As efficient reflectors of sunlight, they screen the ground from some of the energy that it would ordinarily receive from the sun. Although the winds in the stratosphere carry the aerosols rapidly around the globe in either an easterly or westerly direction, the movement of aerosols north and south is always much slower. In the case of an eruption at high latitudes, most of the aerosols simply remain confined to a wide zone surrounding the polar cap for several years. As the photograph shows, their ancient lava flows appear intermingled and moss-covered today. Such older eruptions have been investigated by using various research techniques.
Scientists measure the visible lava and ash exposures in the field.
Sulfuric acid fallout over polar areas can be detected in deeply drilled ice cores having clearly marked annual layers of ice. The anomalous cooling appears in stunted annual growth rings of north temperate trees and, much more importantly, in contemporary reports compiled by competent witnesses. For the oldest historic eruptions, however, these compilers may not be scientifically trained or even first-hand witnesses. In both eruptions, the cloud of aerosols from the eruption traversed northern Europe on the prevailing westerly winds, and dimmed and reddened the sun. This continued for months.