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We know a great deal about historical climate and its variations from various geo­ logical studies. There are two points worth remarking on. One is that the climate changes frequently and radically, but that the degree of variation and even sense of variation depends on the time scale which we are considering. Secondly, that this is a most unusual geological period for the Planet Earth; we are living in a period of mountain building and glaciations, whereas during most of the last 250 million years (m.y.) there was little ice and little topography. A good view of climate change of the last hundred m.y. can be gained by looking at the paper of Kellogg. We are now in a period of extensive glaciations. The previous interval occurred 300 to 250 m.y. ago, when even the Sahara was glaciated. (Of course, it was at that time near the position of the South Pole; we know that 300 m.y. ago the continents had not broken apart and formed one land mass.) Apparently between 250 and 20 m.y. ago there was little ice on the Earth, even at Antarctica. Continental basins were flooded by shallow seas. This was the period when plant life and marine life proliferated and when most of our fossil fuels were laid down.

Environmental sciences. --- Environmental Science and Engineering. --- Environment, general. --- Atmospheric Sciences. --- Environmental science --- Science --- Environment. --- Atmospheric sciences. --- Atmospheric sciences --- Earth sciences --- Atmosphere --- Pollution.

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Environmental Sciences and Forestry. Environmental Management --- Environmental Pollution --- Environmental Pollution.