existence of a significant relationship between the effects of an eruption and long-term climatic conditions.

Atmospheric Environment

Scientists have speculated for centuries on the impact of volcanic eruptions on the climate. Historical figures such as Benjamin Franklin have noted the possible link between an eruption and climatic conditions. Two years after the 1783 Laki eruption, in Iceland, Franklin suggested that the blue haze that lingered over the city of Paris during his stay there was caused by the Icelandic event. He believed the “fog,” as he termed it, caused a decline in the temperature by absorbing portions of sunlight that otherwise would have reached the ground surface.

Technological advances such as satellite imagery have made it possible for scientists to pinpoint more precisely which types of matter injected into the atmosphere by a volcano have the greatest effect. Initially, researchers believed it was the amount of gas and ash hurled into the atmosphere that determined the impact. However, scientists have come to learn that it is the types of gases that are the critical determining factor. The molten rock of a volcano contains a variety of gases that are released into the atmosphere before, during, and after an eruption. They range from water vapor to dense clouds of sulfur. Other gases include carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, fluorine, chlorine, carbon monoxide, and hydrochloric acid, along with smaller amounts of other chemical compounds.

Exposure to the acid gases such as sulfur dioxide, hydrogen sulfide, and hydrochloric acid can be life threatening, while exposure to fluorine is particularly dangerous. One of the most serious hazards posed by a volcano is the large amount of carbon dioxide it can emit during an eruption. Since it is heavier than air, carbon dioxide tends to settle in lower elevations. As a result, numerous people residing near erupting volcanoes have been asphyxiated. When a volcano erupts, its immediate effect on the local area is evident, as it spews a great amount of ash, volcanic gases, and heat into the atmosphere. Violent eruptions usually are accompanied by thunderstorms, lightning, and torrential rains. In some cases, the intense heat can generate powerful whirlwinds, strong enough to topple nearby trees and destroy wildlife. Volcanic plumes also contain droplets of water in which acid gases have dissolved. These droplets eventually fall to the ground in the form of acid rain, which can have a corrosive effect on a variety of metal objects and other materials. Exposure to significant amounts of volcanic gases also can have a lethal effect on most varieties of vegetation.

Along with the research into the makeup of gases has come a greater awareness of the long-term effects on climatic patterns. The volcanic dust released into the lower atmosphere can create a temporary cooling effect by producing an ash cloud that blocks the sunlight. However, the ash particles are quickly washed out by the abundant supply of water and rain contained in the lower atmosphere. Most ash clouds stop rising at the troposphere, where they encounter increased air temperatures. Dust particles thrown into the stratosphere can linger for several weeks or months before they settle back to the surface.

For many years scientists believed that volcanic ash was the most important factor in climatic changes. They have since discovered that volcanic gases exert the greatest influence on climatic conditions. Specifically, the key factor is the conversion of the gases into sulfuric acid aerosols that can remain in the atmosphere for months. The droplets, though tiny, are able to reflect significant amounts of sunlight and have been detected at altitudes as high as 25 kilometers. They are able to cool the troposphere by reflecting solar rays back into space and can warm the stratosphere by absorbing infrared radiation. The reflective effect is particularly strong in cloudless areas. It has been estimated that the aerosols can increase the planet’s reflectivity, or albedo, by nearly 20 percent. Although aerosols eventually grow large enough to fall back to the surface from their own weight, the process can take years because of the rarefied and dry conditions in the stratosphere, which slows their growth. Aerosol cloud eruptions occur on average once every ten to twenty years, with a major eruption taking place every one hundred years.

Only volcanic eruptions that emit significant amounts of sulfur compounds influence the global climate. Smaller eruptions can create atmospheric effects similar to larger explosions if they release large concentrations of sulfur-rich elements directly into the stratosphere. Also, eruptions occurring at high latitudes have less impact on climatic conditions than those at lower latitudes, where air currents are greater. The aerosols travel more quickly around the globe when moving in an easterly or westerly direction. If they are traveling in a north-south direction, the movement is much slower, with many of the aerosols becoming confined for years in a zone surrounding the polar cap.

Measurement Methods

There are two primary indices that are used to measure the probable impact of volcanic activity. The first is the Dust Veil Index (DVI), which is based on an estimate of the volume of material injected into the atmosphere, surface temperatures, and the amount of sunlight reaching the surface. This index is derived from observations made at midlatitudes and thus is not entirely representative of the global scenario. The second index is the Volcanic Explosivity Index (VEI), which is based on magnitude, intensity, dispersion, and destructiveness. Other measurements of impact include tree-ring records, ice-core readings, and solar radiation measurements.

In addition to the impact indices, four primary methods are used to determine the volume and composition of gases that are emitted during large volcanic eruptions. The first is an examination of the composition and amount of aerosol layers in polar ice cores that are identified as being related to volcanic eruptions. Second is a comparison of gases from eruptions and glass inclusions in crystals that had formed in the volcanic rock prior to the event. The third is measurements of eruptions from satellite imagery, and the fourth is measurements of volcanic aerosols from the surface. Scientists have had some success with using a laser instrument that sends out a pulsing light beam, which reflects back when it detects aerosols. This method enables researchers to construct a profile indicating the density and height of the aerosols.

Ice core readings are especially beneficial in providing a clear record of older eruptions. Atmospheric aerosols from nearly every historic event have shown up in deep drillings of the polar ice. Since they tend to accumulate in layers, the ice cores offer clear annual records of climate and weather. As the aerosols fall to the ground surface over the poles, they begin to soil the surface snow with acid fallout in a process called sedimentation, by which thin layers of debris are formed. Over time, the snow becomes compacted into glacial ice and can be detected through electrical conductivity measurements. Some ice core discoveries have provided records going back more than 100,000 years.

Satellite measurements have enabled scientists from the U.S. Geological Survey to determine the amounts and compositions of gases emitted by several active volcanoes in the United States. Satellite sensors detected up to 1 million metric tons of sulfur dioxide tossed into the stratosphere during the main eruption of Mount St. Helens in 1980. Of particular benefit are the satellite observations conducted by the National Aeronautics and Space Administration’s (NASA) Total Ozone Mapping Spectrometer (TOMS) instrument, which have helped to measure sulfur dioxide levels in the atmosphere following major events. TOMS was instrumental in tracking the band of sulfur dioxide across the Pacific produced by the eruption of Mount Pinatubo in the Philippines in 1991. Altogether, TOMS has made more than one hundred observations of volcanic events, including a major eruption of Chile’s Cerro Hudson volcano in 1991. These measurements allow scientists to compare volcanic emissions of sulfur dioxide with injections of the gas from industrial plants and other human-based activities. Through comparative studies of volcanic activity, researchers are able to examine the effects of past and future eruptions with the aim of determining whether human or natural activities ultimately pose the greater threat to the environment.

Historical Record

Though less documented than modern eruptions, the larger historic events offer substantial proof of volcanic effects on climate for the simple reason they were bigger and left a more easily detectable trail of evidence.

By drilling into the sea floor south of Haiti and uncovering evidence of ash, researchers concluded that massive volcanic eruptions in the Caribbean Basin more than 55 million years ago created a sudden temperature inversion in the ocean waters that led to one of the most dramatic climatic changes in history. Scientists discovered distinctly colored volcanic ash layers that were far different from the sediments located above and below them. The period when the ash layers were deposited corresponds with a time of rapid warming globally. The presence of the ash indicates that a gigantic eruption took place just as the warming began. Scientists believe that the dust and gases from the eruptions initially cooled the atmosphere, increasing the density of seawater to the point where it sank into the deep ocean. The descending water, in turn, warmed the ocean floor and melted deposits of methane sediments, which then bubbled up into the air, creating a greenhouse effect that warmed the world. Evidence also indicates that the process resulted in the extinction of nearly one-half of all deep-sea animals, victims of asphyxiation because of the lower solubility of oxygen in the suddenly warmer waters. Conversely, the evolution of new plant and animal species, including many primates and carnivores, was accelerated. Scientists already were aware that volcanic eruptions had occurred in the North Atlantic Ocean nearly 61 million years ago and believe that the Caribbean Basin event somehow may have acted in connection with these earlier eruptions.

In another major undersea discovery, scientists suspect that the islands of Tonga and Epi, located about 1,930 kilometers east of Australia, were the products of a massive eruption that took place around the year 1453. During their research, scientists found that the entire stretch of sea floor separating the two islands was a crater more than 11 kilometers in width. They also uncovered charred vegetation that was carbon-dated between 1420 and 1475. To further narrow down the date of the eruption to 1453, they analyzed ice cores from Greenland and Antarctica; tree-ring records from California, Europe, and China; and reports of worldwide crop conditions during the period.

In 1815, the volcano Mount Tambora erupted in Indonesia, precipitating one of the clearest examples of an eruption-induced global cooling event. The volcano emitted a massive column of solid material into the upper atmosphere. The aerosol veil extended to both hemispheres with effects that lasted well into the following year. The aftereffects were such that the year 1816 came to be known as “the year without a summer.” In some regions of New England, up to 15 centimeters of snow fell in the month of June. There were numerous other reports of abnormally cool weather, including record low temperatures that forced people to wear coats and gloves in July. The average temperature in the Northern Hemisphere was reduced by as much as 0.5 degree Celsius, and parts of the United States and Canada experienced unusual summer frosts and crop failure. In Europe, the unusually cold readings resulted in widespread famine, though at the time the connection with the volcanic veil went unrecognized. Researchers identified the Tambora eruption from evidence uncovered in ice cores in Antarctica and Greenland. The event coincides with the fact that the decade between 1810 and 1820 is considered perhaps the coldest on record.

The famous 1883 eruption of Krakatau in Indonesia marked perhaps the first time researchers became fully involved on a worldwide basis with a volcanically induced atmospheric event. Much of the global interest could be attributed to the advances in telegraphic communication, which enabled scientists to share their observations of the spectacular sunsets and other visible phenomena arising from the eruption. Measurements indicated that Krakatau generated a cloud of approximately 21 cubic kilometers of matter. Witnesses in the area recalled dramatic displays of lightning in the cloud veil and a strong odor of sulfur in the air. Researchers believe that the ash cast into the upper atmosphere by the eruption and the ensuing dust veil led to worldwide decreases in incoming solar radiation. Mean annual global temperatures fell close to 0.5 degree Celsius in 1884, with the cooling period extending through the remainder of the 1880s. In 1884, there was a marked increase in the number of storms in the United States. Record snowfalls, an unusually high number of tornadoes, torrential rains, and severe flooding caused widespread damage. The abnormal atmospheric conditions attributed to Krakatau included brilliant sunsets and a blue or green tinge to the sun, depending on which part of the globe the observation was made. In 1888, the Royal Society of London published a volume that documented the eruption and formally established the connection between major volcanic activity and subsequent changes in worldwide atmospheric conditions.

By historic standards, the eruption of Mount Pinatubo may appear insignificant, but it stands as a watershed event in the ability of scientists to monitor the interaction of a volcanic cloud and the upper atmosphere. The eruption is believed to have sent nearly 20 million tons of sulfur dioxide and ash about 20 to 27 kilometers into the atmosphere, resulting in 30 billion kilograms of sulfuric acid aerosols. For several months the TOMS instrument tracked the sulfur cloud created by Pinatubo with images verifying that its particles circled the globe in about three weeks, forming an almost continuous band.

The global impact of the Pinatubo eruption was significant. The year following its eruption turned out to be one of the coldest on record. Temperature measurements in the lower and middle atmospheres indicated a change of nearly 0.5 degree Celsius between 1991 and 1992. By 1994, readings revealed that the volcano’s effect had waned and that global temperatures had returned to previous levels.

Significance

Much has been learned and much remains a mystery concerning the chemical and physical processes that occur between a volcanic eruption and climatic change. It is an area of intense study because of the belief among scientists that the balance of the global climate is dependent on the phenomenon of volcanism. The atmosphere essentially was developed through intermittent volcanic emissions of carbon dioxide and water vapor, along with nitrogen and possibly methane. There is no reason to believe that the relationship between the two forces has changed in any dramatic way.

Volcanic eruptions can be an agent for global cooling or global warming, depending on their interactions with other environmental elements. The historical record indicates that when the atmospheric balance is threatened by natural or human forces, the consequences can be severe. To most people, fractions of degrees may not appear significant, but in the grand scheme of the environment, they can produce dramatic effects. During the ice ages, the global temperature was only about 5 degrees Celsius cooler than it was at the close of the twentieth century. Scientists believe that a global rise in temperature of as little as 3 degrees Celsius could bring about dramatic changes, including accelerated glacial melting, rising sea levels, more frequent and more severe storms, and droughts. The temperature increase during the twentieth century is considered by many as evidence that the human production of greenhouse gases such as carbon dioxide is affecting the climate. However, it also is believed that multiple eruptions of large volcanoes over a long period of time can raise the carbon dioxide levels enough to cause substantial global warming. To add to the equation, studies also indicate a possible association between other volcanic vapors and the depletion of the ozone layer. A few researchers have even advanced the idea that there is a link between volcanism and El Niño, the periodic warm ocean conditions that appear along the tropical west coast of South America. A number of eruptions preceded El Niño in years past, leading to speculation that volcanic gases may trigger or strengthen the phenomenon.

Major events such as the Caribbean Basin volcanic eruption pose a special problem for climatic equilibrium. The sudden warming of the deep ocean resulting from a series of large volcanic eruptions is a scenario that scientists believe could recur, causing a major disruption of atmospheric circulation. In attempting to trace the connections between volcanic activities and climate, scientists have begun to think in global terms and to look upon their task as an interdisciplinary effort. In so doing, they have been able to make significant strides in developing the depth of understanding necessary to form reasonably accurate forecasts of future catastrophic events.

Principal Terms

aerosol: an aggregate of dispersed gas particles suspended in the atmosphere for varying periods of time because of their small size

atmosphere: the thin layer of nitrogen, oxygen, and other gases surrounding Earth, whose density decreases rapidly with height

climate: the sum total of the weather elements that characterize the average condition of the atmosphere over a long period of time for any one region

greenhouse effect: the retention of solar heat in the lower atmosphere caused by the absorption and reradiation of infrared energy from the surface by various gases, creating an insulating effect similar to a greenhouse

ozone layer: a region of the stratosphere, around 60 kilometers in altitude, containing ozone that absorbs ultraviolet radiation from the sun

stratosphere: the atmospheric layer above the troposphere, characterized by little or no temperature change with altitude

sulfur dioxide: a colorless, nonflammable, suffocating gas formed when sulfur is oxidized

tropopause: the transition zone at the top of the troposphere between the troposphere and the stratosphere

troposphere: the lowest layer of atmosphere where temperature generally declines with altitude, containing about 95 percent of the mass of the atmosphere and the site of most atmospheric turbulence and weather features

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