Do the eruptions of volcanoes, like Mt. St. Helens in Washington State sixteen years ago, generally make the Earth warmer or cooler?

The catastrophic eruption of Mt. St. Helens in May of 1980, ranks among the most important natural events of the twentieth century in the United States. Because Mt. St. Helens is in a remote area of the Cascades Mountains, only a few people were killed by the eruption, but property damage and destruction totaled in the billions of dollars. The height of Mt. St. Helens was reduced from about 2950 m (9677 ft) to about 2550 m (8364 ft) as a result of the explosive eruption on the morning of May 18. The eruption sent a column of dust and ash upwards more than 25 km into the atmosphere, and shock waves from the blast knocked down almost every tree within 10 km of the central crater. Massive avalanches and mudflows, generated by the near-instantaneous melting of deep snowpacks on the flanks of the mountain, devastated an area more than 20 km to the north and east of the former summit, and rivers choked with all sorts of debris were flooded more than 100 km away. The area of almost total destruction was about 600 sq. km. Ash from the eruption cloud was rapidly blown to the northeast and east producing lightning which started many small forest fires. An erie darkness caused by the cloud enveloped the landscape more than 200 km from the blast area, and ash could be seen falling from the sky over the Great Plains, more than 1500 km distant.

Despite the havoc caused by the eruption of Mt. St. Helens, temperatures around the world or even across the U.S. did not noticeably change. In fact, most volcanic eruptions are not weather makers. It's only the major eruptions that have the potential to change the weather and climate. Mt. St. Helens was big news to us here in the states, but worldwide, it was considered to be only about a one in seven year event. Perhaps eight to ten times a century, an eruption is powerful enough to affect the climate. For this to happen the ash, dust and gases ejected by the volcano, must be shot-up into the stratosphere. Once there, they can then be escorted around the globe by the strong winds characteristic of this level of the Earth's atmosphere.

There are several different types of volcanic eruptions. Most do not erupt explosively. Hawaiian-type volcanoes, such as Mauna Loa, produce large out-pourings of balsaltic lava in which the gases are usually liberated without explosive force. Strombolian volcanoes (named after Mt. Stromboli on the island of Sicily in the Mediterranean Sea) consist of more-or-less regular explosions of moderate intensity that throw out an incandescent lava. Vulcanian-type volcanoes, of which Mt. Vesuvius in Italy is an example, are more explosive than Strombolian volcanoes and their lava flows are also more viscous (thick). The eruption cloud of this type volcano can produce an abundance of ash. Pelean-type volcanoes (named after Mt. Pelee on the island of Martinique in the Carribean Sea) are the most explosive. With this type of volcano, the magma is expelled in a gaseous form and there are no lava flows. The eruption is violent and often is associated with the final phase of activity - a final fling before the volcano becomes extinct. This is the only type of volcano that can have global climate consequences. In the last 50 years or so, there have been about seven infamous eruptions of sufficient magnitude to produce a near-global veil of dust and ash. Only Agung in 1963, and Mt.Pinatubo in 1991, however, have resulted in cooler global temperatures. It's difficult to sort out natural variations in the global temperature record with contributions from significant events such as a major eruption. For instance, even after a devastating eruption, the global temperature may be warmer than the long-term average temperature, but it may be cooler than it would have been if no eruption had occurred. The so-called "year without a summer" is attributed to the eruption of Tambora in the East Indes in 1815. This was an epic eruption. In New England, for example, there were summer frosts and poor crops. However, this was not the coldest summer ever recorded in New England. Cooler temperatures have been observed without volcanic eruptions. Apparently, even the largest eruptions have cooled the Earth only by about a degree or two Celsius.

What is the mechanism wherby volcanic eruptions can make the Earth cooler? The volcanic clouds that reach into the stratosphere are composed of particulates and gases, including sulfur dioxide. Millions of tons of sulfur dioxide in the stratosphere can be readily converted to sulfuric acid particles (referred to as aerosols), which reflect energy coming from the Sun back to space. This solar energy is therefore not available to heat the Earth's surface and lower atmosphere. Aside from impacting the climate, stratospheric dust aerosols can create striking sunsets and sunrises. For more than a year following the eruption of El Chicon in Mexico in 1983 and Mt. Pinatubo in the Philippines five years ago, the skies at evening and dawn displayed sometimes brilliant colors. The volcanic dust is of the right size to scatter some sunlight towards the surface, and is high enough in the atmosphere to still be illuminated by sunlight for a period of time after the sun has set for Earth-bound observers.