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Volcanoes and the Weather

Pinatubo91_lateral_blast_plume_pinatubo_06-15-91.jpg
View to west from Clark Air Base of Pinatubo's lateral blast cloud of eruption of 5:55 am. U.S. Geological Survey Photograph taken on June 15, 1991, by Rick Hoblitt.
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Among the more prominent theories of events that have triggered global climatic changes and lead to repeated glaciation are: (1) known astronomical variations in the orbital elements of the Earth (the so-called Milankovitch theory); (2) changes in energy output from the Sun; and (3) increases in volcanism that could have thrown more airborne volcanic material into the stratosphere, thereby creating a dust veil and lowered temperatures.

The years 1980, 1981, and 1982, for example, saw several major volcanic eruptions adding large quantities of particulate volcanic material and volatiles to the stratosphere, including the catastrophic eruption of Mount St. Helens, Washington, on May 18, 1980, and a large eruption of Mount Hekla, Iceland, on August 17, 1980. The 1982 series of eruptions from El Chichón volcano, Mexico, caused death and destruction in the populated area around the volcano, but a further reaching impact may result from the effect on Earth's climate because of the enormous ejection of volcanic material into the stratosphere.

The potential climatic effect of the Laki volcanic eruption in Iceland in 1783, the largest effusive (lava) volcanic eruption in historic time, was noted by the diplomat-scientist Benjamin Franklin in 1784, during one of his many sojourns in Paris. Franklin concluded that the introduction of large quantities of volcanic particles into the Earth's upper atmosphere could cause a reduction in surface temperature, because the particles would lessen the amount of solar energy reaching the Earth's surface. The catastrophic eruption of the Tambora volcano, Indonesia, in 1815 was followed by a so-called "year-without-a-summer." In New England, for example, frost occurred during each of the summer months in 1816.

The June 1991 eruption of Mount Pinatubo was global. Slightly cooler than usual temperatures recorded worldwide and the brilliant sunsets and sunrises have been attributed to this eruption that sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that drifted around the world. The sulfur dioxide (SO2) in this cloud -- about 22 million tons -- combined with water to form droplets of sulfuric acid, blocking some of the sunlight from reaching the Earth and thereby cooling temperatures in some regions by as much as 0.5 degrees C. An eruption the size of Mount Pinatubo could affect the weather for a few years.

A similar phenomenon occurred in April of 1815 with the cataclysmic eruption of Tambora Volcano in Indonesia, the most powerful eruption in recorded history. Tambora's volcanic cloud lowered global temperatures by as much as 3 degrees C. Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In parts of Europe and in North America, 1816 was known as "the year without a summer."

Volcanic eruptions are thought to be responsible for the global cooling that has been observed for a few years after a major eruption. The amount and global extent of the cooling depend on the force of the eruption and, possibly, its latitude. When large masses of gases from the eruption reach the stratosphere, they can produce a large, widespread cooling effect. As a prime example, the effects of Mount Pinatubo, which erupted in June 1991, may have lasted a few years, serving to offset temporarily the predicted greenhouse effect.

Global cooling often has been linked with major volcanic eruptions. The year 1816 often has been referred to as "the year without a summer". It was a time of significant weather-related disruptions in New England and in Western Europe with killing summer frosts in the United States and Canada. These strange phenomena were attributed to a major eruption of the Tambora volcano in 1815 in Indonesia. The volcano threw sulfur dioxide gas into the stratosphere, and the aerosol layer that formed led to brilliant sunsets seen around the world for several years.

However, there is some confusion about the historical evidence that global cooling may be caused by volcanic emissions. Two recent volcanic eruptions have provided contradictory evidence on this point. Mount Agung in 1963 (Indonesia) apparently caused a considerable decrease in temperatures around much of the world, whereas El Chichon in 1982 (Mexico), seemed to have little effect, perhaps because of its different location or because of the El Nino that occurred the same year. El Nino is a Pacific Ocean phenomenon, but it causes worldwide weather variations that may have acted to cancel out the effect of the El Chichon eruption.

Cataclysmic eruptions, such as the June 15, 1991, eruption of Mount Pinatubo (Philippines), inject huge amounts of sulfur dioxide gas into the stratosphere, where it combines with water to form an aerosol (mist) of sulfuric acid. By reflecting solar radiation, such aerosols can lower the Earth's average surface temperature for extended periods of time by several degrees Fahrenheit. These sulfuric acid aerosols also contribute to the destruction of the ozone layer by altering chlorine and nitrogen compounds in the upper atmosphere.

As observed after several eruptions, including Agung in 1963 (Indonesia) and El Chichon in 1982 (Mexico), stratospheric warming and lower tropospheric and surface cooling have been documented after the Pinatubo eruption. Labitzke and McCormick (1992) show that warming in the lower stratosphere (16 to 24 kilometers or 30 to 100 mbar) of up to 2 to 3 degrees C occurred within 4 to 5 months of the eruption between the equator and 20degreesNorth latitude, and it was also later noticed in middle northern latitudes (Angell, 1993). The warming distribution closely mirrored the dispersal pattern of the aerosol cloud; this mirroring strongly suggests that the warming was due to absorption of radiation by the aerosols. The warming was more intense in southern temperate-polar latitudes, perhaps due to the presence of aerosols from the Mount Hudson (Chile) eruption. Such temperature changes can influence stratospheric dynamics (Pitari, 1992). Since the peak of stratospheric warming in late 1991, temperatures in the 18- to 24-kilometer region have cooled considerably, passing the average in early 1993; temperatures in 1993 were the coldest ever recorded (Christy and Drouilhet, 1994; Monastersky, 1994) and may be related to ozone destruction in the lower stratosphere. Stratospheric temperatures also plummeted and stayed cooler than average for 7 years after the El Chichon eruption.

Several experiments have measured the radiative climate forcing of the Pinatubo aerosols. The NASA Earth Radiation Budget Experiment (ERBE) recently provided the first unambiguous direct measurement of the climate forcing on a large scale in both hemispheres (Minnis and others, 1993), an average radiative cooling of 2.7 W/m² by August 1991. Direct solar beam reductions of 25 to 30 percent were measured at widely distributed stations by Dutton and Christy (1992), while Stowe and others (1992) showed from AVHRR-derived optical depth measurements that the globally averaged net radiation at the top of the atmosphere may have decreased by about 2.5 W/m² in late 1991. These values translate into a global cooling of at least 0.5 to 0.7 degrees C, as seen in the global and Northern Hemisphere temperature records by September 1992 (Dutton and Christy, 1992). A net cooling effect of approximately 0.3 degrees C was estimated as a result of the El Chichon aerosol (Angell and Korshover, 1983; Handler, 1989), but the overall potential cooling caused by the El Chichon cloud was moderated by warming assiciated with El-Nino-Southern Oscillation (Angell, 1988, 1990). Pinatubo had a much larger radiative influence than El Chichon in the Southern Hemisphere (Dutton and Christy, 1992). Pinatubo's cloud caused about 1.7 times the global radiative forcing of El Chichon, making the estimated cooling of 0.5 degrees C a more robust figure.

One possible opposite effect, leading to surface warming, may have been caused by stratospheric to tropospheric transport of aerosols, due to aerosol-induced changes in atmospheric dynamics, and in a theoretical study Jensen and Toon (1992) suggest that this process may cause higher than usual amounts of cirrus clouds in the upper troposphere. Warmer than average winters and cooler than average summers over continental Northern Hemisphere areas have been documented and modeled after several eruptions, including Pinatubo, and this appears to be part of the normal Northern Hemisphere response after volcanic aerosol events (Groisman, 1992; Robock and Liu, 1994).

Sulfur dioxide gas, continuously emitted during Kilauea's current long-lived eruption, has resulted in persistent volcanic smog ("vog") in downwind areas. Vog, as well as acid rain, forms when sulfur dioxide reacts with atmospheric moisture. Vog aggravates respiratory problems, and acid rain damages crops and corrodes metal. On the Island of Hawai'i, the drinking water of many homes with rain-water catchment systems has been contaminated by lead leached by acid rain from roofing and plumbing materials.

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the surface through small openings called fumaroles. Ninety percent of all gas emitted by volcanoes is water vapor (steam), most of which is heated ground water (underground water from rain fall and streams). Other common volcanic gases are carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulfur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be adsorbed onto volcanic ash particles that later fall to the ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and also contaminate domestic water supplies.

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