Wow, I have never gone to 3 parts! I can't even remember where I started!
Ok, I remember. We had 2 dials, the strength of which overwhelms anything that mere humans can do. There really is a third dial, which is a big mystery, and we have no data on it. This is the Solar Input dial, and it can vary by means of changes in the Sun's output, earth wobble, or orbital variations. It works on a much shorter period than the other two, and might be responsible for warm and cold decades, and centuries. For example, nobody knows why the 30's were warmer than now (although everybody knows why were getting warm now!!).
Our two main dials are very slow, and changes take 10's of millions of years. Over the last few billion years, the fossil record shows big swings in both water and carbon.
The variations are caused by our giant plate tectonics machine. When we have an 'average' world (right now!), the oceanic crust subducts under places like Peru. This conveyor belt of junk contains huge amounts of water, and carbonate rocks. Normally, it dives at a fairly steep angle, until it gets deep enough to cook. Then it spews out water and carbon dioxide (along with other nasties), in the big line of volcanoes, right over the cooking zone.
Our recycling factory, however, has not worked at a steady pace over geologic time. Sometimes, the plates move so fast that the cooking process is interrupted, and huge cold slabs of water and carbon get 'frozen' under the continents. This is the time when the 'dials' go down.
Of great interest to me, was the last time when all the continents glumped together a couple of hundred million years ago. When this happened, most subduction stopped, and all the cold slabs were under a giant thermal blanket of fluffy continental rock, and they cooked. Huge amounts of water and carbon were released, and we had the age of the dinosaurs. When the continents broke up again, the cooking stopped, and the world became average again. The poor dinos were not happy when the big lump hit the earth, and did not recover. The mammals started their party!
The final mystery is whether these two dials are independent, or linked together in some manner. It is possible to have subducted slabs that are full of water, and not carbonate, and vice versa. The last big cooking (dinosaurs) had both, but there were really bizarre climate changes in the earlier fossil records. I like to think that they can be twiddled independently, and more science will prove or disprove this hypothesis.
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