Global Effects of Explosive Volcanism : TOMS Results from the El Chichon & Mt. Pinatubo Volcanic Eruptions

TOMS Images from El Chichon and Mt. Pinatubo.

This composite shows false-color images of the SO2 clouds on the second day after each eruption.

Following the devastating eruption of Mt. Pinatubo in the Philippines in June 1991 there has been a resurgence of interest in the global effects of volcanism. Mt. Pinatubo was the second largest eruption of this century and produced approxmately 10 cubic kilometers of rock and ash (called tephra by volcanologists). This is enough to bury the District of Columbia to a depth of 128 feet, according to a recent Washington Post article. As well as tephra, the eruption also injected a twenty million ton sulfur dioxide (SO2) gas cloud into the stratosphere. This gas cloud was chemically converted into a sulfuric acid aerosol, which was predicted to cause a 0.5íC global temperature decrease. Although the temperature decrease is hard to identify due to natural climate variability, the aerosols caused a measurable stratospheric temperature rise and a drop in the direct solar beam at the Earth's surface. Some have blamed both the recent Midwest floods and the increased size of the Antarctic ozone hole on Mt. Pinatubo aerosols.

Volcanologists attempt to estimate the magnitude of volcanic eruptions by looking at criteria such as tephra amount and crater size. A variety of researchers have then compared such data from historical eruptions to climate records, and also to temperature data derived from tree rings and ice cores. Result show that some large historic eruptions were followed by temperature decreases. However, not all large historic eruptions were followed temperature decreases, and not all historic temperature decreases were preceded by large eruptions. A variety of other factors, such as volcano latitude and season of eruption, will need to be taken into account before the volcanism-climate relationship can be fully understood.

In order to establish which factors are important we can take advantage of the remote sensing and ground based instruments available today and understand the effects of eruptions for which we have detailed data sets. If relationships between volcano-type and explosivity can be found, then it may be possible to calibrate the historic record of volcanism , using the recent remotely-sensed volcanic data. Current work at the Goddard Space Flight Center under the supervision of A. Krueger (Code 916) and L. Walter (Code 900) involves relating SO2 to explosivity, as well as analysing in detail data from important eruptions such as El Chichon and Mt. Pinatubo. SO2 is important because in the atmosphere it chemically reacts to form a sulfuric acid aerosol. If the amount of SO2 is known then the amount of aerosol produced can be calculated.

Data from Total Ozone Mapping Spectrometer (TOMS) instruments is used to measure and track SO2 gas clouds from explosive volcanic eruptions. The TOMS measures reflected ultraviolet (UV) radiation and can measure both ozone and sulfur dioxide. The ability of TOMS to detect SO2 gas was first noticed after the eruption of El Chichon, Mexico in April 1982. Abnormally high ozone values were found across Mexico at the same time as the eruption. These false high-ozone values were due to SO2 gas from the volcano. SO2, like ozone, absorbs UV light and the SO2 was being incorrectly interpreted as extra ozone.

Only the most powerful volcanic eruptions can produce stratospheric eruption clouds. Well-known volcanoes such as Mauna Loa in Hawaii and Mount Etna in Sicily, erupt at relatively constant rates but with little explosive force. Gas clouds from these volcanoes remain in the troposphere, and only rarely are stratospheric clouds produced. In the troposphere the gas and aerosols are rapidly removed by rain. Volcanoes like these are called non-arc volcanoes and lie at regions of tectonic plate spreading and over "hot spots". Large explosive eruptions such as Mt. Pinatubo and El Chichon, produce stratospheric clouds and the aerosols produced can remain in the stratosphere for several years. The majority of such explosive volcanoes are classed as arc volcanoes, and lie close to subduction zones at tectonic plate boundaries. The Pacific rim is surrounded by such volcanoes from Chile to Alaska, and from Indonesia to Kamchatka.

We can consider El Chichon and Mt. Pinatubo as typical arc volcanoes. These are characterized by long dormant periods punctuated by intense explosive eruptions of variable intensity and duration. In this case both volcanoes were dormant for almost 600 years before their most recent eruptive activity began. During the month before the largest eruptions, there were a sequence of much smaller eruptions, some of which produced small SO2 clouds detected by TOMS. The ground-based monitoring of these precursor eruptions allows volcanologists to predict explosive volcanic eruptions, as was done successfully by the US Geological Survey and Philippine Institute of Volcanology and Seismology at Mt. Pinatubo.

El Chichon and Mt Pinatubo are the two largest eruption clouds measured by TOMS, but over 100 other eruption clouds have also been measured. The final explosive eruptions from El Chichon and Mt. Pinatubo produced approximately seven and twenty million tons of SO2 respectively, as measured by TOMS. The clouds from each eruption were tracked for over a month and gradually faded from TOMS' view, as the SO2 was converted to sulfate aerosols. Other measurements made at the same time indicate the rapid formation of an aerosol layer which persisted for several years. Stratospheric aerosols have only recently returned to pre-Pinatubo levels.

The Volcanic Explosivity Index, or VEI, is the volcanologists version of the Richter scale, and rates eruptions by explosivity, from 0 (low explosivity, eg Mauna Loa) to 8 (very high explosivity; at least 100 times bigger than Pinatubo; none recorded in last 10,000 years). For example, El Chichon rates as VEI 5, whilst Mt. Pinatubo rates as VEI 6. TOMS data can be combined with VEI data to try to relate SO2 output with explosivity or volcano type. Analysis of TOMS data for the Nimbus-7 period (1978 - 1993) show a large variation of SO2 output (and hence aerosols) with explosivity. This means it is very hard to predict the SO2 output of an eruption, even if the explosivity is known. For example, the eruption of Mt. St. Helens in 1980 (VEI 5), produced only a tenth as much SO2 as El Chichon (also VEI 5). The lack of correlation between explosivity and SO2 output complicates the work of those trying to look at the historical effects of volcanism upon climate.

Volcanic aerosol particles scatter and absorb a fraction of incoming solar radiation, as well as absorbing a fraction of outgoing terrestrial radiation. The change in global temperatures caused by the aerosols from El Chichon and Mt Pinatubo is estimated to be 0.2íC and 0.5íC. However both these values lie within the natural variability of temperature . The estimations are based upon Global Circulation Model (GCM) results and temperature analyses that attempt to remove other sources of climate variability, such as the El Nino/Southern Oscillation (ENSO). ENSO's are quasi-periodic changes in sea surface temperatures in the Pacific and cause an increase in global surface temperatures for a few years. The co-incidence of an ENSO with an eruption can possibly mask the effects of the eruption. For example, an ENSO after El Chichon meant that no distinct temperature decrease was observed. Feedbacks in the climate system mean that we cannot expect the climate effects of an ENSO and an eruption to add in a linear fashion. It has even been proposed that volcanic stratospheric aerosols can trigger an ENSO to occur earlier than it would otherwise, although this is treated with great scepticism by most researchers. If this is true then the recent Midwest floods can be attributed to the eruption of Mt Pinatubo.

Volcanic aerosols have also been implicated in ozone depletion. A year after an eruption, stratospheric aerosols even from equatorial volcanoes can be distributed to polar latitudes and studies show volcanic aerosols can catalyze ozone-destroying chemical reactions. The largest ever ozone hole was measured by TOMS at the end of 1993, and the hole was larger than predicted. If this unexpectedly large increase in the size of ozone hole is due to Mt. Pinatubo aerosols, we may possibly see a return to predicted ozone hole size changes in the next few years, as the Mt. Pinatubo aerosols fall out of the stratosphere.

Recognition and measurement of the volcanic climate signal is vital to understanding global climate change. Before we can quantify any climate change due to human activity we must quantify natural sources of short-term climate variability. The primary source of such variability is from explosive volcanism. TOMS data can both quantify recent volcanic activity and illuminate historic volcanic activity in terms of SO2 output.

Ian E. Sprod

February 1994

Further Reading

(containing many useful references)

Bluth, G.J.S., C.C. Schnetzler, A.J. Krueger,and L.S. Walter. The Contribution of explosive volcanism to global sulphur dioxide concentrations, Nature, Vol 366, Pg 327-329, 25 November 1993.

Sato, M., J.E. Hansen, M.P. McCormick and J. B. Pollack. Stratospheric Optical Depths, 1850-1990, Journal of Geophysical Research, Vol. 98, No. D12, Pg 22,987-22,994, December 20 1993.