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magnitude of the sunspot cycles. In the case of the medieval Nile floods, he found a reversed relationship.

3. VOLCANIC DUST AS A TRIGGERING FACTOR

S.C. Porter (1986) demonstrated a close relation between the pattem of the northern hemisphere glacier variations during the last millennium and the volcanic aerosol production, as found in the Greenland ice cores. He thus maintains that sulfur-rich aerosols generated by volcanic activity are a primary factor in forcing climate change on the decade level. He found that glaciers advance whenever acidity levels rise sharply from background values to reach concentrations >1.2 meq. H+/kg above background. The glaciation lags by about 10-15 years behind the beginning of the acidity increase.

R.S. Bradley et. al. (1987) found extremely sharp drops in temperaturę, particularly in fali months, occurred after several major volcanic eruptions. High temperatures are sometimes associated with major El-Nino years. When the two occur morę or less simultaneously their influence is minimized

R.A. Bryson and R.U. Bryson (1996, 1998) developed a methodology of archaeo-climatic modeling of palaeo-climates based on a series of hierar-chical steps, each of which relies on certain underlying assumptions. Accord-ingly they calculated various parameters such as the ratę of loading by vol-canic dust, impact on optical depth and insolation of dated volcanic events. These they used to predict ice volume, glacial area and albedo, the values of which were combined to model seasonal hemispheric mean temperatures, which could then be used to ascertain the timing of major atmospheric circu-lation features. Synoptic climatology was then used to determine the relationship between contemporary monthly precipitation or temperaturę and perti-nent circulation features. This was applied to the modeled past monthly positions of the relevant circulation features in order to calculate past precipitation or temperaturę at specific intervals. The Brysons* generał conclusion is that volcanic eruptions were the main triggering factor for climate chang-es during the Holocene. They suggest that the explosions, strong enough to load the atmosphere with a quantity of aerosols, which affected atmospheric optical depth, modulated also insolation. By a series of calculations they ar-rived at time-series for atmospheric circulation features at different locations.

K.B. Taylor and B.F. Molnia (1993) have measured the electrical conduc-tivity (ECM) of the layers of a core in the summit of the Greenland ice sheet. This is affected by their dust content, which by neutralizes their acidity. The