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What was Earth's climate like millions years ago? The scientists are now desperate to get the answer in the context of current global warming. The Earth is assumed to be free of ice during 80-90% of its known history. However, climate of those ancient times is poorly studied. Scientists from RAS Geology Institute decided to fill the vacuum and carried out 12 palaeoclimatic reconstructions for various periods of the Cretaceous (135-65 mln yrs ago), described most fully with geological, paleogeographical, geochemical and biological objects. The scientists summarized pieces of available information and came up with climatic zones, existing in the times of dinosaurs.
Ancient climatic conditions turned out to have nothing in common with modern ones. Even familiar zones were located in other latitudes, thus had specific light conditions, precipitation, prevailing winds and cyclone tracks. Thus, both polar regions had climatic conditions compared to modern temperate ones. This hypothesis is supported by plant and shellfish residues and also by oxygen isotope analysis. According to calculations, average annual temperature at Alaska region has been about +10îC, and the warmest month temperature has balanced around +20îC. Such conditions are quite similar to current temperature regime in Eastern and Southern Europe and New Zealand.
However, cold snaps and seasonal freezing of some ancient seas sometimes occurred, which can be seen on occasional sediments of sea ice. In contrast to modern climatic conditions, Cretaceous high-latitude climate had polar night and day interchange, and its vegetation period lasted up to half a year. These were the conditions under which a grand coal-bearing belt has been formed. Mid latitudes were warm and moist. Average annual temperatures rarely dropped below +20î, which is indicated by fossile tree ferns, thermophilic insects and numerous sea coral reefs.
In the first half of the Cretaceous mid latitudes (sometimes from 45î South to 45î North, i.e. from Africa to the Black Sea) were mainly occupied by vast dry areas. Within these areas continental margins were covered with salt basins, often bordered with barrier reefs. Equatorial rainforests seemed to be absent at that time. They appeared only during the second half of the Cretaceous, when climatic conditions of a narrow equatorial region of dry areas became hot and humid. This was when coals, bauxites, kaolinic layers started to form on the equator, and the amount of thermotolerant plant pollen in the deposits dropped significantly. The Cretaceous ended with vast humid areas near the equator as well as in mid latitudes.
No one doubts that the unique climatic zonation of the Cretaceous indicated warm conditions of the biosphere. But still many facts remain mysteries to the scientists. The example is warm polar night phenomenon, which provided food for herbivorous dinosaurs. Numerous hypotheses describe how ice and no-ice ages had changed on Earth. The scientists from Geology Institute tend to think that main reason for the no-ice Cretaceous is global albedo (the fraction of extinction which is scattered) decrease. Due to the ocean incursion, forest territories' growth and global greenhouse effect the Earth partly lost its ability to reflect solar radiation, which led to climate warming. "Warm biosphere" phenomenon studies will help to shed light on Earth's future under fast global warming conditions, the scientists report.
