Volcanism refers to the process where molten rock (magma), normally located below Earth's surface, and subsurface gases are transported to the surface during an eruption of a volcano.
A volcanic eruption can be violent, with gases and magma being expelled from the tip of the volcano (or from the side if portions of the volcano explode, as happened during the May 18, 1980, eruption of Mount St. Helens in Washington). The explosive force can propel material far into the atmosphere, which can alter the penetration of light to the surface locally or, in the case of a massive eruption, globally. For example, the June 1991 eruption of Mount Pinatubo in the Philippines produced a transient cooling of the atmosphere that offset the predicted course of global warming for several years.
Volcanism may affect climate by contributing to the destruction of ozone in the stratosphere. Declining atmospheric ozone, as exemplified by the “ozone hole” over the continent of Antarctica, is allowing more genetically destructive ultraviolet radiation to reach Earth's surface.
Historical Background and Scientific Foundations
Volcanoes have been a part of Earth's history from the beginning of the planet. Indeed, formation of new lands relies on the hardening of material extruded from volcanoes. If 2007 is typical of most years, then there are approximately 1,500 active volcanoes worldwide.
Volcanoes represent a pathway from the molten subsurface of Earth to the planet's surface. The subsur-face material is extruded fairly constantly from some volcanoes, and flows downhill from the volcano as a river of lava. In other volcanoes, eruptions occur less frequently, when the pressure inside the volcano builds to an extent that the molten magma and gases are propelled outward.
This latter type of eruption produces a cloudy looking plume that contains ash, gases, and subsurface air. The ash is too heavy to remain suspended in the air for long, and tends to settle back to the surface within several days. In contrast, the volcanic gases that are spewed out can remain aloft for years as an aerosol, and can spread via air currents.
The most abundant volcanic gases are water vapor, sulfur dioxide (SO2), and carbon dioxide (CO2). Sulfur dioxide is especially relevant to climate. The gas reacts with water vapor in the atmosphere to form sulfuric acid (H2SO4), which falls to the surface as acid rain.
Aerosols composed of sulfate reflect incoming sunlight back to space. The reduced amount of solar heat reaching Earth's surface produces cooler temperatures locally in the days following an eruption and, if the aerosol is dispersed more widely, can lower the average temperature of a much larger area.
Analysis of ice cores obtained from glaciers and the size of tree rings, which can provide climate data dating back hundreds of thousands of years, indicates that increased volcanic activity may have been an important factor in the cold spell from 1550 to 1850 that is known as the Little Ice Age.
Volcanism may also contribute to the depletion of atmospheric ozone. Experiments have shown that sulfuric acid can react with chlorofluorocarbons (CFCs) and convert the CFCs into forms that accelerate ozone destruction. It is not clear whether a similar reaction occurs in the stratosphere, but there is no reason why it should not.
The atmospheric spread of sulfur dioxide following major eruptions has been tracked by the National Aeronautics and Space Administration (NASA) using an orbiting instrument called the Total Ozone Mapping Spectrometer (TOMS). The atmospheric consequences of more than 100 major volcanic eruptions have been observed using TOMS. The studies have confirmed that the cooling of the atmosphere is short-lived, lasting several years at most. This transient cooling is termed radiative forcing.
Impacts and Issues
A volcanic eruption can release millions of tons of sulfur dioxide gas to the atmosphere. The particles are so small that they remain suspended as an aerosol.
The effect on climate can be dramatic. An example occurred in 1815, when the Tambora volcano erupted in Indonesia. The sulfur dioxide added to the atmosphere produced brilliant sunsets around the world for the next several years, and contributed to a cool summer along portions of the eastern United States and in Europe. During that summer, areas of New Hampshire experienced frost, which does not normally occur until mid-October.
WORDS TO KNOW
ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.
AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.
EXTRUDE: To push out or force out a plastic substance such as lava or mud.
ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.
RADIATIVE FORCING: A change in the balance between incoming solar radiation and outgoing infrared radiation. Without any radiative forcing, solar radiation coming to Earth would continue to be approximately equal to the infrared radiation emitted from Earth. The addition of greenhouse gases traps an increased fraction of the infrared radiation, reradiating it back toward the surface and creating a warming influence (i.e., positive radiative forcing because incoming solar radiation will exceed outgoing infrared radiation).
STRATOSPHERE: The region of Earth's atmosphere ranging between about 9 and 30 mi (15 and 50 km) above Earth's surface.
TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.
ULTRAVIOLET RADIATION: The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5% of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.
WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.
Another particularly notable example was the aerosol from the eruption of Mount Pinatubo in 1991. The eruption cloud that was 11.2 mi (18 km) wide and 18.6 mi (30 km) high sent approximately 20 million tons of sulfur dioxide and ash into the atmosphere. The aerosolized material stayed aloft through the following year and restricted the penetration of sunlight so much that the average global temperature in 1992 was the coolest in 30 years.
Although examples such as Mount Pinatubo show that volcanism can have a dramatic effect on the global atmosphere, the long-term significance to temperature is probably negligible. Indeed, the Intergovermental Panel on Climate Change (IPCC) concluded in its 2007 Assessment Report that human-related activities are very likely the prime influence to the warming of the atmosphere that has occurred since the mid-twentieth century, overriding the transient temperature decreases related to volcanic eruptions.
More importantly, volcanism contributes large quantities of sulfur and sulfur dioxide, and so is a factor in acid precipitation, which has destroyed millions of trees in northern latitudes of North America and Europe, and can hinder crop production. In poorer areas of the world, the hindered production of food in the years following a major eruption can be serious.
Scientists are modeling volcanic activity based on data obtained from TOMS and other measuring devices. Gaining a better understanding of the influences of volcanism on climate may help reduce the hardship that volcanism-related climate change has on the undeveloped and developed world.
Cowie, Jonathan. Climate Change: Biological and Human Aspects. Cambridge: Cambridge University Press, 2007.
Emanuel, Kerry. What We Know About Climate Change. Boston: MIT Press, 2007.
Tanaka, Shelley. Climate Change. Toronto, Canada: Groundwood Books, 2006.
Brian D. Hoyle
1. A theory of the 18th and 19th centuries, based initially on fieldwork in the French Auvergne and associated with Nicolas Desmarest and James Hall. They proposed that volcanic rocks were produced from molten material, and that volcanoes had formerly existed where basalts, etc. are now present.
2. See VOLCANICITY.