Friday, February 02, 2018

Carbon Fingerprint

Lignite Coal.
Decaying blocks of black coal along the bank of the Missouri River, McKenzie County, ND. Photo 7/15/13

This coal is from a geologic stratum called the Sentinel Butte Formation. Lignite is a soft, brownish coal with fewer calories and more moisture than other coals, making it an intermediary between peat and bituminous or anthracite coal. The Sentinel Butte Formation is from the late Paleocene Epoch in the early Cenozoic Era. This epoch began after the C-T extinction event, which marked the extinction of dinosaurs and the advent of mammals. It is old. 
It has been observed that that plains Indians did not burn much of this coal, nor did the Corps of Discovery as they sailed by these bluffs two hundred years ago. It was in 1804 that William Clark remarked that the coal was of "inferior quality." Their fuel of choice was firewood cut from the cottonwood, ash, elm, and juniper trees that were plentiful along the Missouri River. This is for good reason. The smokes of hickory, mesquite, apple, alder, cherry and other woods have long been popular flavorings in grilled foods, but we have yet to see any food products on the market that advertise "Natural Coal Smoke Flavor", nor do we expect to see any in the near future. The hapless homesteaders did make use of the lignite, perhaps out of the desperation of the times, and by 1939, at the height of the Great Depression, there were some 309 mines operating in North Dakota. But today there are only six coal mines out there and two of those produce oxidized lignite, not for burning. None for food. 
The distinction between coal smoke and wood smoke goes beyond successful backyard bar-b-ques. While coal and wood both originate as plant matter and terminate as fuel, the antiquity of the plant matter accounts for a significant difference in the chemistry of the fuel, a distinct carbon fingerprint. Here is how it works: Humans have a symbiotic relationship with plants. We produce carbon dioxide as waste matter, plants consume carbon dioxide as food. Carbon comes in about 15 sizes, called isotopes. Three are naturally found on earth: C12, C13, and C14. The latter is a radioactive isotope, unstable, formed by cosmic radiation, ultimately decaying into an isotope of Nitrogen. This is a slow deterioration; the half-life of C14 is 5,730 years.
While living, plants incorporate carbon in their tissues and the balance of carbon isotopes in their tissues reflects the balance of carbon isotopes in the atmosphere. When a plant dies, they no longer take in carbon and the carbon that remains in their tissues is fixed, set. From this point, the unstable C14 continues to decay, slowly depleting the dead tissues of C14.  
In the short term, what this means is, when a dead cottonwood limb is thrown into a campfire, and the wind shifts and casts the plume of campfire smoke over the campers, they are inhaling smoke almost fully stocked with busy, buzzing, radioactive C14 atoms. Only a few atoms have been lost to decay. However, when a chunk of Paleocene coal is thrown into a campfire, and the wind shifts and the smoke descends upon the ashen, hacking campers, they inhale a lighter, decadent smoke, one depleted of C14 atoms.
In the mid-term, the empirical evidence is there has been a dramatic increase in carbon in the earth's atmosphere. Currently, we are above 400 ppm. That's a 100% increase since the ice age and a level 25% higher than anything for the past 400,000 years. Where is all that smoke coming from?
The answer is in the chemistry of the carbon. The atmospheric carbon, while increasing in quantity, has been steadily decreasing in C14 in the past century; it is being depleted of C14. This could not be from the burning of firewood or rain forests, grassland or forest fires, or burning other plant matter of recent origin. These would produce smoke that has nearly the full stock of C14, which would not deplete the atmosphere of C14. This must be from the combustion of carbon sources that are depleted in C14. Ancient plant matter has lost its C14 through radioactive decay. Their combustion products lack C14 and thereby deplete the atmosphere of C14. This is the source of the smoke. This is what is called "fossil fuel." Coal, natural gas, petroleum. Old plant matter.
Ah, the smoking gun.
In the long-term, it is estimated that there are over one trillion tons of extractable coal reserves on earth, which would last 150 years at present consumption rates. At the same time, combustion of these reserves in that time frame would contribute to a novel set of environmental conditions on earth. As Foster (2017) states, "Humanity's fossil-fuel use, if unabated, risks taking us, by the middle of the twenty-first century, to values of CO2 not seen since the early Eocene...If CO2 continues to rise further into the twenty-third century, then the associated large increase in radiative forcing, and how the Earth system would respond, would likely be without geological precedent in the last half a billion years."
Without precedent. At which point, all trees may be ancient plant matter, food may grill spontaneously, coal-flavored dishes may be standard fare, and visibility would be reduced to a few smoggy meters. Let us cough. This remodeled earth will have a brand new look, but with a decidedly retro feel. We will name our new home Venus.

Gavin Foster et al. Future climate forcing potentially without precedent in the last 420 million years. Nature Communication. April 2017.

M. S. Baxter and A. Walton. A Theoretical Approach to the Suess Effect. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences. Vol. 318, No. 1533 (Aug. 11, 1970), pp. 213-230

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