The spreading of suspended particles of ash in the atmosphere, particularly if they are rich in sulfur, can actually create a protective haze in the high atmosphere. The sulfur particles mix with water vapour to form clouds with sulfuric acid droplets. These are said to stay aloft for years and contribute to the cooling of the atmosphere by absorbing solar radiation and reflecting it back into space. This may give scientists and governments a little more time to find ways to reduce the harmful impact of increasing carbon dioxide emissions from human activity. Even as we struggle to find ways to reduce these emissions, scientists are starting to turn to "geoengineering" looking for ways to shield the earth from the harmful rays of the sun.
While volcanoes may damage the ozone layer, it's also very clear that certain significant eruptions in the past have contributed to recorded periods of cooler weather.
The 1991 eruptions of Mount Pinatubo in the Philippines and Mount Hudson in Chile released a gigantic sulfuric oxide cloud that scientists say caused the reduction of global mean temperatures by as much as one degree Celsius for the following two years. It may not sound like much, but such a reduction is significant. To appreciate the change, let's look at the opposite scenario: think of what would happen if the temperature were to swing the other way. An increase of the global mean temperature by one degree takes us to the edge of fateful precipice: many climatologists say an increase of just two degrees would spark an unstoppable chain reaction of global warming that would increase the amount of methane in the atmosphere, choke our food supply and rapidly doom our planet. So, in contrast, a reduction of global temperatures by one degree is a big deal.
A similar global cooling phenomenon was recorded after the 1883 eruption of Krakatoa (Krakatau) in Indonesia when the world experienced very cool weather and dramatic sunsets.
The image shows the smoke plume from the Mt. Etna volcano, in Italy.
For information on climatic effects of volcanic activity, including the two eruptions cited above, click here.
For a primer on climate engineering, click here.
The story of the Krakatoa eruption is here.