Pinatubo Climate Investigation
Volcanic eruptions, if large enough, can blast gas and dust into the Earth's lower stratosphere. The strong winds at these altitudes, about 15-25 km, rapidly blow the volcanic material around the world. The main gas emitted by the volcanos, sulfur dioxide, over a period of weeks combines with oxygen and water to form sulfuric acid gas. This gas then condenses into fine droplets or "aerosols" that form a haze, similar to the haze that often reduces visibility in the United States, Europe and other industrialized regions (the tropospheric haze originates mainly from the sulfur in fossil fuels).
Volcanic haze scatters some of the incoming sunlight back to space, thus reducing solar heating of the Earth's surface. This changed heating of the Earth is an example of a "climate forcing". If we can understand how the Earth's climate responds to such volcanic forcing, it may help us predict how climate will change in response to other forcings, such as increasing greenhouse gases.
The idea that volcanos effect climate is not new. Benjamin Franklin, in May of 1784, wrote of a constant "dry fog" in 1783 which so dissipated the sun's ray's that "when collected in the focus of a burning glass, they would scarce kindle brown paper". Franklin correctly attributed the dry fog to a large Icelandic volcano that erupted in 1783, and he argued that the reduced solar heating was the cause of unusual cold in 1783-4. He even dicussed the concept of the climate feedback that would result from increased snow cover.
The "Pinatubo" research team at GISS, a group of scientists, students and educators, is studying the effect of volcanos and other forcings on climate. Their special focus is the 1991 Mt. Pinatubo eruption, which produced the greatest volcanic aerosol haze in the twentieth century.
With a single volcano it may be hard to identify a climate "signal" among the large amount of weather and climate "noise", that is, the unforced chaotic fluctuations of the atmosphere and ocean. So the Pinatubo team first looked at the average climate response after the five largest volcanos this century. They found (Figure 1) that there was a small cooling, about 1/4°C (1/2°F), which peaked 1-2 years after the eruption. This tends to confirm that volcanos do cause a small global cooling.
A detailed test of climate models will be possible for the Pinatubo eruption, because of global satellite measurements of the volcanic aerosols and the climate change. A primitive climate model used to predict the effect of Pinatubo immediately after the eruption did a fairly good job, as shown in Figure 2, although observed cooling was somewhat less than predicted. The GISS Pinatubo team is now using a more sophisticated climate model for a comprehensive investigation.
Hansen, J., et al. 1996. A Pinatubo climate modeling investigation. In The Mount Pinatubo Eruption: Effects on the Atmosphere and Climate (G. Fiocco, D. Fua, and G. Visconti, Ed.). NATO ASI Series Vol. I 42, pp. 233-272. Springer-Verlag. Heidelberg, Germany.