The Evolution of Climate
9781465685537
213 pages
Library of Alexandria
Overview
Geologists very early in the history of their science, in fact as soon as fossils began to be examined, found indisputable evidence of great variations in climate. The vegetation which resulted in the coal measures could have grown only in a sub-tropical climate, while over these are vast remains of ice-worn boulders and scratched rocks which obviously have been left by ice existing under polar conditions. Such records were not found only in one region, but cropped up in juxtaposition in many parts of the world. Remains of sub-tropical vegetation were found in Spitzbergen, and remains of an extensive ice-sheet moving at sea-level from the south were clearly recognized in central and northern India. At first it was simply noticed that the older fossils generally indicated a warmer climate, and it was considered that the early climate of a globe cooling from the molten state would be warm and moist, and so account for the observed conditions. It was recognized that the ice remains were relatively recent, and so far as a cause for the Ice Age was sought it was considered that astronomical changes would be sufficient. It was only when geologists began to find records of ice ages far anterior to the Carboniferous Age, and astronomers proved by incontrovertible observations and calculations that changes in the earth’s orbit, or its inclination to that orbit, could not account for the ice ages, that the importance and inexplicability of the geological evidence for changes of climate came to be clearly recognized. During the last few years much study has been given to “palæoclimatology,” but such a study is extremely difficult. Only a very small fraction of the total surface of the earth can be geologically examined, and of that fraction a still smaller proportion has up to the present been studied in detail. There has been a great tendency to study intently a small region and then to generalize. The method of study which has to be employed is extremely dangerous. A geological horizon is determined by the fossils it contains. Wherever fossils of a certain type are found the strata are given the same label. Isolated patches correlated by their fossils are found in different parts of the world, and it is frequently assumed not only that these rocks were laid down at the same time, but that the conditions which they indicate existed over the whole of the earth’s surface simultaneously. Thus geologists tell us that the climate of the Carboniferous Age was warm and damp; of the Devonian Age cool and dry; of the Eocene Age very warm; of the Ice Age very cold. But has the geologist given sufficient attention to the climatic zones during the various geological climates? It is true that the geologist has definitely expressed the view that in certain ages climatic zones did not exist; but from a meteorological point of view it is difficult to see how the climate could have been even approximately the same in all parts of the world if solar radiation determined in the past as in the present the temperature of the surface of the earth.
