Geology 309 - Lecture 26


Volcanoes and Climate

Atmospheric gases and their effects on climate

Impact of some major historic eruptions

Volcanoes and Ozone Depletion

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. The troposphere can be scrubbed by rain. It is the gases (especially Cl) that reach stratosphere that are a problem; for example the eruption of Pinatubo in 1991. But, interestingly, there was no increase in stratospheric Cl from 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.

So volcanoes do not play a big direct role in ozone depletion. However, they may play a harmful indirect role. Scientists have found that sulfate 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 aerosol 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. 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.

How do we go backwards to affects of volcanoes?

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