Erupting volcanoes produce a myriad of gases and particles and provide a significant challenge to understand and simulate their atmospheric influences. Simulations of past volcanic eruptions provide an important test of the performance of climate models and provide further confidence in their future predictions. Many of the processes which influence climate also influence the local weather. Particularly explosive volcanoes can also decrease stratospheric ozone.
Volcanic Activity and the Atmosphere
Atmospheric influences of volcanoes occur due to several processes. General outgasing from the Earth's crust occurs almost continuously on climate time scales. Aditional effects arise from "explosive volcanoes" of which the largest have influences which reach the stratosphere, approaching 20 km (12 miles) in altitude.
The Volcanic Explosivity Index (VEI, Newhall and Self, 1982) is a measure of the volume of erupted material. The scale is logarithmic, with VEI1 corresponding to 0.001 cubic km, and VEI6 corresponding to 100 cubic km. Although it is not a direct measure of the different categories of ejecta, to some extent, the largest VEI coincide with the largest climate influences.
In the last century, the most significant eruptions were Santa Maria (1902, VEI 6), Agung (1963, VEI 5), El Chichon (1982, VEI 5) and Pinatubo (1991, VEI 5). Climate influences were observed for several years following each eruption.
Ash and Gas Emissions from Volcanoes
Particles such as ash and dust have a short life span in the atmosphere as they are heavier than air. They are removed from the atmosphere by rain, or fall to the ground by gravity. The eruption of the Eyjafjallajokull volcano in Iceland had a substantial impact on aircraft flight patterns in 2010 but its influence on the global atmosphere was small. Aerosols contribute to the haze in the atmosphere and the redenning of the skies seen at sunrise and sunset. They also provide condensation nuclei for clouds.
Volcanoes emit some CO2, but the human production of CO2 exceeds this natural source, by a factor of at least 100 (Gerlach, 1991, 2010). SO2 gas released from volcanoes, undergoes a sequence of atmospheric reactions and is somewhat longer-lived in the atmosphere than volcanic ash. SO2 is emitted into the lower atmosphere and reacts with OH (produced from water) to form sulphuric acid droplets (often referred to as sulphate aerosols). These have a very small size but have a major influence on the climate. The sulphuric acid droplets absorb and reflect energy from the sun, preventing it from reaching the surface. For the major eruptions the lower stratosphere is warmed and the surface of the Earth is cooled.
The globally averaged cooling after the eruption of Mt. Pinatubo in the Philippines was about 0.2C (0.3F), for a period of up to 2 years. This was enough to offset temporarily about a decade worth of climate warming from greenhouse gas increases.
Mitigating Climate Change
The influence of sulphur aerosols in cooling the surface has been suggested as a possible way of mitigating climate change: by artificially pumping SO2 or sulphate into the lower stratosphere. However, such a technology is not yet established nor is it necessarily desirable (e.g. Robock et al., 2009). For example, one of the unintended consequences would be a reduction in the amount of ozone in the atmosphere, which would increase the damaging effects of surface ultraviolet.
Implications for Climate Prediction of Past and Eruptions
About 20 years ago, the human induced global warming theory was becoming established and stimulated the formation of the Intergovernmental Panel on Climate Change (IPCC). Simulations of the time, though, were not entirely consistent with the observations which showed a lack of significant warming.
It was soon recognised that Mt. Pinatubo and indeed El chichon had distorted the observational record. Once these eruptions were taken into account, a delayed (but stronger) warming was simulated once the Pinatubo influence on the atmosphere had decayed.
With volcanic processes much better represented, simulations of climate are now much more reliable. Of course, over the next century, an explosive volcano comparable to Mt. Pinatubo might occur, and this might again distort the future climate of the Earth. Although the volcano itself would be unpredictable, it is likely that its influence would be short-lived and after a period of a few years, the climate would revert to the level that it would have had without the eruption.
References
Gerlach, T. M. (1991), Present-day CO2 emissions from volcanoes, Eos Trans., AGU, 72(23), 249,
254-255.
Gerlach, T. M. (2010), Volcanic versus anthropogenic carbon dioxide: The missing science, Earth, 55(7), 87.
Newhall, C. G. and Self, S. (1982), The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism, Journal of Geophysical Research, 87 (C2): 1231–1238. doi:10.1029/JC087iC02p01231.
Robock, A., et al. (2009), Benefits, risks, and costs of stratospheric geoengineering, Geophysical Research Letters 36 (19), L19703. doi:10.1029/2009GL039209
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