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How do we know the CO2 comes from fossil fuels

by Paul Kando

Carbon dioxide (CO2) in the atmosphere is like a planetary sweater. If Earth had no atmosphere, the heat energy gained from solar radiation would escape directly back into space. Atmospheric gas molecules, like CO2, keep that from happening. When infrared light hits a CO2 molecule, the molecule absorbs the light, trapping the heat, and keeping the planet comfortably warm. Too little CO2 would let the planet cool down; too much overheats the planet, changing its life sustaining climate. The same can be said of other greenhouse gases, including methane, nitrous oxide and water vapor.

Ice Core
photo credit: Heidi Roop, National Science Foundation

A quarter of the CO2 in the atmosphere comes from fossil fuels and the percentage is growing. How do we know this? – by looking at the relative proportions of different carbon isotopes in the atmosphere. All elements have a characteristic number of protons in their atoms. Isotopes are variants with the same number of protons but a different number of neutrons in each atom, impacting the atomic weight.

About 99% of the carbon in the atmosphere is Carbon-12, with the lightest mass or weight. The rest is Carbon-13 and Carbon-14. The relative abundance of these three isotopes tells us a lot about the source of the carbon in a given CO2 molecule. Since humans started adding CO2 to the atmosphere by fossil fuel burning and deforestation, the percentage of these slightly heavier carbon atoms in the atmosphere has gotten smaller.

Carbon-14 is radioactive and over time it decays into nitrogen. It takes over 5,700 years for half of it to decay: the so-called half life of Carbon-14. This means that the amount stored in plant tissue decreases over time. Fossil fuels like coal and oil are mostly decomposed remains of plants that lived millions of years ago. Since these remains are so old, virtually all their Carbon-14 content has changed into nitrogen. So, as we add CO2 to the atmosphere by burning fossil fuels, we increase the total amount of CO2, but not of Carbon-14. Since humans started burning fossil fuels over two centuries ago, the percentage of atmospheric Carbon-14 has gotten smaller. This shrinking percentage confirms fossil fuel burning’s contribution to the increase of atmospheric CO2.

Carbon-13 is not radioactive, but it too provides useful information. Carbon-13 CO2 molecules are larger than those of Carbon-12, and plants have more difficulty turning them into food. For this reason, plants contain a smaller fraction of Carbon-13 than the atmosphere. And because fossil fuels come from plants, they too contain less Carbon-13 than the atmosphere. Indeed, with the burning of fossil fuels the atmospheric percentage of Carbon-13 has been decreasing.

Tracking these two heavier carbon atoms, by analyzing air bubbles trapped in ice cores, tells us that about 25% of the CO2 now in the atmosphere came from the burning of fossil fuels and the destruction of living plants, e.g., by clearing rain forests. This total is about the same amount that scientists have estimated should be in the atmosphere, based on how much mankind has burned – another reason for confidence that the extra CO2 in the atmosphere has human origins.

The oldest ice cores provide an 800,000-year-old record of Earth’s climate. We know they’re that old because each season’s snowfall has slightly different properties than the last. These differences create annual layers in the ice that can be used to count the age of the ice, just like annual rings reveal the age of a tree. The ice core shown contains a dark layer of volcanic ash that settled on the ice sheet some 21,000 years ago.

Only we humans can stop the flow of excess CO2. Alas we cannot stop the feedback effects our contributions have already unleashed. The longer we keep burning fossil fuels, the more we compound the climate problem our children and grandchildren will face. Denying this may be convenient – but isn’t it also, literally, a crime against humanity?