Climate change as a result of volcanic eruptions?

The effects on the global climate generated by even a gigantic eruption like that of Mt. Tambora only last for a few years. The temperature fluctuation caused by eruptions of such dimensions barely exceeds 1°C. This transient phenomenon can hardly be regarded as climate change. But there have been much larger eruptions in the geological past. Early man also witnessed such events.

The most dramatic of these must have been the eruption of Mt. Toba on Sumatra about 73,500 years ago. Estimates of the ejecta volume range from 2,000 to 6,000 km3 — compared with 18 km3 ejected by Krakatau in the famous eruption of 1883, 50 km3 by Tambora in 1815, and only 6 km3 by Pinatubo in 1991. The Toba eruption produced a caldera — a collapse crater — measuring 100 x 30 km, which today is filled with a lake. The volume ejected would have been sufficient to cover the whole of India with a layer of ash one metre thick.

With the aid of ice cores it is possible to reconstruct the amount of ejected gases and the length of time they remained in the atmosphere. According to these investigations, such large amounts of sulphur gases were ejected that up to 5 billion tonnes of sulphuric acid aerosols were formed. Compare this with the "mere" 150 million tonnes ejected by Tambora. At least 90% of the sun's radiation was blocked out, producing an estimated drop in the mean global temperature of between 5° and 6°C — as much as 15°C in tropical latitudes — which lasted for at least six years. Such a drop in temperature would have resulted in ice age conditions and must have had a grave impact on the human population of the time by reducing photosynthesis and the availability of food — effects that would have been felt much farther afield than the immediate physical effects of the eruption on Sumatra and the surrounding areas.

With the aid of DNA analyses, evolution research has shown that humanity went through a critical phase at this time, at the end of which only a few thousand humans are thought to have survived. Although there is no conclusive proof of it, there seems to be a connection between this evolution crisis and the Toba eruption.

In the last two million years, three similarly massive eruptions have occurred at intervals of between 550,000 and 800,000 years in the Yellowstone region in the United States. The caldera has a diameter of 80 km today, and the Yellowstone National Park bears witness to the ongoing activity in the region. But even these gigantic events are no measure of what volcanic forces can really do. By far the largest ejecta volumes are released in connection with the — albeit less eruptive but nevertheless immense — lava plateaus, which are formed by what are known as flood basalts.

As to their dimensions, they have an eruption rate typically equivalent to that of the entire midocean ridge system today, which has a length of 70,000 km. This involves volumes of several hundred thousand cubic kilometres, ejected over a period of up to several million years, but concentrated in much shorter phases of weeks or years. One of these flood basalt provinces is Deccan Traps in India with an area of over 500,000 km2. It was formed 65 million years ago, i.e. at the Cretaceous-Tertiary boundary, and is therefore associated with the mass extinction of the dinosaurs which marked this transition. Current evidence favours the rival hypothesis of meteorite impact.

Generally speaking, if large meteorites or comets crash onto the earth, the climatic effects are likely to be similar to those caused by very large volcanic eruptions. The meteorite that tore open the Chixculub crater on the coast of Yucatán was an estimated 10 km in diameter. A correspondingly large volume of debris and dust particles was flung into atmosphere by the impact. In the case of this Cretaceous-Tertiary boundary event, the possibility cannot be ruled out that both of these mechanisms, meteorite impact and volcanic activity, were responsible.

In summary, there is no denying that the climate has been and will continue to be influenced by large volcanic eruptions or eruption periods. A new Yellowstone eruption would doubtlessly have a global impact — and statistically speaking, the time is ripe even if there are no acute signs of a pending eruption whatsoever. Conversely, there are hypotheses that postulate an influence of the climate on volcanic activity. They claim that the earth's mantle is destabilised by the load on the lithosphere either being increased by colossal ice masses in ice ages or being decreased by the ice melting in warm periods, which lead in both cases to greater volcanic activity.