Global Temperatures

The Carboniferous Period

occurred from about 354 to 290 million years ago.
cf. UCMP Berkeley That's when the coal seams were laid down, and also quite a lot of limestone. Both of these processes sequestered carbon. Note that the use of limestone to make Portland cement and therefore concrete releases some of this carbon dioxide, independently of the heat energy used in the kiln. The process took 64 million years.

Temperature

The average global atmospheric temperature dropped from about 20°C to about 12° C in that period. It had, in the latter part of the Devonian, dropped from 22°C for a total of 10° C, according to the graph shown below, from Monte Hieb's excellent page at Plant Fossils of West Virginia.

I would conclude therefore, that if we burn all the coal that was then laid down, we can expect a global temperature rise of perhaps 5 to 10° C, suitable for the living organisms of the early or middle Carboniferous, but not necessarily birds, mammals, or even dinosaurs.
It would be catastrophic, because we are doing it more than a hundred thousand times faster than the Carboniferous change.


But Monte Hieb argues that, since there is not a strong correlation in the graph between carbon dioxide concentration and temperature over the longer period shown, the present global warming hypothesis (which would put West Virginia's coal industry out of business) must be flawed.

I think that he is ignoring the fact that methane is an even stronger greenhouse gas than carbon dioxide, and that the discrepancies might be due to variations in methane concentration. The sudden peak in temperature at the Permian/Triassic boundary coincides with a mass extinction. which could have caused a sudden increase in the activity of methanogenic bacteria. Such bacteria decompose dead organic material in anoxic conditions like submerged sediments. They are responsible (by also producing sulfur and ammonia compounds) for the stink of wet, inadequately turned (and therefore relatively anoxic) compost heaps. Or it might have been rapid decomposition of methane clathrate hydrates.

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