About a century ago, A.L. Chizhevsky, great Russian biophysicist, was the first to pay attention to the correlation between people's death-rate and the solar activity cycles.
However, in the course of time, it became clear that some powerful natural factor of terrestrial origin interfered in the process. It turned out that there existed a global filter that distributed the solar ultraviolet's influence heterogeneously on various regions of the Earth. This filter turned out to be the ozone layer of the terrestrial atmosphere, the layer being located in the stratosphere. Absorbing the ultra-violet radiation of the Sun, the ozone layer protects the biosphere from the destructive short-wave part of ultra-violet radiation.
Specialists of the Institute of Atmosphere Optics, Northern Branch, Russian Academy of Sciences, took advantage of the fact that excess ultra-violet radiation caused stress to growing trees, the annual rings becoming more dense than the usual ones as the thickness growth of the trunk slowed down. At the end of the last century, geophysicists learned to measure the ozone content both from the Earth and from outer space. That gave the Tomsk researchers an opportunity to compare the ozone observation data within the last 20 years and the respective density of annual rings within the same period. This effort resulted in a formula, which allowed to calculate the ozone concentration in the atmosphere based on dendrochronological data - through measuring the annual rings kept in the International WSL Bank.