The total number of active volcanoes in the world is 455, with an estimated 80 or more being submarine. The greatest concentration is in Indonesia, where 77 of its 167 volcanoes have erupted within historic times.
Throughout history our planet has occasionally been witness to tremendous volcanic eruptions. For instance, on June 8, 1783, there was the giant eruption of Laki. Thanks to this Icelandic volcano, Benjamin Franklin may have been the first scientist to recognize the effect of volcanic eruptions on the climate. While serving as ambassador to France, Franklin noticed a “. . . constant. . . universal fog” that had drifted across Europe and into parts of Asia following the Laki eruption. The winter of 1783-84 was unusually severe in Europe. In a paper read before the literary and Philosophical Society of Manchester on December 22, 1784, Franklin proposed that a connection existed between the fog over Europe, the cold winter, and the volcanic eruption in Iceland the previous year.
On April 7, 1815, the titanic eruption of Mount Tambora, on the island of Sumbawa in Indonesia emitted a cloud of dust and sulfuric acid which some believe gave rise to the famous “year without a summer” in the United States the following year. The amount of material blasted into the stratosphere was 100 times greater than that of Mount St. Helens. It was, in fact, the greatest amount of ejecta released into our atmosphere in more than 3,000 years.
Then came the eruption of Tambora’s neighbor, Krakatau, August 27,1883, an explosion so great that rocks were thrown to a height of 34 miles, and 10 days later dust fell at a distance of 3,313 miles. It was an explosion that has been estimated to have had about 26 times the power of the greatest H-bomb test.
More recently have been the gigantic eruptions of Guatemala’s Santa Maria on April 24, 1902, Gunung Agung on Bali in 1963, EI Chichón in Mexico in 1982, and the Philippines’ Mount Pinatubo in 1991. These eruptions all managed to drive into the stratosphere, long-lived ash veils of noteworthy significance to world weather; producing vivid, fiery sunsets; weakening the Sun’s direct rays by anywhere from 5 to 20 percent; and chilling the global-scale average temperature by a few fractions of a degree.
THE GREENHOUSE “DEFECT”?
We know that volcanic veils like Agung’s, EI Chichón’s, and Mount Pinatubo’s took something away from the warmth of the Sun’s rays and tended to cool the lower levels of the atmosphere. We can also draw on the theory that tells us to expect such effects when unusually large numbers of very small particles of any origin, of any kind, are present in the upper atmosphere. Sulfur dioxide and sulfuric acid droplets are especially long-lived in the stratosphere. Because each volcanic veil survives only a year or two in the atmosphere, the cooling effect tends to be a concentrated in the first year or two following the eruption that produced it. But some slight cooling may persist several years after that because the oceans are slow to respond to the cooling effect, and equally slow to recover from it.
The 15 to 20 million tons of sulfur dioxide ejected into the atmosphere by the massive eruption of Mount Pinatubo on June 15, 1991, promptly reversed the trend of global warming, considered to be a possible sign of the greenhouse effect. Mean global temperatures were depressed by 1/2 to 1º C for the next several years. (One scientist has speculated that EI Niño is actually an oceanic response to pollutants emitted by large volcanic eruptions, such as Mount Pinatubo, thus hinting at a connection between the two phenomena.)