Volcanoes and Climate, Past and Present
Have you ever wondered why some winters, or even some summers, are exceptionally cold? The fact is that most of this strange weather is due to natural climate variability. On occasion, though, a volcano explodes and, to our consternation, the weather becomes even chillier than during the normal downturns. Remember the huge Mount Pinatubo blowup in the Philippines in 1991? Recall the cold weather in much of North America during the following year?
In powerful eruptions like that of Mount Pinatubo, ash particles and sulfur gases are violently discharged by the volcano into the stratosphere, the region of the atmosphere lying above the layers where our familiar weather takes place. Because of their weight, the ash particles settle down to the ground in a matter of days, but the sulfur gases combine with traces of water vapor to form tiny sulfuric acid aerosols. These droplets are so small that many of them can stay aloft for several years. As efficient reflectors of sunlight, they screen the ground from some of the energy that it would ordinarily receive from the sun. Although the winds in the stratosphere carry the aerosols rapidly around the globe in either an easterly or westerly direction, the movement of aerosols north and south is always much slower. In the case of an eruption at high latitudes, most of the aerosols simply remain confined to a wide zone surrounding the polar cap for several years.
Volcanoes erupt in one of two basic styles: lava outflows from fissures in the ground and ash explosions from vents on top of volcanic mountains. During the past eleven centuries, the world has seen two enormous fissure eruptions: the Eldgjá (Fire Chasm) eruption in 934 and the Laki, or Skaftareldar (Skaftar Fires), eruption in 1783, both in southern Iceland. As the photograph shows, their ancient lava flows appear intermingled and moss-covered today.
Such older eruptions have been investigated by using various research techniques. Scientists measure the visible lava and ash exposures in the field. Sulfuric acid fallout over polar areas can be detected in deeply drilled ice cores having clearly marked annual layers of ice. The anomalous cooling appears in stunted annual growth rings of north temperate trees and, much more importantly, in contemporary reports compiled by competent witnesses. For the oldest historic eruptions, however, these compilers may not be scientifically trained or even first-hand witnesses.
Using published historical documents, Richard Stothers at GISS has traced the climatic and demographic consequences of the great Eldgjá and Laki eruptions. In both eruptions, the cloud of aerosols from the eruption traversed northern Europe on the prevailing westerly winds, and dimmed and reddened the sun. This continued for months. King Henry of Saxony noted the ominous presence of the thick dry fog in 934, and Benjamin Franklin observed it scientifically in France in 1783. Both eruptions were followed by a very cold winter, poor harvests the next summer, severe famine, and a widespread disease epidemic. Conditions remained bad for 5 to 8 years after Eldgjá, but only for 2 to 3 years after Laki. The Eldgjá eruption, though slightly smaller than Laki, injected far more aerosols into the middle stratosphere, where they persisted for a longer time than did Laki's aerosols at lower layers of the atmosphere.
The climatic aftereffects of the largest historic eruptions are very important for verifying our knowledge of the aftereffects of the smaller, though better documented, modern eruptions. While the older data may be poorer in quality and quantity, the old climatic signals were bigger, and therefore more easily detectable, because the eruptions were larger. The eruption of Mount Pinatubo in 1991 may have looked impressive to us, but, by historic standards, it was just a little puff.
Stothers, R.B. 1996. The Great Dry Fog of 1783. Climatic Change 32, 79-89.
Stothers, R.B. 1998. Far reach of the tenth century Eldgjá eruption, Iceland. Climatic Change 39, 715-726.