Volcanoes are neither the cause nor the solution to the current global warming crisis...
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Aerosols produced red sunsets and sunrises for astronauts aboard the
Space Shuttle for up one year after the 1991 eruption of Mt. Pinatubo.
Symonds, Rose, Bluth, and Gerlach concluded that stratospheric injection of sulfur dioxide (SO2) is the principal atmospheric and global impact of volcanic eruptions via
SO2 + OH + 3H2O -> H2SO4 (l) + HO2
The SO2 converts to sulfuric acid aerosols that block incoming solar radiation and contribute to ozone destruction. The blocked solar radiation can cause global cooling.
The amount of SO2 released by volcanoes is much less compared to man-made sources but the impact of some eruptions might be disproportionately large. The gases emitted by most eruptions and by man-made sources never leave the troposphere, the layer in the atmosphere from the surface to about 10 km.
However, volcanic gases reach the stratosphere, a layer in the atmosphere from about 10 km to about 50 km in altitude, during large eruptions. This relationship is complicated by the fact that the elevation between the volcano summit and the distance to the troposphere/stratosphere decreases with latitude. So, some smaller eruptions at higher latitudes can eject as much SO2 gas into the stratosphere as larger eruptions closer to the equator.
Factors influencing the amounts of SO2 in the stratosphere were described and modeled by Bluth and others (1997). For eruptions in the last 25 years, El Chichon and Mount Pinatubo emitted the greatest amounts of SO2 into the stratosphere. El Chichon produced 7 Mt of SO2 and Mount Pinatubo produced 20 Mt. Both of these volcanoes are at low latitudes but they both had high eruption rates.
The importance of latitude is obvious for four of the next five volcanoes that had a major influence on SO2 amounts in the stratosphere. Hudson, St. Helens, Alaid, and Redoubt are all at latitudes greater than 45 degrees, where the distance to the stratosphere is less. The eruption rate of Hudson was comparable to El Chichon and Mount Pinatubo. However, the eruption rates of St. Helens, Alaid, and Redoubt where an order of magnitude less. These volcanoes emitted 1, 1.1, and 0.2 Mt of SO2. The other eruption was at Ruiz, which had a high eruption rate, comparable to El Chichon and Mount Pinatubo, but is near the equator. Ruiz emitted 0.7 Mt of SO2. Bluth and others (1997) used the changes in aerosol optical depth as a measure of the impact of the eruptions.
The impact of eruptions may not last very long. The aerosols in the stratosphere from mid-range eruptions (St. Helens, Alaid) settled back to the troposphere in about 5-8 months (Kent and McCormick, 1984). For large eruptions like El Chichon it takes about 12 months for SO2 levels in the stratosphere to return to pre-eruption levels. Pinto and others (1989) suggested that at high eruption rates aerosols tend to make larger particles, not greater numbers of same size aerosol particles. Larger particles have smaller optical depth per unit mass, relative to smaller particles, and settle out of the stratosphere faster. These self-limiting effects may restrict the total number of particles in the stratosphere and may moderate the impact of volcanic clouds (Rampino and Self, 1982; Pinto and others,1989).
More complicated patterns of warming and cooling have been found on regional scales. Robock and Mao (1992) found warming over Eurasia and North America and cooling over the Middle East and northern Africa during the winters after the 12 largest volcanic eruptions from 1883-1992. For eruptions in the tropics the temperature changes were noted in the first winter after the eruption. For eruptions in the mid-latitudes changes were observed in the first or second winter after the eruption. For eruptions in high latitudes changes were observed in the second winter after the eruption. Robock and Mao (1992) proposed that heating of the tropical stratosphere by the volcanic aerosols led to an enhanced zonal winds. The zonal winds heated some areas while blocking of solar radiation cooled other areas.
The 1982 eruption of El Chichón produced one of the largest sulfuric acid plumes this century. The eruption was immediately followed by an El Niño event in 1982-83, the largest El Niño of the century up to that time. Roback and others (1995) used three different models to see if the eruption volcanic eruptions might trigger or enhance the El Niño. One model, involving mid-tropospheric heating, did result in a weakening of the trade winds. This change was consistent with observed surface winds north of the Equator in the eastern Pacific Ocean. However, Roback and others (1995) noted that the 1982 El Niño event had started before this wind anomaly. Furthermore, they pointed out that only trade wind collapses in the western equatorial Pacific can initiate El Niños. They concluded that the El Chichón eruption and the large El Niño event were a coincidence.
Handler (1984) looked at all VEI = 4 or more historic eruptions to see if they were associated with El Nino events. He sorted the volcanoes by latitude. Low latitude (20 N to 20 S) eruptions are associated with an increase in sea surface temperatures in the eastern tropical Pacific Ocean (see above figure), the area where El Nino events begin. The warmer temperatures last up to three seasons after the eruptions. Handler called for further theoretical work on these observations.
Ozone is a gas made of three oxygen atoms. Ozone is bluish in color and harmful to breathe. Most of the Earth's ozone (about 90%) is in the stratosphere. The stratosphere is a layer in the atmosphere from about 10km to about 50km in altitude. Ozone is important because it absorbs specific wavelengths of ultraviolet radiation that are particularly harmful to living organisms. The ozone layer prevents most of this harmful radiation from reaching the ground.
As concern grew over depletion of ozone in the stratosphere scientists examined the role of volcanoes. They noted that the gases emitted by most eruptions never leave the troposphere, the layer in the atmosphere from the surface to about 10km.
Hydrogen chloride released by volcanoes can cause drastic reductions in ozone if concentrations reach high levels (about 15-20 ppb by volume)(Prather and others, 1984). As the El Chichon eruption cloud was spreading, the amount of HCl in the cloud increased by 40% (Mankin and Coffey, 1984). This increase represents about 10% of the global inventory of HCl in the stratosphere. Other large eruptions (Tambora, Krakatau, and Agung) may have released almost ten-times more HCl into the stratosphere than the amount of chlorine commonly present in the stratosphere (Pinto and others, 1989). At least two factors reduce the impact of HCl, chlorine appears to be preferentially released during low-levels of volcanic activity and thus may be limited to the troposphere, where it can be scrubbed by rain. Hydrogen chloride may also condense in the rising volcanic plume, again to be scrubbed out by rain or ice. Lack of HCl in ice cores with high amounts of H2SO4 (from large eruptions) may indicate ambient stratospheric conditions are extremely efficient at removing HCl. Thus, most HCl never has the opportunity to react with ozone. No increase in stratospheric chlorine was observed during the 1991 eruption of Mt. Pinatubo.
Volcanoes account for about 3% of chlorine in the stratosphere. Methyl chloride produces about 15% of the chlorine entering the stratosphere. The remaining 82% of stratospheric chlorine comes from man-made sources, mostly in the form of chlorofluorocarbons.
Although volcanic gases do not play a direct role in destroying ozone they may play a harmful indirect role. Scientists have found that particles, or aerosols, produced by major volcanic eruptions accelerate ozone destruction. The particles themselves do not directly destroy ozone but they do provide a surface upon which chemical reactions can take place. This enhances chlorine-driven ozone depletion. Fortunately, the effects from volcanoes are short lived and after two or three years, the volcanic particles settle out of the atmosphere.
Study of ozone amounts before and after the 1991 eruption of Mt. Pinatubo show that there were significant decreases in lower stratospheric ozone (Grant and others, 1994). The amount of ozone in the 16-28 km region was some reduced by 33% compared to pre-eruption amounts. A similar reduced amount of ozone was measured in the summer of 1992.