Volcanoes and Climate Change

Mount Tambora dominates the small Indonesian island of Sumbawa in the Flores Sea east of Java. In 1815 it exploded in the largest eruption in the last 20,000 years, ejecting between 125 and 175 cubic kilo meters of pumice (a type of rock formed by volcanoes) and ash. The year after the eruption, 1816, would become known as the "year without a summer." Climate is influenced by many processes operating simultaneously, so the precise effect of Tambora is not easily gauged. The eruption was coincident with a span of several decades, from about 1790 to 1830, of colder climate apparently caused by a considerable decrease in solar activity. The years 1812 to 1818 were among the coldest of these, and this may have been primarily a result of the change in solar activity. It is also possible that the other large eruptions that occurred in that decade-Soufriere Hills (Montserrat, 1812), Mayon (Philippines, 1814), Colima (Mexico, 1814), and Beerenberg (North Atlantic, 1818)-contributed to the cold conditions. Sparseness of climate records adds to this uncertainty. Much of our knowledge comes from tree rings and other natural recorders of time and climate.

How, exactly, do volcanic eruptions modify climate? Until recently, it was thought that dust in the form of particles of volcanic ash simply blocked incoming sunlight, and thus that the explosiveness of an eruption and the amount of solid material blown into the atmosphere were important parameters. We now know, however, that the effects of dust are negligible because it simply does not remain in the atmosphere long enough to block sunlight. Far more important is the amount of sulfur dioxide injected into the stratosphere, the base of which is about 10 to 15 kilometers high. In the atmosphere, sulfur dioxide reacts rapidly with water vapor to form sulfuric acid. Sulfuric acid forms an aerosol, a cloud of submicroscopic droplets less than a ten-thousandth of a millimeter in diameter-small enough that the droplets do not rapidly settle. In contrast, in the troposphere, which extends from the stratosphere to Earth's surface, precipitation rapidly washes out sulfuric acid and other pollutants. (This cannot happen in the stratosphere because it is above the clouds.) The aerosol in the stratosphere absorbs some solar radiation, thereby causing cooling of the troposphere. Fortunately for life, the aerosol remains in the stratosphere for only a few years, so the effect of cataclysmic eruptions on climate is temporary.

The 1815 Tambora magma (liquefied rock found under the surface of Earth) was unusually rich in sulfur, and the eruption injected about 85 million tons of sulfur dioxide into the atmosphere. This is less than the current annual anthropogenic (created by humans) production of 130 million tons. However, anthropogenic sulfur dioxide has little influence on climate because it is confined to the troposphere and is rapidly removed as acid rain. Other eruptions that injected large amounts of sulfur into the stratosphere were Krakatau (Indonesia, 1883), Gunung Agung (Indonesia, 1963), El Chichon (Mexico, 1982), and Mount Pinatubo (Philippines, 1991). All were followed by several years of unusually cool climate.

Sulfuric acid aerosols are also a major factor in the destruction of the ozone layer, which occupies the lower part of the stratosphere. For example, the Pinatubo eruption is estimated to have caused a 15 to 25 percent decrease in ozone in the high latitudes. Exactly how the aerosol affects ozone is not precisely known. One possibility is that the droplets provide sites for the breakdown of otherwise inert chlorine compounds and in that way enhance the formation of atomic chlorine, which is known to destroy ozone.

The Tambora magma was also rich in chlorine. The fate of chlorine in the atmosphere is not well understood. Hydrochloric acid, which has hydrogen and chlorine atoms, is soluble (easily dissolved) in water, absorbs ash particles, and thus probably is removed from the atmosphere as water or ash rain. Even if hydrochloric acid does reach the stratosphere, its effect on ozone is unclear because hydrochloric acid itself does not react with ozone. Anthropogenic chlorofluorocarbons reside in the stratosphere for long periods of time, however, and the atomic chlorine formed as chlorofluorocarbons break down clearly does destroy ozone.