Saturday, June 8, 2013

Soda Pop and Your Carbon Footprint

Published in the Sun June 8, 2013

In the discussion of climate change, why do you ignore non-fossil fuel sources of carbon dioxide emissions such as the breath of humans and animals, wildfires, campfires, and carbonated drinks?


SG, Georgetown, TX


Let’s look at the carbonated drinks first.  The carbon dioxide (CO2) that makes the satisfying fizz when you pop a top is actually a product of the petrochemical industry, so drinking soda does contribute to your carbon footprint, but just a tiny amount.  A 12 ounce can of soda contains about 2.2 grams of CO2.  You would have to drink 400 gallons of soda to equal the amount of CO2 released from burning one gallon of gasoline in your car.


What about breathing?  Seven billion people exhale a lot of carbon dioxide.  Fortunately, the CO2 that we exhale comes from the breakdown of food that we burn for energy.  And the food that we ate came from plants, which made the carbohydrates from CO2 taken out of the atmosphere.  This recycling of carbon means that the CO2 going into and out of plants and animals has no net effect on the total amount of CO2 in the atmosphere.  Wildfires and campfires are part of the same carbon cycle.  Plant material (wood) is burned in the presence of oxygen and turned into carbon dioxide, but without passing through the inside of an animal.


The carbon dioxide that results from burning oil or coal is a different matter entirely.  The plants and animals that were destined to become fossil fuels were buried underground 300 million years ago.  They had removed carbon from an atmosphere that was higher in carbon dioxide than our atmosphere today.  When we dig up petroleum and coal to power our cars and homes, we release that ancient carbon that was sequestered away so long ago and the total amount of carbon dioxide in the atmosphere increases.


Just as scientists can judge the age of an ancient bone or rock by measuring the decay of radioactive carbon-14 (carbon dating), carbon-14 can tell us whether atmospheric carbon dioxide came from living plants and animals or whether it came from ancient deposits of petroleum or coal.  Ninety-nine percent of the carbon in the world exists as the stable isotope carbon-12.  A few carbon atoms in the atmosphere are hit by cosmic rays from the sun and become radioactive carbon-14.  That means that the carbon breathed in and out by modern plants and animals contains a tiny (one in a trillion) amount of carbon-14.  In contrast, the carbon in fossil fuels has been stored away for millions of years, and the radioactive carbon-14 has had time to completely decay into non-radioactive isotopes.  The CO2 that comes from burning fossil fuels contains no carbon-14.  As the levels of CO2 have risen over the last century, the proportion of carbon-14 has decreased, indicating that the new CO2 in the atmosphere is coming from ancient sources.


Last month the level of CO2 in the atmosphere hit 400 parts per million, a 43% increase since the mid 1850’s.  The last time carbon dioxide levels were this high was 2.5 million years ago, at which time there were no human beings.  It will be quite an interesting experiment to see what happens as CO2 levels continue to rise.

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