Climatic effects of volcanic eruptions
In New England, 1816 was called 'the year without a summer', with average temperatures in June 7 °F (4 °C) below normal. The consequence was serious crop failures. The situation was even worse in Europe; disastrous crop failures led in places to famine, and the price of grain shot up dramatically. This unusual climate has been widely attributed to the eruption of Tambora in the Sunda Arc in the previous year. It should, however, be pointed out that there were other cold years in the early nineteenth century for which no volcanic explanation is available. Dust and sulphur dioxide (SO2) from volcanoes form a sort of sunscreen. Sulphur dioxide is actually a greenhouse gas and its initial effect is to cause warming. However, it quickly reacts with water to produce sulphate aerosols that backscatter and absorb the Sun's radiation. Such effects are localized unless gases are injected into the lower stratosphere. They are thence rapidly dispersed around the hemisphere. Global cooling is well recorded in historical times, but the effect is usually only for one to two years because of the rapid 'rain-out' of aerosols. Dust from volcanic ash is less important, because it is usually washed out much more quickly.
Besides sulphur dioxide, carbon dioxide is volumetrically the most important gas from volcanoes. Because of the greenhouse effect, it produces global warming and, significantly, it stays much longer in the atmosphere than sulphur dioxide (Fig. 8.2). The greenhouse effect results from the fact that, whereas incoming light from the sun is of short wavelength, the energy radiated from the Earth is of long wavelength, and the more carbon dioxide there is in the atmosphere, the less transparent it is to outgoing radiation. Among the other volcanic gases, only chlorine (Cl) is postulated to occur in suffi-
cient quantities to cause environmental harm as a result of local acid rain, in the form of hydrochloric acid, and ozone depletion. It is, however, rapidly removed from the atmosphere (Fig. 8.2). As is evident, carbon dioxide cannot be removed rapidly. The estimated yearly output of volcanic carbon dioxide is about 1011 kg, a figure dwarfed by the burning of fossil fuels, for which the equivalent figure is 1013 kg. Both values are minute in comparison with the amount of carbon in the atmosphere and oceans. Single eruptions are thus unlikely to cause any noticeable increase in this important greenhouse gas, but because of its long residence time the cumulative effect could be considerable.
The largest explosive eruption in the recent geological past was of the Toba volcano in Sumatra, 73,000 years ago. This left an elongated caldera about 50 km long and 100 kilometres across. Ash was spewed over much of the Indian Ocean, leaving a layer on the ocean floor up to 10 centimetres thick at a distance of 1300 miles. The eruption has left a clear record in a Greenland ice core, but oxygen isotope data from this core lend no support to any postulation of significant climatic change recorded in the seas of this time. This could well be bound up with the high thermal capacity, and hence thermal inertia, of the oceans. Even a fall in atmospheric temperature of 3-5 °C over a few years is unlikely to change sea-surface temperatures. The effect could, though, be more marked for terrestrial environments.
We have so far confined our attention to the explosive volcanoes that produce silica-rich lava and ash. In terms of the volume of material emitted, the most important volcanic activity is, however, from more or less non-explosive flood basalts, which are silica-poor, erupted from fissures. These form the bulk of what are called Large Igneous Provinces (LIPs for short), none of which has been formed in the past few million years (Fig. 8.3). Basalts are much richer in sulphur dioxide (SO2) than the more silica-rich lavas of explosive volcanoes, and so the global cooling effect of the sulphate aerosols they produce is considerably greater. The only substantial
example of flood-basalt fissure eruptions in modern times is that of Laki, in Iceland. Eight months of eruptions started in June 1783, with the ejection of enormous quantities of sulphur dioxide. It has been estimated that a hundred million tons of sulphuric acid rain descended over this period. This is approximately the same as the amount of acid rain that falls today on the whole Earth in an entire year. A significant cooling took place in the northern hemisphere; it was 7 °F cooler than average in eastern North America during the following winter, although more normal temperatures returned during the next two to three years. The European winter was especially harsh, and a bluish haze extended from Iceland as far as Siberia and North Africa. Acid rain destroyed most of Iceland's crops and 75 per cent of its livestock, and the effects reached Norway and England. That remarkable polymath Benjamin Franklin, serving at that time as American Ambassador in Paris, was the first person to suggest a relationship between volcanism and climatic cooling, on the basis of the Laki eruptions.
Since fissure eruptions are the most quiescent form of volcanic activity, it has been doubted whether they can inject gas into the stratosphere. Only the presence of what are called 'fire fountains' along the length of the fissures provides a potential mechanism, especially in high latitudes where the troposphere is thinnest.
We can sum up at this point by saying that volcanic activity in the recent past is likely to have affected the climate on a timescale ranging from a few months to several thousand years (Fig. 8.2). The shorter-term effects rank as weather rather than climate, and longer-term effects are not clearly demonstrated. This does not, however, take into account ancient Large Igneous Provinces, which exceed by many orders of magnitude the volcanic activity of historic times. Many LIPs have been shown to record brief bursts of activity over a period of about a million years, and recent improved radio-metric dating generally shows a close correlation with mass extinctions. Indeed Vincent Courtillot, a geophysicist at the University of Paris (Denis Diderot) enthusiastically writes that the correlation is 'almost perfect'. Let us now explore the relationship in some detail.
Continue reading here: Floodbasalt provinces and mass extinctions
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