Carl Sagan, Cement And The Greenhouse Gas Effect

By Richard F Cronin

Going all the way back to Carl Sagan’s original essay on the runaway warming greenhouse gas effects from CO2 making Earth into another Venus turned out the following:

“Like Venus, the Earth also has about 90 atmospheres of carbon dioxide; but it resides in the crust as limestone and other carbonates, not in the atmosphere. If the Earth were moved only a little closer to the Sun, the temperature would increase slightly. This would drive some of the CO2 out of the surface rocks, generating a stronger greenhouse effect, which would in turn incrementally heat the surface further. A hotter surface would vaporize still more carbonates into CO2, and there would be the possibility of a runaway greenhouse effect to very high temperatures. This is just what we think happened in the early history of Venus, because of Venus’ proximity to the Sun. The surface environment of Venus is a warning: something disastrous can happen to a planet rather like our own.”

Emphasis “A hotter surface would vaporize still more carbonates into CO2”.

Under the 90 atmospheres pressure the surface of Venus is 462 C.

Carbonates such as limestone (CaCO3) and soda ash (NaCO3) were formed on earth over eons of accumulation from crustaceans like minute hard shelled plankton, krill, and corals. The earliest life forms greatly exceeded organic life.

Carbonates do not “vaporize”. They decompose.

A lime kiln takes crushed limestone and heats it to a temperature of 900 to 1000 C to undergo the following reaction:

CaCO3 @ 900 C —> CO2 + CaO ( “quick lime”).  Quick lime is then further processed to make cement.

To achieve what Carl Sagan described would mean heating every limestone quarry and soda ash deposit to 900 C in order to release the CO2.  Then things would cool back down to 462 C.  Except that Earth is further away from the Sun and has water.

I don’t believe Carl Sagan ever studied much industrial chemistry and certainly not cement manufacture.

The Minoans are credited as the first to produce cement by heating limestone.  The Minoan civilization lasted from approx. 2700 BC to 1450 BC and flourished under the Minoan Warming Period. The horrific volcanic explosion on Thera between 1642 to 1540 BC is deemed to be a major blow leading to the demise of Minoan society.

So there you have it. CO2 from dabbling in cement manufacture plus a big volcano did the Minoans in.

No doubt the early attempts at industrialization was the worse of the two.  ** eyes roll **


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