Fire and Ice:
Volcanic Eruptions and Global Climate Change

The greatest immediate dangers and possible changes to the global climate arise from volcanoes that erupt on time scales of hundreds to thousands of years. Several 'cataclysmic' eruptions of the past few centuries have resulted in a volcanic-forcing of climate and a marked cooling of Earth's surface temperatures, with wide-ranging consequences.

The detrimental impacts of volcanism may extend beyond regional effects on populations and the evironment to influence the planet as a whole and specifically its climate. Our present understanding how volcanic eruptions affect climate derives from a much improved understanding of eruptive processes, revised interpretations of written accounts of famous historic eruptions, and new geologic and climate-proxy evidence of the effects of volcanism in pre-historic and historic times. These all lend support to the concept of `volcano-forcing' as a mechanism of climate change.

Any sustained explosive eruption which generates a high-altitude (stratospheric) eruption plume has the potential to influence global climate. Such eruptions are termed 'Plinian' and are characteristic of those at subduction-related volcanoes. Tephra, and hot gases released from the magma, are forcefully ejected into the atmosphere, forming a low-level eruption cloud above the vent. Initially, the momentum of this cloud allows it to propagate upward to heights of many hundreds of metres. However, in order for the cloud to reach higher stratospheric altitudes, its density must be reduced below that of the surrounding atmosphere. This will occur if sufficient atmospheric air is entrained into the cloud and heated. Buoyancy forces will then dominate and strong vertical convection will allow the cloud to continue to rise tens of kilometres until it reaches an altitude where its density becomes equal to that of the surrounding atmosphere. Here, the plume will begin to spread laterally forming a characteristic stratospheric 'umbrella' cloud.

Volcanic ash carried to high altitudes in the convecting plume rains out from the base of the umbrella cloud over a period of several days to months, blanketing the landscape below it. Due to the short residence time of ash in the stratosphere, the effect of the particulate loading on Earth's global radiation budget is negligible. Rather, it is the gas content of the plume, specifically the sulphur dioxide loading, that is of importance. Once injected into the stratosphere, sulphur dioxide gradually reacts to form sulphuric acid aerosols, clouds of fine liquid droplets dispersed in air. These aerosols may persist for several years during which time they reduce the receipt of solar radiation at the Earth's surface by scattering and absorbing solar radiation. Their effect is to reduce tropospheric temperatures. The degree of cooling is a function of both the optical density (the extent to which aerosols scatter and absorb solar radiation) of the aerosol and its latitudinal distribution as it spreads with the atmospheric circulation. Eruptions at equatorial latitudes are nicely positioned to disperse aerosols into both hemispheres and so have far greater potential to affect global climate.

In recent years, eruptions such as the 1982 El Chichón and 1991 Pinatubo eruptions have had small but measurable effects on global climate. Sulphuric acid aerosols from these eruptions encircled the Earth in less than three months, reducing mid-latitude northern hemisphere temperatures by 0.3-0.5EC.

The overall tendency of upper-atmospheric winds is to spiral towards the poles where downward movement of air masses aids sedimentation into the ice caps. As a result, sulphate concentrations in polar ice cores provide a direct measure of the deposition of volcanically produced sulphuric acid. Ice layers showing high sulphate concentration correlate to well-documented recent eruptions, and such `acid spikes' can be used to identify probable climate-forcing volcanic events in early historic and pre-historic times. Identified are the eruptions of Taupo (New Zealand) in 181, Huaynaputina (Peru) in 1600, Tambora (Indonesia) in 1815, Krakatau (Indonesia) in 1883 and Gunung Agung (Indonesia) in 1963, amongst many others.

Historical accounts of some of these events describe 'volcanic winters', characterised by deteriorating climatic conditions and markedly cooler summer temperatures for up to three years following the eruption. The effects of a volcanic winter can be devastating, resulting in widespread crop and livestock losses, famine and deaths of thousands of people.

Some eruptions produce acid spikes at both poles, indicating widespread circulation of sulphur-rich aerosols. These eruptions are assumed to be of global importance and may be intimately linked with major climatic global perturbations. One such eruption occurred 74,000 years ago at Mt Toba in Indonesia, during a period of climate change associated with the last glacial cycle. The eruption produced a stratospheric sulphuric acid loading in excess of one billion tonnes, resulting in an estimated 3-5EC decrease in global surface temperatures for up to 3 years, but more importantly, a lowering of summer temperatures of >10EC in areas adjacent to those of established ice-cover. This temperature change would have been sufficient to tip the balance toward annual snow accumulation and the establishment of a snow-albedo feedback mechanism. Dramatic increases in the planetary surface albedo would thus have triggered rapid glacierization and an acceleration of the global cooling already in progress.

The Toba eruption may itself have been triggered by a lowering of sea levels associated with the onset of glaciation. Throughout the Quaternary (the last 2 million years), periods of increased global volcanic activity, especially at volcanoes along coastlines, appear coincident with periods of rapidly fluctuating sea levels. During low sea levels, hydrostatic pressures acting to confine magma at depth are reduced, so allowing magma to ascend through new and reactivated crustal fractures caused by increased crustal tension. During periods of high sea level, the undermining of a volcanic edifice and periodic flank collapses may trigger decompression related explosive eruptions. Indeed, studies at Mt Etna, on the coast of Sicily, suggest that past explosive eruptions are linked to rapidly changing sea levels in the Mediterranean area. Given that more than 90% of all active volcanoes are located within 20 km of the sea, the predicted rise in global sea levels attributable to global warming may have important implications for future eruptive activity, both in terms of eruption periodicity and impact on global climate.

References:

 

	GO BACK TO OCCURRENCE AND TYPE