Volcanic activity weather and climate

Many of the major volcanic eruptions in historical times have been followed by short-term variations in climate which lasted only as long as the dust veil associated with the eruption persisted. The most celebrated event of this type was the cooling which followed the eruption of Tambora in 1815. It produced in 1816 'the year without a summer', remembered in Europe and North America for its summer snowstorms and unseasonable frosts. Its net effect on world temperature was a reduction of the mean annual value by 0.7°C, but the impact in mid-latitudes in the northern hemisphere was greater, with a reduction of 1°C in mean annual temperature and average summer temperatures in parts of England some 2-3°C below normal (Lamb 1970). The eruption of Krakatoa in 1883 was also followed by lower temperatures which made 1884 the coolest year between 1880 and the present (Hansen and Lebedeff 1988).

Increased volcanic activity may have been a contributing factor in the development of the Little Ice Age—which persisted, with varying intensity from the mid-fifteenth to the midnineteenth century. The eruption of Tambora falls within that time span, and other eruptions have at least a circumstantial relationship with climatic change. A volcanic dust veil may have been responsible for the cool, damp summers and the long, cold winters of the late 1690s in the northern hemisphere, which ruined harvests and led to famine, disease and an elevated death rate in Iceland, Scotland and Scandinavia (Parry 1978). Lamb (1970) has suggested that dust veils were important during the Little Ice Age because their cumulative effects promoted an increase in the amount of ice on the polar seas, which in turn disturbed the general atmospheric circulation. He also points out, however, that contemporaneity between increased volcanic activity and climatic deterioration was not complete. Some of the most severe winters in Europe—such as those in 1607-08 and 1739-40—occurred when the DVI was low, and the period of lowest average winter temperatures did not coincide with the greatest cumulative DVI. Thus, although increased volcanic activity and the associated dust veils can be linked to deterioration between 1430 and 1850, it is likely that volcanic dust was only one of a number of factors which contributed to the development of the Little Ice Age at that time.

Major volcanic episodes in modern times have usually been accompanied by prognostications on their impact on weather and climate, although it is not always possible to establish the existence of any cause and effect relationship. The Agung eruption produced the second largest DVI this century, but its impact on temperatures was less than expected, perhaps because the dust fell out of the atmosphere quite rapidly (Lamb 1970). It is estimated that it depressed the mean temperature of the northern hemisphere by a few tenths of a degree Celcius for a year or two (Burroughs 1981), but such a value is well within the normal range of annual temperature variation. The spectacular eruption of Mt St Helens in 1980—enhanced in the popular imagination by intense media coverage— promoted the expectation that it would have a significant effect on climate, and it was blamed for the poor summer of 1980 in Britain. In comparison to other major eruptions in the past, however, Mt St Helens was relatively insignificant in climatological terms. It may have produced a cooling of a few hundredths of a degree Celcius in the northern hemisphere, where its effects would have been greatest (Burroughs 1981). The eruption of El Chichón in Mexico in 1982 produced the densest aerosol cloud since Krakatoa, nearly a century earlier. Within a year it had caused global temperatures to decline by at least 0.2°C and perhaps as much as 0.5°C (Rampino and Self 1984). However, the cooling produced by El Chichón may have been offset by as much as 0.2°C as the result of an El Niño event which closely followed the eruption, and effectively prevented cooling in the southern hemisphere (Dutton and Christy 1992) (see Figure 5.5). Past experience suggested that the eruption of Mount Pinatubo would also lead to lower temperatures. It was blamed for the cool summer of 1992 in eastern North America, and by September of that year it was linked with reductions in global and northern hemisphere temperatures of 0.5°C and 0.7°C respectively (Dutton and Christy 1992). Estimates by modellers studying world climatic change indicate that such cooling would be sufficient to reverse—at least temporarily—the global warming trends characteristic of the 1980s (Hansen et al. 1992).

Although volcanic activity is most commonly associated with cooling, there is some evidence that it may also cause short-term, local warming.

Figure 5.5 Monthly mean global temperature anomalies obtained using microwave sounding units (MSU). The events marked are: A—La Niña; O—El Niño; E—El Chichón; P—Pinatubo

Figure 5.5 Monthly mean global temperature anomalies obtained using microwave sounding units (MSU). The events marked are: A—La Niña; O—El Niño; E—El Chichón; P—Pinatubo

Source: After Dutton and Christy (1992)

Figure 56 Mean anomalies of surface air temperature produced a similar pattern, and Groisman

(°C) for the period 2 to 3 years after volcanic suggests that the unusually mild winter of 1991-

eruptions with volcanic explosivity power n, . it,- i- c approximately equal 92 in central Russia was an indication of a to the Krakatoa eruption, (a) Winter (b) Summer. positive anomaly associated with Mount Points show the stations used. Dashed regions depict n Pinatubo. The warming is seen as a result of the egative anomalies egative anomalies

Source: From Groisman (1992)

Groisman (1992) has compared temperature records in Europe and the northeastern United States with major volcanic eruptions between 1815 and 1963. The results show that in western Europe, and as far east as central Russia and Ukraine, statistically significant positive temperature anomalies occur in the winter months some 2 to 3 years after an eruption the size of Krakatoa (see Figure 5.6). The analysis of data following the eruption of El Chichón greater frequency of westerly winds over Europe, which owe their development to an increase in zonal temperature gradients following the general global cooling associated with major volcanic eruptions.

Volcanoes have the ability to contribute to changes in weather and climate at a variety of temporal and spatial scales. At a time when the human input into global environmental change is being emphasized, it is important not to ignore the contribution of physical processes, such as volcanic activity, which have the ability to augment or diminish the effects of the anthropogenic disruption of the earth/ atmosphere system.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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