(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Twenty years ago, Mount Pinatubo awakened from a long sleep to produce Earth’s second largest eruption in more than half a century. Volcanologists have learned a lot about deep eruptive processes and the hazards of large explosive events by studying the Pinatubo eruption.

The climactic event of June 15, 1991, produced the largest sulfur dioxide (SO2) eruption cloud ever measured — at least 17 million tonnes. This aspect of Pinatubo’s 1991 activity riveted the attention of climatologists and atmospheric scientists around the world, who estimated that the event caused a global temperature decrease of about 0.5 degree Celsius (0.9 degree Fahrenheit) over the ensuing year.

Scientists have long known that the vigorous injection of gas and ash into the stratosphere can cause changes in global weather patterns and climate by reflecting sunlight away from Earth and heating the upper atmosphere.

Diplomat-scientist Benjamin Franklin speculated about these effects following the 1783 Laki eruption in Iceland, which caused “a constant fog over all Europe, and a great part of North America” for several months.

After the largest observed eruption in recorded history — the 1815 eruption of Indonesia’s Tambora volcano — New England and parts of Europe experienced a “year without a summer” as ash circled the globe in 1816.

The Pinatubo eruption, along with those of Mount St. Helens in 1980 and El Chichon in 1982, provided opportunities for climatologists and atmospheric scientists to test, revise, and improve climate models.

By using the live strong signals produced by well-monitored events at different places on the globe, the scientists have been able to interactively test their ability to “predict” climate-forcing results that might occur for these eruptions and then adjust the models to what actually happened.

The improved climate models have, in turn, helped improve our understanding of the role of human-generated pollution, including greenhouse gases and ozone, in the lower and upper atmosphere. These results have helped steer policy decisions regarding pollution regulation and strategies used to reduce emissions.

The predictive ability of these models makes them an appealing tool for use in proposing “what if” scenarios. For instance, climate scientists have been quick to use the Pinatubo eruption analogy to propose potential “geoengineering” solutions to global warming.

Geoengineering can generally be defined as deliberately modifying the environment, including the atmosphere, for the intended purpose of counteracting global warming caused by the buildup of greenhouse gases.

One such solution proposes using jet aircraft to inject millions of tonnes of either sulfur dioxide or tiny sulfuric acid aerosol droplets directly into the stratosphere. The gas or aerosol would theoretically act as Pinatubo’s eruptive plume did, scattering sunlight energy while producing minor stratospheric warming.

An alternative proposal to block infrared radiation and cool the planet advocates the use of engineered particles. These nanoparticles, like the sulfur dioxide or sulfuric acid aerosol droplets discussed earlier, would need to be “flown” up to the upper atmosphere and released.

Geoengineering the atmosphere carries complications beyond simply getting particles into the stratosphere. As the saying goes, “What goes up must come down.”

Adding millions of tonnes of corrosive sulfur dioxide or sulfuric acid aerosol particles to the upper atmosphere can be expected to produce unanticipated, as well as modeled, results.

In the case of engineered nanoparticles, the suggested chemical composition of these particles incorporates compounds of aluminum, barium and titanium. The environmental effects from these particles when they eventually fall back to Earth is unknown.

The environmental implications of geoengineering intervention on Earth’s climate are likely to be global, but not necessarily uniform. As in the case of Pinatubo, light-scattering particles injected into the stratosphere by humans won’t recognize or heed international borders. A temperature decrease in one area may well bring about an increase elsewhere, or a modification to critical rainfall patterns.

Participants in the 2010 Asilomar International Conference on Climate Intervention Technologies recommended that the principles of climate intervention research should promote the collective benefit for humankind and the environment.

Unlike volcanic eruptions, during which almost nothing can be done about the hazards themselves, humans who consider climate intervention have both the capability and the responsibility of carrying out measured action to prevent unintended consequences.