(c) 2015 by Barton Paul Levenson
Some global warming deniers will go so far as to admit that global warming is happening, and even that carbon dioxide (CO2) is a greenhouse gas. But how do we know it's coming from human technology? Are the greenies just trying to make us all feel guilty for driving cars and having electricity?
No. There is clear evidence that the increase in carbon dioxide in recent years comes from human technology, and to a good approximation, from nowhere else.
The fraction of CO2 in the air was roughly stable at 260-280 parts per million by volume from antiquity to about 1750. Then we started burning coal, and later on, oil. Since then, CO2 has risen to 400 ppmv, a 43% increase.
We know the new CO2 is mostly from fossil fuels from its radioisotope signature. In the biosphere, a small, constant background level of 14C is constantly formed from cosmic ray bombardment of 14N, nitrogen being 78% of our atmosphere. That radioactive 14C gets into everything organic--trees, grass, animals, the ocean, the soil, you and me. But it's radioactive, half-life about 5,570 years, so it decays, and there's a balance between creation and destruction.
Organic material buried for long periods of time has lost all its 14C. Fossil fuels are 100-300 million years old and have no 14C to speak of. Neither does the new CO2 coming into the air. This was first pointed out by Hans Suess in a research paper in 1955, and confirmed by Revelle and Suess in 1957.
The new CO2 is also deficient in 13C, a stable isotope of CO2. Because it is slightly heavier than the more common 12C, plants tend to take up more 12C than 13C. Fossil fuels are formed from dead plant matter from the carboniferous area, mostly ferns, cycads, and other plants that fell into swamps in that period. So the source of the new CO2 is very old plant material--i.e., fossil fuels.
"Stoichiometry" is the part of chemistry dealing with weights and balances. For example, let's say you have a lump of coal that is pure carbon (unlike real coal, which has plenty of impurities). The equation for its combustion is:
C + O2 => CO2
The respective molecular weights of carbon, molecular oxygen, and carbon dioxide are 12.0107, 31.9988, and 44.0098 AMUs, respectively. So if you've got 12 kilograms of carbon and burn it with 32 kg of oxygen, you get 44 kg of carbon dioxide. Unavoidable. When you burn carbon-containing fuels, you get carbon dioxide. Matter is conserved in non-nuclear reactions. So where do the deniers think all the fossil fuel exhaust burned since 1750 went?
Some has actually been taken up by natural sinks--new plant growth, increasingly acidic oceans, etc. But about 45% of it has remained in the air. And a pulse of carbon dioxide lasts about 200 years, on average. Archer and Brovkin (2008) showed that 20-60% of it persists "for a thousand years or longer." That's where it went.
Archer, D.; Brovkin, V. 2008. The millenial atmospheric lifetime of anthropogenic CO2. Climatic Change 90, 283-297.
Revelle, R.; Suess, H.E. 1957. Carbon Dioxide Exchange between Atmosphere and Ocean and the Question of an Increase of Atmospheric CO2 During the Past Decades. Tellus 9, 18-27.
Suess, H.E. 1955. Radiocarbon Concentration in Modern Wood. Science 122, 415-417.