Volcanic activity and climate variations

Explosive volcanic events inject fine-grained ash and dust into the atmosphere. This commonly leads to short-lived temperature drops due to reduction of incoming radiation. Dust particles may also act as foci for formation of water droplets and thereby cloud formation. Sulphur volatiles are, however, more significant in terms of climate change. In the atmosphere these are converted into sulphuric acid and these aerosols result in cooling of the lower troposphere by back-scattering of long wavelength radiation. The residence time of these aerosols is commonly between one and five years. After the Mt. Pinatubo eruption in the Philippines in 1991, solar radiation declined by up to 10 per cent and surface temperatures in the northern hemisphere dropped approximately 1°C (Handler and Andsager, 1994). Porter (1986) found temperature reductions following major volcanic eruptions of about 1.5°C during historical times.

Instrumental temperature records over the past 200 years and satellite data for the last 20 years have combined to show that explosive volcanic eruptions that emit large amounts of sulphur-rich gases into the stratosphere can reduce global temperatures by about 0.3°C over a period of 3-4 years following the eruption. The global cooling is not, however, homogeneously distributed. In addition, the number and type of eruptions that have occurred during the last 200 years is limited, and evaluating the impact of particular types of eruptions under different climatic modes is restricted to modern climatic conditions. Consequently, a complete understanding of the volcanism-climate system requires a multidisciplinary approach beyond the instrumental temperature data time period (e.g. Zielinski, 1998).

The volcanic records are developed through the evaluation of the direct products of the eruption, from terrestrial archives, ice core records, and atmospheric phenomena linked to the presence of aerosols in the stratosphere (e.g. red sunsets, dimmed lunar eclipses, dry fog). Volcanic records may in addition be deduced from proxy data (e.g. tree rings and coral records) of the climatic cooling resulting from a specific eruption. One of the most reliable records of past volcanic activity comes from continuous, high-resolution records of geochemical and conductivity variations in ice cores (see Chapter 3). Additional verification of source eruptions can be achieved through tephrochronological investigations. The stratospheric loading and optical depth for a particular eruption provide information for models and postulate the climate forcing of the eruption. High-resolution ice-core records from Greenland and Antarctica from the last 400 to 2000 years demonstrate the potential of eruptions to influence past climate and hence to modify future climate. In addition, these records show that several closely spaced eruptions may have a climatic impact on decadal time-scales. The ice-core records of volcanism also support the hypothesis that rapid climatic changes during glacial growth and decay periods can enhance crustal stresses, leading to increased periods of volcanic activity (Sejrup et al, 1989).

Studies from terrestrial archives provide information on the composition of magma, volume erupted, and dispersal direction(s). Correlation of particular volcanic eruptions between terrestrial deposits, marine sediment cores, and ice cores provides distinct time markers for different proxy records. Compilations of volcanic records suggest that the global record of volcanism is incomplete, preventing prediction of the global and regional climatic impact of future volcanic eruptions.

A network of circum-hemisphere tree-ring density chronologies related to annual summer temperature variations has been combined into a single time-series for northern high latitudes and the northern hemisphere. Based on this well-dated, high-resolution composite time series, Briffa et al. (1998) suggested that large explosive volcanic eruptions produced cooling events in the northern hemisphere during the last 600 years. The significant temperature effect of some events, such as in 1816, 1884 and 1912, are apparent. The most severe short-term northern hemisphere cooling event of the past 600 years occurred in 1601, probably as a result of the ad 1600 eruption of the Huaynaputina volcano in Peru (Shanaka and Zielinski, 1998).

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