Spectacular images of volcanic eruptions have recently appeared in the media, including an eruption of the Taal volcano in the Philippines, which began January 12.
At any given time of the year, there are about 50 active volcanoes across the globe, but rarely does one affect global weather and climate.
Much of the ash from volcanic eruptions like Taal and Mount St. Helens in Washington in 1980 reach only into the troposphere, which extends about 6 miles from the Earth’s surface. There it can stay airborne for no more than a week due to precipitation processes, wind and gravity. The amount of solar energy reaching the surface is reduced by the ash plume, just like a cloud would do, resulting in cooler temperatures in the vicinity of the volcano. The resulting daytime high temperatures can be reduced by several degrees when compared with nearby areas unaffected by the ash.
To have a global impact, the volcano must eject debris into the stratosphere — the layer of the atmosphere that extends approximately 6 to 30 miles above the Earth’s surface. There it can last for a couple of years and be spread over the entire globe.
If the eruption also emits large amounts of sulfur dioxide, chemical reactions in the atmosphere can form tiny sulfuric acid droplets. These droplets, particularly if formed in the stratosphere, can reside for a few years and cause a cooling of the global surface temperatures because they reflect solar energy back to space.
This was the case with Mount Pinatubo in the Philippines in 1991. The
Pinatubo eruption spewed sulfur into the stratosphere, which during the
next 15 months decreased the average global temperature by about 1
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month. Send them your questions at email@example.com or firstname.lastname@example.org.