What is the impact of volcanic eruptions on climate? In an examination of this question, Briffa and Jones (1998) used tree-ring wood density data from over 380 boreal forest locations in the Northern Hemisphere as surrogates for summertime temperatures, as they looked for effects of large volcanic eruptions on climate going back to 1400 AD. This exercise indicated that major volcanic eruptions seemed to produce significant Northern Hemispheric cooling the year after their occurrence, with the largest of the temperature declines they detected estimated to be 0.81°C in 1601, the year following the eruption of Huaynaputina in Peru.
Of the six centuries examined, the 17th century experienced the greatest number (six) of climatically significant eruptions. Strong temperature anomalies suggested there were three other major volcanic eruptions during the late 17th century that are not reported in historical accounts. It is also noteworthy that every Northern Hemispheric temperature departure of 0.3°C or more since 1400 (19 total events) followed a confirmed major volcanic eruption. This study thus suggests there is a strong linkage between volcanic activity and large-scale temperature variability, and it may help to explain the cool temperatures of the Little Ice Age, as it illustrates how closely-spaced multiple eruptions could reduce hemispheric temperatures on decadal and multi-decadal time scales, as well as how a lack of such eruptions could result in periods of warmer global temperatures.
Building on this perceived relationship, Hyde and Crowley (2000) analyzed statistical properties of climatically significant volcanic eruptions over the last 600 years in an effort to estimate the probability of future eruptions. This exercise led them to conclude there was a 35 to 40% probability of a volcanic eruption occurring with the capability of producing a radiative perturbation of -1 Wm-2 or larger in the following 10 years. Such an eruption was estimated to produce a 0.1°C to 0.15°C cooling over a 2-3 year period, while the probability of a larger Mt. Pinatubo-scale eruption with a radiative perturbation of -3 Wm-2 was estimated to be 15 to 25%. In light of these findings, the two scientists stated they were concerned that if a major volcanic eruption were to occur sometime in the next decade (which seems a reasonable possibility), it could "mask the CO2 effect and complicate discussions on a greenhouse gas protocol."
Taking a more cautious view of the subject were Sadler and Grattan (1999), who examined a number of issues related to the linking of volcanic activity with various spatial and temporal events in climatic, historical and palaeoecological records. They concluded that although volcanoes can have a significant effect on proximal climates, their global impact is less well understood. The message of their paper is thus one of caution in jumping to conclusions about climate change and its connection to volcanic explosions. They say, for example, that "a run of bad summers, an increase in sea ice off America, narrow rings in dendrochronological sequences, and neoglaciations can all be linked to temporally convenient climate forcing by volcanic aerosols." In addition, they note that "speculation as to the likely effect of these eruptions on fauna and flora and human societies may involve further supposition." They also point out that "the role of precursor climatic and/or environmental conditions is frequently overlooked," and that "it is valid to question whether the relationships established are fortuitous rather than dependent." As one of the scientists they quote has aptly phrased it, "an eruption here, a destruction there, a plague somewhere else -- all are too easily linked in hasty surmise."
Yet another indication that the climatic effects of volcanic eruptions may not be as well established as some might like to believe is provided by the study of [not yet posted] Douglass and Knox (2005), who "determined the volcano climate sensitivity and response time for the Mount Pinatubo eruption, using observational measurements of the temperature anomalies of the lower troposphere, measurements of the long wave outgoing radiation, and the aerosol optical density." Their analysis revealed "a short atmospheric response time, of the order of several months, leaving no volcano effect in the pipeline, and a negative feedback to its forcing."
One of the issues raised by these results, according to Douglass and Knox, concerns "the origin of the required negative feedback." With respect to this question, they report that "negative feedback processes have been proposed involving cirrus clouds (Lindzen et al., 2001)," and that "Sassen (1992) reports that cirrus clouds were produced during the Mt. Pinatubo event." In addition, they note that the adaptive infrared iris concept of Lindzen et al. "yields a negative feedback factor of -1.1, which is well within the error estimate of the feedback found by us." They also note that the short intrinsic response time they derived (6.8 ± 1.5 months) "confirms suggestions of Lindzen and Giannitsis (1998, 2002) that a low sensitivity and small lifetime are more appropriate" than the "long response times and positive feedback" that are characteristic of the reigning climate paradigm.
Clearly, there still remain some significant uncertainties and disagreements about the climatic effects of volcanic eruptions, just as there remain significant uncertainties and disagreements about their biological implications (see Volcanic Eruptions (Biological Impact) in our Subject Index).
References
Briffa, K.R., Jones, P.D., Schweingruber, F.H. and Osborn, T.J. 1998. Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years. Nature 393: 450-454.
Douglass, D.H. and Knox, R.S. 2005. Climate forcing by the volcanic eruption of Mount Pinatubo. Geophysical Research Letters 32: 10.1029/2004GL022119.
Hyde, W.T. and Crowley, T.J. 2000. Probability of future climatically significant volcanic eruptions. Journal of Climate 13: 1445-1450.
Lindzen, R.S., Chou, M.-D. and Hou, A.Y. 2001. Does the earth have an adaptive infrared iris? Bulletin of the American Meteorological Society 82: 417-432.
Lindzen, R.S. and Giannitsis, C. 1998. On the climatic implications of volcanic cooling. Journal of Geophysical Research 103: 5929-5941.
Lindzen, R.S. and Giannitsis, C. 2002. Reconciling observations of global temperature change. Geophysical Research Letters 29: 10.1029/2001GL014074.
Sadler, J.P. and Grattan, J.P. 1999. Volcanoes as agents of past environmental change. Global and Planetary Change 21: 181-196.
Sassen, K. 1992. Evidence for liquid-phase cirrus cloud formation from volcanic aerosols: Climate indications. Science 257: 516-519.
Last updated 8 June 2005