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Name: Lauren
Status: Other 
Grade: Other
Location: OH
Country: United States
Date: Spring 2010

Sometimes they say that volcanoes can cause Earth's temperature to cool because of the sulfur particles in the atmosphere. other times I hear that they cause warming because of the build up of CO2. For instance, the theory for initial cause of the Permian Extinction, was that the eruptions in Russia (basalt?) caused the earth to heat up due to the increase in CO2. However, recently in Iceland the same sort of eruptions, to a lesser degree, occurred and resulted in cooling.

I can understand that shield volcanoes do not decrease temperature since they are typically not very explosive. But otherwise, it seems sometimes the same type of volcanic eruptions are credited with warming and sometimes with cooling the earth. Could you clarify this for me?

Certainly. The effects are basically as you state, for the reasons that you state. They act on different time scales.

Sulfates and other suspended aerosols stay in the stratosphere, blocking incoming solar radiation, for a year or so. Even though the stratosphere is very stable compared to the troposphere (the atmospheric layer in which we live), the suspended particles do eventually settle out on the time scale of a year or so.

Currently, carbon dioxide has a residence time in the atmosphere of about 100 years. So let us say that we have major volcanic eruptions every five years or so. So every five years there will be a year of cooling due to volcanic aerosols, which soon dissipate. Over time, though, carbon dioxide accumulates, making a long-term warming trend.

The Permian extinctions may be related to an upsurge in volcanism. There were also Precambrian episodes of extreme global cooling ("Snowball Earth") that are thought to have been ended only by radiation trapping by accumulated volcanic carbon dioxide. In the "Snowball Earth" case, the residence time of CO2 was much longer than it is not, because the icy conditions were not very conducive to carbon dioxide-consuming photosynthesizers. So, over the eons, the carbon dioxide could accumulate, and accumulate, until it tipped Earth's the climate regime. Any temporary effect of volcanic aerosols in the long run was overwhelmed by the greenhouse accumulation.

So, in summary, the issue is one of time scale. The different processes operating on Earth, such as photosynthesis, biomass burning, fossil carbon release, carbonate weathering, and so on affect the time scale of the different processes, and they can also "feed back" in complex ways.

Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming

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