|MadSci Network: Earth Sciences|
What you are asking is a very long and complicated story. And when scientists are unravelling a story like this about the distant past, there are some things and times that they know a lot more about than others. There are many clever ways of collecting evidence, but the sort of evidence we can get can only tell us a few little bits when we go to the very distant past. The earth is, according to the best evidence we can collect, about 4.5 billion years old. As far as the composition of the atmosphere is concerned, there are three main periods, with small variations within each period. In the very early times, the atmosphere was made of mainly hydrogen and helium, with a little bit of methane. The atmospheres of the giant outer planets, Jupiter, Saturn, Uranus, and Neptune are still very much like this. But the Earth was a smaller and warmer planet, and its gravity was not strong enough to hold on to these very light gases for very long -- millions of years maybe, but not billions. The hydrogen/helium atmosphere was lost very quickly on the overall time scale of things. So the atmosphere gradually thinned and changed its composition. Hydrogen and helium were lost, and were replaced by gases that came out of volcanoes, and from the weathering of rocks. The main gases in the atmosphere were nitrogen, carbon dioxide (although that was probably mostly dissolved in the oceans), and a little bit of argon. The atmospheres of Venus and Mars are still pretty much like this today. But there were two things that were a little bit different and quite remarkable about the earth. The first is that the temperature was just right for liquid water to remain about on the Earth's surface, and for oceans to form. Venus was too hot, and Mars was too cold. The second was that life got started, probably in some of the shallow water of the oceans near the edge of the land. Some of the early life forms learnt a remarkable trick. Water contains hydrogen and oxygen atoms; carbon dioxide contains carbon and oxygen atoms. By rearranging the chemical bonds between these atoms, organic substances like sugar and starch, which also contain carbon, hydrogen, and oxygen atoms, could be made. It required energy from the sunlight to make this happen, and when it was done, there were quite a lot of oxygen atoms left over, which joined up in pairs to make oxygen gas. The first living creatures to do this were a sort of seaweed slime, known as blue-green algae; later the whole great variety of green plants developed on the basis of the same sort of chemistry. The sugar and starch were useful both as growth material for the living creatures, and as a source of energy that they could use. As green plants became more numerous, and particularly when they began to colonize the land surfaces, the composition of the atmosphere changed again, because all of the carbon dioxide was being used up, and oxygen was being added as a waste product. And of course other creatures multiplied which could use the sugars and starches in green plants for food, and oxygen from the air to react with it to produce their energy. So for about the last 2 billion years the atmosphere has contained a fairly large amount of oxygen, along with nitrogen and a little argon, and only a tiny bit of carbon dioxide. I do not have a really good reference I can give you. The one I mainly use "Chemistry of the Natural Atmosphere" by Peter Warneck, Academic Press 1988, chapter 12, is very technical, and I think you would find it too hard to read. "The Ages of Gaia" by James Lovelock, 1986, is a bit less technical, but still quite heavy going. Now, to get on to the temperature of the Earth: because there is plenty of evidence that liquid water has always been present in large quantities on the earth's surface (water worn pebbles are quite different to pebbles produced by glaciers, wind erosion, or other weathering processes, for example), we know that the average surface temperature on the earth has never been more than about 30 deg C nor less than about 5 deg C. I will start out with the most recent tiny fraction of the Earth's history. For the last 300 000 years or so, we have a very detailed record from an analysis of ice cores drilled in Antarctica and Greenland. In these two places, fresh snow accumulates on top of old ice, and gradually compresses it down into an icesheet that is now several kilometres thick. It forms a frozen record of the recent past -- the shallower ice refers to more recent times, the deeper ice to older times. For the top bit, you can apparently count ice layers like tree rings, because of changes in ice structure due to slight surface melting in the direct sunshine of summer months; deeper down this structure disappears, and you have to rely on other clues to get accurate dates. Air composition shows up in trapped air bubbles in the ice; temperature in the relative amounts of two different isotopes of oxygen in the ice; dust content of the ice tells how strong the prevailing winds were, and marks major volcanic eruptions; and trapped pollen grains tell what sort of plants were growing in countries not too far from the ice -- obviously there would be almost none of them in ice ages, and a lot more in warmer times. This record shows us three 'ice ages', each of which lasted for tens of thousands of years, and three warmer and briefer 'interglacials' where the climate was much warmer for a relatively shorter time. We are at present in the third of those interglacials. When we go earlier than this ice record, we have to rely on the evidence of fossils to make deductions about the climate. One good example is that where I am living in Victoria Australia, we have fossils of dinosaurs from about 70 million years ago. At that time this part of the land was very close to the South Pole. But the dinosaurs look as though they were adapted to living in forests. So it seems fair to suppose that at that time the world was a fair bit warmer than it is today. The South polar icecap either did not exist, or was much smaller. It is only from those sort of deductions that we know much about the temperatures in those earlier times. The other thing we do know is that the average surface temperature, now about 15 deg C, never got to be above about 30 deg C, because then there would have been extra carbon dioxide and water vapour in the atmosphere which would have led to runaway warming to even hotter temperatures.
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