Although human activity is known to be affecting the Earth’s climate, there are some natural influences too. One of these is the impact of volcanic eruptions, which have wide-ranging and complicated effects. In detail, the consequences are different in different parts of the atmosphere, but the most significant effect is often seen as global cooling [2, 5]. However, this shouldn’t be seen as a natural counteraction to the global warming caused by humans.

There are several examples of volcanic eruptions having noticeable effects on climate. Two of the best known cases are: the eruption of Tambora, Indonesia in 1815 which was a major cause of an average global temperature decrease leading to widespread food shortages in the Northern Hemisphere in 1816, “the year without summer” [3, 7], and the 1991 eruption of Pinatubo in the Philippines, after which cooler average global temperatures were recorded for up to three years [3]. There are many factors that determine what effect, if any, an eruption has on global climate. For example, in order to trigger the cooling described above, an eruption has to be powerful enough to send particles into the stratosphere, about 20 km above the surface of the Earth, so that these particles can spread across the whole planet [6], where they can remain for several years [3]. These are significantly large eruptions – neither of the recent eruptions of Eyjafjallajökull in Iceland and Puyehue-Cordón Caulle in Chile which caused such disruption to air travel reached these heights [4]. Eruptions of this size do not occur as frequently as smaller eruptions that fail to reach stratospheric levels. The location of a volcano will also control how significant its eruptions are for changes in climate – the pattern of global atmospheric circulation means that eruptions from volcanoes nearer the equator will have a greater effect on the climate than those nearer the poles [6].

Various types of particles and gases are added to the atmosphere by eruptions, but cooling occurs because one erupted gas, sulphur dioxide (SO₂), is converted by reactions in the atmosphere into sulphuric acid. This forms aerosols that increase the amount of reflection of radiation from the sun back into space. These particles and the ash produced by eruptions also form a haze around the Earth and reduce the amount of sunlight reaching the Earth’s surface. This cools the lower atmosphere, and therefore the planet, down [1, 2, 3],and has been seen to last for 2-3 years after eruptions [8].

As well as the cooling effect, it is well documented that volcanoes also emit carbon dioxide (CO₂). This is a well-known greenhouse gas and therefore CO₂ emissions from volcanoes have the potential to enhance global warming [5]. However, the amount of CO₂ produced each year by volcanic eruptions is about 10,000 times less than the amount added to the atmosphere from anthropogenic sources [1] and as yet there has been no detectable global warming of the atmosphere by CO­₂ from volcanoes [5].

Therefore, volcanic contributions to atmospheric CO₂ levels are insignificant compared to those from human sources, and although the most significant effect of volcanic eruptions on global climate is cooling as a result of the injection of SO₂ into the stratosphere and subsequent reactions [2, 5], these are infrequent and short-term variations lasting a few years, compared to continuing global warming trends caused by human activity. Consequently, there is no evidence that volcanic activity is either a detractor from the importance of reducing anthropogenic carbon emissions, or a natural counterbalance to it.

[1] Harpp, K., 2005. http://www.scientificamerican.com/article.cfm?id=how-do-volcanoes-affect-w

[2] McGee, K.A. et al., 1997. http://pubs.usgs.gov/of/1997/of97-262/of97-262.html

[3] Robock, A., 2000. Reviews of Geophysics, 38, 191–219. http://climate.envsci.rutgers.edu/pdf/ROG2000.pdf

[4] Simkin, T. and Siebert, L., 2002-. www.volcano.si.edu

[5] USGS, 2011. http://volcanoes.usgs.gov/hazards/gas/climate.php

[6] Viner, D. and Jones, P., 2000. http://www.cru.uea.ac.uk/cru/info/volcano/

[7] Wikipedia, 2011. http://en.wikipedia.org/wiki/Year_Without_a_Summer

[8] Wolfe, J., 2000. http://earthobservatory.nasa.gov/Features/Volcano/