VOLCANIC eruptions have long been known to influence global climate, besides their obvious local effects. They emit large clouds of aerosols and sulphur-containing gases into the lower and upper atmospheres, affecting albedo, cloud formation and causing optical phenomena. They also cause widespread oceanic disturbance (tsunamis) which can destroy settlements large distances from the eruption. Although there is now conclusive evidence from the satellite record of temperature effects from volcanic eruptions and from ENSO, there is, so far, no evidence of global warming from an increase in greenhouse gases.
The table below lists prominent volcanic eruptions since 1815. They were compiled from Robock  and Zielinski et al. . Besides the date and the name of the volcano is given the Volcanic Explosibility.Index (VEI) and the Dust Veil Index (DVI) which are measures of the possible climatic disturbance of the eruption. Also is given the sulphate measurement from the ice core studied by Zielinski et al. which is a measure of the sulphur contribution to the atmosphere.
It will be seen that there were far more large volcanic eruptions towards the end of the nineteenth century, and at the beginning of the twentieth century than at any time since. It appears, from the study of early temperature records  that this period was unusually cool; and it seems probable that the high level of volcanic activity contributed to. this coolness. It would seem inappropriate to select this exceptionally cool period as a datum line by which to evaluate the possible existence of global warming by the greenhouse effect. Recently the behaviour of the atmosphere following the eruption of Mount Pinatubo in 1991 has been adduced as evidence for the lack of global warming, but it seems to have been forgotten that the beginning region of the global temperature records used to evaluate supposed global warming included eruptions such as those of Krakatoa (1883) and several others.
It should be pointed out that the Volcanic Explosibility Index (VEI) and the Dust Veil Index (DVI) are rather crude measures of volcanic activity. It would seem, from the ice-core measurements, that several recent, but apparently less violent eruptions, may have provided large amounts of sulphur gases.
Although there has always been a belief that volcanic eruptions lead to reduction in global temperature, there has been until recently surprisingly slim evidence for this belief. For example, the eruption of the volcano Tambora on the Indonesian island of Sumba in April 1815, which has been called “the largest and deadliest volcanic eruption in recorded history” has been held responsible for the “Year without a Summer. of 1816, when temperatures were unusually low all over the globe. Yet, as has been shown by Angell and Korsover , the eruption came at the end of several years of falling temperatures, as measured at several places, and it was followed by immediate temperature rises for all of them, except for the measurements at Yale University where the rise was delayed by only a year.
A more recent example was the eruption of El Chichon in April 1982 which was followed by the record breaking hot year of 1983, 0.21ºC above 1982. Although there are also examples of temperature falls following major volcanic eruptions, they are only slightly in excess of the rises .
The possible effects of volcanic eruptions on climate were almost totally ignored by the IPCC in their Report “Climate Change.6” in 1990. The only mention of them was as a possible contribution to “natural variability”. Angell  has presented evidence that global cooling from volcanic eruptions can usually be demonstrated if the temperature data are corrected for the influence of the El Niño Southern Oscillation which has a comparable effect on climate to volcanic eruption, and thus may cancel it or enhance it.
The attitude of the IPCC to the influence of volcanic eruptions underwent a change before the publication of the “Climate Change 1992. supplementary Report, as the result of the eruption of Mount Pinatubo in the Philippines in June 1991. This eruption could be studied in much greater detail than any previous one, by satellite and other instruments. It was calculated that the negative radiative forcing associated with the eruption of Mount Pinatubo exceeded the magnitude of the positive forcing associated with greenhouse gases during the second half of 1991, and remained significant through 1991 and much of 1992 [9.10,11].
The effects seem, however, to have dispersed by the beginning of 1994. The temperature effects are evident in the record of the NASA Satellite Global Monthly Temperature Anomalies from January 1979 to December 1994.
It will be seen, to begin with, that there has been no distinguishable overall change in global temperature over this whole period. It is also apparent that immediately after the eruption of Mount Pinatubo in June 1991 there was a rise in temperature of 0.2.ºC. Then, from August 1991, there was a fall of 0.7ºC until August 1992, when there was a rise of 0.3ºC by January 1993, followed by another fall of 0.3C by January 1993, a further drop of 0.3ºC by May 1993, and then a rise of 0.55ºC by January 1994.
This complex behaviour has been oversimplified by McCormick et al . If the 12 month running means are taken, there was a drop of 0.45ºC between October 1991 and June 1993, followed by a rise of 0.25ºC by December 1994. But were all of these changes due to Mount Pinatubo? It should be noted that the fall in temperature between December 1988 and January 1989 of 0.7ºC is not attributable to volcanic action; and we still have the temperature rise after El Chichon in 1982 as a contrary example.
The apparently large temperature drop after the eruption.of Mount Pinatubois less evident in annual or monthly figures. The annual global temperature anomalies from Jones et al.12 are 1990, 0.39ºC, 1991, 0.35ºC, 1992, 0.17ºC, 1993, 0.21ºC and 1994, 0.31ºC. Since Pinatubo erupted halfway through 1991 it can hardly be held entirely responsible for the 0.18ºC drop from 1991-1992, and subsequent to that the temperature has crept up slightly. It is considered that the 1994 figure is no longer influenced by Pinatubo. Mount Pinatubo has therefore not had the profound effect on the climate that was predicted, and it seems that any measured effects have now dispersed.
1. Robock, A., “The Volcanic Contribution to Climate Change of the Past 100 Years”, in ” Greenhouse Gas-Induced Climate Change “(Ed M.E. Schlesinger) 1991, Elsevier, pages 429-443
2. Zielinski, G.A., et al. 1994, “Records of Volcanism Since 7000 BC from the Greenland Ice Core.” Science, 264, 948-951.
3. Weber, G-R., 1994, “Long-Term European Temperature Variation Between 1525 and the Present”, “Proceedings of the Air and Waste Management Association Conference on Global Climate Change.” (C.V. Matthai and G. Stensland, Eds.), Phoenix Arizona
5-8 April pages 120-157.
4. Angell, J.K., and Korshover, J., 1985, ”J. Climate and Applied Meteorology” .24,937-951.
5. Stommel, H and E., 1979 “The Year without a Summer”, “Scientific American”, 240, 134-141
6. Houghton, J.T., Jenkins,G.J., & Ephraums, J.J. (Eds.) 1990, “Climate Change: The IPCC Scientific Assessment”. The Intergovernmental.Panel.on Climate Change. Cambridge University Press .
7. Angell, J.K., 1988. “Impact of El Ni<\q>o on the Delineation of Tropospheric Cooling Due to Volcanic Eruptions”. “J. Geophysical Research”, 93, 3697-3704.
8. Houghton, J.T., B.A. Callander, and S.K. Varney. 1992. “CLIMATE CHANGE 1992: The Supplementary Report to the IPCC Scientific Assessment. “Cambridge University Press
9. McCormick, M.P., Thomason, L.W., & Trepte, C.R., “Atmospheric Effects
of the Mt Pinatubo eruption”, 1995, “Nature,” .373, 399-404.
10. Minnis, P., et al., 1993, “Radiative Climate Forcing by the Mount Pinatubo Eruption”. “Science” 259, 1411-1415.
11. Dutton, E.G., and Christy, J.R., 1992. “Geophys. Res. Letters” 19, 2313-2316.
12. Jones, P.D., Wigley, T.M.L., and K.R. Briffa, 1994, pp 603-608 in T.A. Boden, D.P. Kaiser, R.J. Sepanski and F.W. Stoss (Eds) “Trends ’93” Carbon Dioxide Information Analysis Center, Oakridge, Tennessee
This article was written by Vincent Gray of Wellington, New Zealand, in March 1995 and is republished here with his permission.