Volcanic Eruptions (Indonesia) #1

Overview of Volcanic Eruptions and its Climatic Effects

The Ozone Effect:


Intense sunlight in the stratosphere (above 12 km) produces bluish colored ozone (O3) by naturally breaking down normal oxygen molecules (O2) into two highly reactive oxygen atoms (O). Each oxygen atom then quickly bonds with an oxygen molecule to form ozone. Ozone absorbs UV radiation, and in the process ozone is changed back into an oxygen molecule and an oxygen atom. A balance exists in ozone destruction and production, so that an equilibrium concentration exists in the stratosphere.

This equilibrium has probably existed throughout much of geologic time. Recently, however, an ozone hole has been detected in the stratosphere over Antarctica, presumably due to the atmospheric build up of ozone-destroying CFCs by humans. Ozone depletion has resulted in a greater penetration of ultraviolet radiation on the earth's surface, which is harmful to life on earth because it damages cellular DNA. The ozone effect does not appear to have a direct influence on global temperatures.

The Greenhouse Effect:


Certain gases, called greenhouse gases (primarily carbon dioxide and water vapor; but also methane, N2O, and CFCs), allow short wavelength radiation from the sun (UV and visible light) to penetrate through the lower atmosphere to the earth's surface. These same gases, however, absorb long wavelength radiation (infrared), which is the energy the earth reradiates back into space. The trapping of this infrared heat energy by these greenhouse gases results in global warming. Global warming has been evident since the beginning of the Industrial Revolution. Most scientists attribute global warming to the release of greenhouse gases through the burning of fossil fuels.

The Haze Effect:
Suspended particles, such as dust and ash, can block out the earth's sunlight, thus reducing solar radiation and lowering mean global temperatures. The haze effect often generates exceptionally red sunsets due to the scattering of red wavelengths by submicron-size particles in the stratosphere and upper troposphere.

Influence of Volcanic Eruptions

On The Ozone Effect:
The halide acid HCl has been shown to be effective in destroying ozone; however, the latest studies show that most volcanic HCl is confined to the troposphere (below the stratosphere), where it is washed out by rain. Thus, it never has the opportunity to react with ozone. On the other hand, satellite data after the 1991 eruptions of Mt.Pinatubo (the Philippines) and Mt. Hudson (Chile) showed a 15-20% ozone loss at high latitudes, and a greater than 50% loss over the Antarctic! Thus, it appears that volcanic eruptions can play a significant role in reducing ozone levels. However, it is an indirect role, which cannot be directly attributed to volcanic HCl. Eruption-generated particles, or aerosols, appear to provide surfaces upon which chemical reactions take place. The particles themselves do not contribute to ozone destruction, but they interact with chlorine- and bromine-bearing compounds from human-made CFCs. Fortunately, volcanic particles will settle out of the stratosphere in two or three years, so that the effects of volcanic eruptions on ozone depletion are short lived. Although volcanic aerosols provide a catalyst for ozone depletion, the real culprits in destroying ozone are human-generated CFCs. Scientists expect the ozone layer to recover due to restrictions on CFCs and other ozone-depleting chemicals by the United Nations Montreal Protocol on Substances that Deplete the Ozone Layer. However, future volcanic eruptions will cause fluctuations in the recovery process.

On The Greenhouse Effect:
Volcanic eruptions can enhance global warming by adding CO2 to the atmosphere. However, a far greater amount of CO2 is contributed to the atmosphere by human activities each year than by volcanic eruptions. T.M.Gerlach (1991, American Geophysical Union) notes that human-made CO2 exceeds the estimated global release of CO2 from volcanoes by at least 150 times. The small amount of global warming caused by eruption-generated greenhouse gases is offset by the far greater amount of global cooling caused by eruption-generated particles in the stratosphere (the haze effect). Greenhouse warming of the earth has been particularly evident since 1980. Without the cooling influence of such eruptions as El Chichon (1982) and Mt. Pinatubo (1991), described below, greenhouse warming would have been more pronounced.

On The Haze Effect:
Volcanic eruptions enhance the haze effect to a greater extent than the greenhouse effect, and thus they can lower mean global temperatures. It was thought for many years that the greatest volcanic contribution of the haze effect was from the suspended ash particles in the upper atmosphere that would block out solar radiation. However, these ideas changed in the 1982 after the eruption of the Mexican volcano, El Chichon. Although the 1980 eruption of Mt. St. Helens lowered global temperatures by 0.1OC, the much smaller eruption of El Chichon lowered global temperatures three to five times as much. Although the Mt. St. Helens blast emitted a greater amount of ash in the stratosphere, the El Chichon eruption emitted a much greater volume of sulfur-rich gases (40x more). It appears that the volume of pyroclastic debris emitted during a blast is not the best criteria to measure its effects on the atmosphere. The amount of sulfur-rich gases appears to be more important. Sulfur combines with water vapor in the stratosphere to form dense clouds of tiny sulfuric acid droplets. These droplets take several years to settle out and they are capable to decreasing the troposphere temperatures because they absorb solar radiation and scatter it back to space.