The White Planet by Raynaud Dominique Lorius Claude Jouzel Jean

Author:Raynaud, Dominique, Lorius, Claude, Jouzel, Jean
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2012-04-02T16:00:00+00:00

Volcanism and Solar Activity: Natural Climatic Forcings

As we get closer to the current period, our knowledge of the evolution of the climate becomes increasingly detailed, both in time and in space. But just as much as the most precise description possible of the evolution of the climate, it is important to look carefully at the phenomena that could be at the origin of it, to specify what we call climatic forcing. In this period of relative stability, certain forcing, which retreats from the forefront when we are looking at the great changes of the last glacial period, must be taken into consideration. This is the case for volcanic eruptions and variations in solar activity, forcings that are recorded in polar ice.

The chemical composition of the ice was modified at each sufficiently large volcanic eruption with, for example, an increase in the content of sulfates. The eruptions in the Northern Hemisphere are clearly recorded in the ice of Greenland, those of the Southern Hemisphere in the Antarctic ice. Some eruptions occurring close to the equator or in the tropics can leave traces in both the north and the south when they are very violent. Beyond the historical archives, polar ice sheets thus enable us to establish a calendar of eruptions that have marked our recent and more distant past, a calendar that in some cases proves more precise than that kept in historical archives. For a long time it was believed that the eruption of the volcano of Santorini, in Greece, whose climatic consequences were felt as far as China and California, occurred in the sixteenth century; the ice in Greenland, dated year by year, place that eruption a century earlier; it is that chronology, confirmed by other methods, that is now considered fact.

The influence of these volcanic eruptions on the climate results from the presence of ash, which darkens the atmosphere for a few weeks, but above all from the formation of sulfur aerosols that result from the oxidation of gaseous compounds emitted during the eruption. These aerosols have an effect over longer periods, as much as several years. Volcanoes are thus related to cooling and, for a limited time, they can, as a result of an increase in the optical thickness of the atmosphere, have negative forcings equivalent to that associated with an increase in the greenhouse effect of anthropogenic origin. This forcing was on the order of 3 Wm−2 for the eruptions of Krakatoa in 1883 and of Pinatubo in 1991 and weaker (2 Wm−2) for those of Agung in 1963–64 and El Chichon in 1982, all of which are relatively well documented. It is more difficult to estimate the variations in optical thickness for eruptions only recorded in the ice as well as the geographic distribution and the duration of associated perturbations, but this is the only approach available for going back in time. This effort of reconstruction has concentrated on the last millennium; with more than 10 Wm−2, the prize goes to the eruption of 1259, to which no named volcano has up to now been associated.

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