eruptions always have a significant effect on the local environment,
but three factors determine whether an eruption will have
an impact on global climate.
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.
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.
weather up there in the stratosphere is also important. Without
winds to carry the volcanic debris, they will remain and settle
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.
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 in the winter of 1992-1993.
A change in average global temperature of 1 or 2 degrees Fahrenheit
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 Fahrenheit
lower than today's.
on a thumbnail picture to learn about another
great moment in global climate change: