Fossil Fuel and Atmospheric Levels of Carbon Dioxide


The atmosphere can handle about 700 billion tons of carbon.  Atmospheric carbon dioxide levels have been increasing since the industrial revolution.  Carbon dioxide is a greenhouse gas which acts like a blanket in the atmosphere to trap heat (see my posting entitled Solar Activity, Greenhouse Gas Levels and Climate Change on Our Earth).  Today the atmosphere contains about 800 billion tons of carbon and it continues to rise.  How do we know that the burning of carbon-based fossil fuels like coal, oil and natural gas contribute to the atmospheric carbon overload? 

Carbon has a unique footprint which allows scientists to determine whether the burning of fossil fuel contributes to the atmospheric carbon overload.  Carbon is composed of three isotopes which are carbon-12, carbon-13 and carbon-14.  Carbon consists mainly of the carbon-12 and carbon-13. A small amount of the carbon atom is the radioactive isotope carbon-14.  In the upper atmosphere cosmic rays from the Sun react with nitrogen to create carbon-14.  Carbon-14 is unstable and over time is converted back to nitrogen.  After 60,000 years there is no carbon-14 remaining in the original sample because it has been completely converted to nitrogen.     

Fossil fuel reservoirs are composed of coal, oil or natural gas and over time these reservoirs are buried deep in the ocean floor or underground.  The carbon atoms found in both the atmosphere and initially in fossil fuel contain all three carbon isotopes (carbon-12, carbon-13 and carbon-14).  After 60,000 years fossil fuel contains only carbon-12 (all of the carbon-14 has been converted to nitrogen) but the atmosphere still maintains a healthy mixture of the three isotopes.  Since it takes millions of years to create fossil fuel, the carbon dioxide that is released into the atmosphere from the burning of fossil fuel would no carbon-14.  If the burning of carbon-based fossil releases carbon dioxide into the atmosphere, the amount of carbon-14 isotope found in atmospheric carbon dioxide should decrease over time. Measurements of the isotopic composition of atmospheric carbon dioxide do indeed demonstrate a steady decline of carbon-14.  Furthermore, fossil fuel also contains a much lower amount of carbon-13 than does the atmosphere.  Over time the amount of carbon-13 found in atmospheric carbon dioxide has decreased. 

Clearly, the atmosphere’s carbon isotopic composition is changing and this change matches the isotope fingerprint of coal, oil and natural gas.  This demonstrates that the burning of fossil fuel is partly responsible for the current atmospheric carbon overload. 

There are three broad types of human activities that contribute to the amount of greenhouse gases (carbon dioxide, methane and nitrous oxide) in the atmosphere: carbon dioxide emissions from the burning of fossil fuels; carbon dioxide emissions from cutting and burning of forest; and, the emission of methane (from livestock and rice cultivation) and nitrous oxide (from fertilizer use).  Burning of fossil fuel accounts for 52-65 percent of the human-induced emissions while deforestation accounts for 12-25 percent and 23 percent comes from methane and nitrous oxide.   

It is easy to understand that we can reduce the atmospheric level of carbon dioxide by converting to non-carbon renewable energy sources.  Stopping deforestation will also reduce carbon dioxide emissions.  In addition, the creation of new forest by planting trees will also help reduce the atmospheric level of carbon dioxide because trees extract carbon dioxide from the atmosphere for photosynthetic activity.  However, continued population growth will make the task of reducing methane and nitrous oxide emissions and deforestation more difficult because the expanding population will require more resources.  It is becoming increasingly apparent that the world population needs to stabilize. 

3/10/2019 1:28:07 PM

You mention an amount the atmosphere can handle. I've seen two sources (Skeptical Science and New Scientist) that say from 750 to 770 GT is what is exchanged each year.Included in that exchange is about half (20 GT) of our emissions. Here's my problem: we emit about 40 GT a year which is around 4% of the total annual emissions. Multiply that times 25 (ratio of 25:1), and you get 1 trillion tons. Subtract our 20 GT that are not absorbed, and you have 980 GT. The difference between 980 and 770 is 210. Those 210 cannot be our cumulative, as we are talking annual. According to the many studies and reports I've seen, it is assumed nature absorbs all it emits, and predictions have been based on that assumption.So, who does the 210 belong to? NATURE. We now have a ratio of 10.5:1, with nature in the lead. So, how is our CO2 responsible for temperature and CO2 increases, when nature emits far more unabsorbed CO2 than we do? Taking the calculation, if you fill a tube with air, and measure the CO2 in it, should we not find 10.5 times more of nature's CO2 than ours? After all, we are told that CO2 is well mixed in the air. In addition, I've seen reports that say nature does not know the difference between a carbon 12 atom it emits from a carbon atom man emits.Therefore,it just absorbs carbon 12 atoms. If that is so, how do we know that nature consistently absorbs about half of our emissions?

2/8/2015 9:27:56 AM

Good article, answers an objection that thinking CD's (Climate Deniers) raise. I would suggest 2 small editting changes. You say: " After 60,000 years fossil fuel contains only carbon-12 (all of the carbon-14 has been converted to nitrogen)." 60,000 years is only 11 half lives of carbon 14, so, though 99.95% of the carbon 14 would be gone, there would still be gazillions of carbon 14 atoms in a gallon of extracted fossil fuel. But there is no need to use 60,000 years as your time period. Since the last major episode of fossil fuel creation was 90,000,000 years ago, that would be a realistic time period (60,000 seeming, to me, to be arbitrary). In that case, we're looking at sixteen thousand half lives which means that carbon 14 levels will have been reduced to a "too small to measure" level on the finest mass spectrometers, giving CD's no toehold to stand on. I would also mention the typo: " Since it takes millions of years to create fossil fuel, the carbon dioxide that is released into the atmosphere from the burning of fossil fuel would no carbon-14." This would be worth fixing to maximize the respect afforded you by your readers. Yours, a humble science teacher, that could not help falling back into "paper correcting mode" : ), Bruce Ratcliffe

3/17/2014 10:02:19 AM


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