Volcanoes

Volcanic eruptions always have a significant effect on the local environment, but three factors determine whether an eruption will have an impact on global climate.

The direction and force of the eruption determines where the debris from the volcano winds up. Unless the gas and dust ends up in the stratosphere - the upper layers of the atmosphere that can convey pollutants around the globe - rain and snow will wash it out of the lower levels of the atmosphere.

In 1980, for example, Mount St. Helen's erupted, spewing a billion cubic meters of ash into the sky over Washington state. But the force of the explosion was largely horizontal, so the eruption had no overall global impact.

The weather up there in the stratosphere is also important. Without winds to carry the volcanic debris, they will remain and settle locally.

Perhaps the most important factor, however, is how much sulfur is in the volcanic plume. Sulfur compounds in the atmosphere block out the light of the sun, cooling the Earth slightly. If these sulfur compounds have reached the stratosphere and have been carried around the globe by winds, an eruption can mean violent and abrupt changes in the weather worldwide.

In 1991, Mount Pinatubo in the Philippines produced ten times as much ash as Mount St. Helens and released more than 25 million tons of sulfur dioxide into the stratosphere. The resulting cloud - which formed a wide band around the planet within about a month - resulted in an overall cooling of the global surface temperature by about 1 degree Fahrenheit. It doesn't sound like much, but according to NASA scientists, even 1 degree can have a significant impact on worldwide weather patterns. In fact, data later revealed that winter weather patterns lasted up to two weeks later the winter of 1992-1993. A change in average global temperature of 1 or 2 degrees F. resulted in the Little Ice Age. For comparison, the Great Ice Age that once gripped the Earth about a million years ago had an average temperature just 9 degrees F. lower than today's.
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