Milutin Milanković and the astronomical theory of climate changes

Abstract

changes in the geological past has become widely accepted only in the first half of the 19th century, due to the work of Playfair, Schimper, Venetz, Charpantier and many others, and somewhat later in particular of L. Agassiz [1]. ey claimed that the moraines present in many alpine valleys, or the erratic granite blocks standing on the geologically unfitting bedrock are due to the fact that “the big ice-sheets like those seen at present-day Greenland once covered all the territories where such stones have been found”. At the same time the expression “Die Eiszeit” was introduced in an ode written by K. Schimper. Even the famous German poet Johann Wolfgang von Goethe back in 1829 states: “to have a lot of ice a cold weather is needed, thus I presume that an epoch of great cold at least over Europe passed”. Still, there were many who opposed the idea of the Ice Ages, and the debate lasted for the entire century. At the very beginning of the 20th century A. Penck and E. Bruckner [4] proposed that the glaciations took place four times in the Quaternary geological period, with three interglacial intervals of unequal duration in between. Although we now know that the climate changes were much more complex than this simple scheme predicts, the fact that the Ice Ages did take place in the past has been firmly established and not seriously disputed aerwards. Different mechanisms have been considered to explain the changes of Earth’s climate, including also astronomical ones. Soon aer the publication of Agassiz’ work, J. Adhemar [2] proposed that the precession of the Earth’s axis of rotation is responsible for the Ice Ages. Although the simple mechanism he considered was soon rejected, Adhemar actually showed that the astronomical and geological phenomena can be related, and that the long term variations of the Earth’s motion can possibly lead to climate changes. e most remarkable early theory of the Ice Ages, which consistently combined the achievements of different sciences, was undoubtedly the theory by J. Croll [3]. Croll correctly interpreted the influence of the eccentricity of Earth’s orbit upon the duration of the seasons and its coupling with the precession of the rotation axis. He was the first to consider the changing obliquity of the rotation axis, thus completing the list of relevant astronomical mechanisms causing climate changes. He also pioneered the idea of the feedback effect due to the reflectance of the incoming radiation from the surface covered by ice, and proposed that the eccentricity-driven amplification of the ocean currents augments the heat exchange between equatorial and polar regions. Croll’s theory at first attracted geologists, but it has soon been found that its results do not match the observations. As later explained by Milankovic, the failure of Croll’s theory is due mostly to the fact that the influence