Gas Emissions And The Greenhouse Effect
The occurrence of volcanic eruptions often has an effect on global temperatures, in this case global heating, due to the emission of certain gases. The mechanism of the greenhouse effect and the gases involved is vital in understanding the process by which emitted gases heat the earth, particularly the role of carbon dioxide. Despite not having as greater effect on global temperatures as human activity, such as burning fossil fuels, a large volcanic eruption may have a significant effect.
The Greenhouse Effect
Despite being perceived as a negative process acting on the Earth, the greenhouse effect acts as a global thermostat, with three main gases reacting within the atmosphere to keep the Earth at a stable temperature. These three gases (carbon dioxide, water vapour and methane), as well as small amounts of nitrous oxides, help to regulate the Earth’s annual surface temperature, of 13°C on average, by absorbing infrared radiation that is emitted from the surface. These gases come from both human activity and natural processes, for example from fossil fuel use and volcanism.
The planet receives its energy in the form of electromagnetic radiation, predominantly UV radiation and approximately 46 to 50% light in the visible region of the spectrum. However, from this energy emitted by the sun, only 70% of the total solar irradiance (TMI) is absorbed by the earth, via land, oceans and the atmosphere, with the other 30% being reflected in various ways, particularly through clouds, atmospheric aerosols and ice[1].
This ongoing process is largely due to the greenhouse gases and their interactions with the incoming electromagnetic radiation. The greenhouse gases, for example carbon dioxide, allow the radiation, which includes lower level UV (UVa and UVb), to pass through the higher levels of the atmosphere due to their transparency to long wave, high frequency solar radiation[2], however become opaque to the emitted infrared radiation that heats the earth’s surface as a result of its lower frequency. Gases in the atmosphere such as CO₂ absorb this infrared radiation as a consequence of its emission, and due to being a linear molecule, the two C=O bonds have the ability to vibrate upon absorption, therefore re-emitting kinetic energy in the form of longer wave infrared radiation. This energy cannot be lost or gained and only transferred, causing it to remain in the lower troposphere or outer layers of the atmosphere, and heating the earth. This whole process keeps the earth at a consistent and manageable temperature; however natural hazards, such as volcanoes, can throw the greenhouse effect out of balance through the slightest change in greenhouse gases.
The Greenhouse Effect as a system
Figure 4: Process of the Greenhouse Effect
This diagram summarises the process behind the greenhouse effect, whereby the greenhouse gases regulate the earth's temperature. Here, the atmosphere initially reflects some radiation, before the infrared emitted by the earth is trapped and redistributed.
The impacts of an eruption on the greenhouse effect
During a volcanic eruption, the main greenhouse gas that is emitted is carbon dioxide. During an eruption, huge amounts of CO₂may be released (approximately 0.26 Gt/y globally[3]), leading to an overall heating of the earth and its atmosphere through an accelerated greenhouse effect, often referred to as global warming. For example, in 1991, Mount Pinotubo released over 0.5 Gt of CO₂during its eruption[3] and the more recent Icelandic volcano, Eyjafjallajokull, giving off 150,000 tonnes of the same greenhouse gas on each day of the eruption[4]. Carbon dioxide and water vapour are the main greenhouse gases released as a result of a volcanic eruption that have the ability to absorb infrared and therefore enhance the greenhouse effect in a negative way. This emission of gases is controlled by the equilibrium within the vent between the exsolved gases and the silicate magma within the volcano (Symonds et al. 1994). Below is a table indicating the mean quantities of gases produced by a volcanic eruption (as percentages).
This table shows that three of these emitted gases have the ability to absorb infrared radiation, two of which are in significant enough amounts to have an effect on the earth’s atmosphere (water vapour and carbon dioxide). Small amounts of methane are also released from a volcanic eruption however these amounts are insignificant due to their negligible percentages. This is also noteworthy in the effect of volcanic eruptions on the atmosphere and the earth’s temperature due to the reaction of methane with the reflected radiation. Due to the structure of methane and its four single bonds, it has a greater ability to absorb and emit approximately 20 times the capability of carbon dioxide, further accelerating the greenhouse effect. This means that should volcanoes emit more methane than they do, their effect on the climate would be much greater. Despite this, the high percentage of water vapour and carbon dioxide released are significant as they allow greater amounts of infrared, originally radiated as UV radiation from the sun, to be absorbed and re-emitted before remaining in the atmosphere for long periods of time. This whole process depicts a positive feedback cycle whereby as more CO₂ and H₂O is released, it strengthens and enhances the greenhouse effect, having an overall negative effect on the environment. However, the extent to which this effect, from a single volcanic eruption, is significant is debateable in comparison with the amount of carbon dioxide emitted by humans.