For the past two centuries, the amount of carbon dioxide in the atmosphere has been rising from 280ppm to over 390ppm in 2010. This is an incontrovertible fact. What has been called into question in recent years is the source of this carbon dioxide; it has been asserted by some that humanity is not responsible for this increase. Yet this position not only makes no sense, it is easily disproved.
The first line of evidence is what most people are aware of. The logic is straightforward: Humanity has been burning large amounts of fossil fuels for energy for the past two centuries. The burning of these fossil fuels releases carbon dioxide and other greenhouse gases into the atmosphere. Over the same time period, atmospheric levels of carbon dioxide have risen by exactly the same amount as has been released by fossil fuel burning. Therefore, the burning of fossil fuels is responsible for the rise in carbon dioxide. QED.
However, this is not enough for some people; they want more evidence. Climate scientists were happy to oblige. But before I can get into two further lines of evidence, I need to review some high school chemistry.
All atoms are made up of protons, neutrons, and electrons. Protons have a positive charge, neutrons no charge, and electrons a negative charge. The number of protons and electrons determines what element the atom is. For example, oxygen always has 8 protons and 8 electrons. If it had 9 protons and 9 neutrons, it would be fluorine. However, while atoms must always have the same number of protons and electrons, the number of neutrons can vary. With oxygen, most atoms have 8 neutrons. However, a small percentage of atoms have 10 neutrons. These different versions of an atom of the same element are known as isotopes.
The element in question with global warming is carbon. The most common isotope of carbon is 12C, which has 6 protons, 6 neutrons, and 6 electrons. This isotope accounts for 98.89% of all carbon on Earth. A second isotope is 13C, which has 6 protons, 7 neutrons, and 6 electrons and accounts for 1.1% of all carbon on Earth. And finally, there is 14C, which is made up of 6 protons, 8 neutrons, and 6 electrons and is only a tiny fraction of all carbon on Earth.
These isotopes matter because fossil fuels are depleted in 13C and 14C. Thus, we can predict that, if fossil fuels are the reason carbon dioxide levels in the atmosphere have risen, the atmospheric concentrations of 13C and 14C must both be falling. But why are fossil fuels depleted in these isotopes?
To explain 13C, we have to remember that coal is made when dead plants do not decay. This is significant because, during photosynthesis, plants process carbon and the isotope of that carbon matters. Plants “prefer” 12C to 13C because 12C is easier to process. Therefore, plants have a smaller percentage of 13C than is found in the environment. This means that coal is also depleted in 13C.
14C is very different. This isotope is significant because it is radioactive, meaning that it decays over time. Every 5,730 years the amount of 14C present is halved. This means that fossil fuels that are millions of years old will have no 14C left.
So what does the data tell us? Below are graphs of the concentrations of 13C and 14C respectively.
As you can see, the concentrations of both isotopes have been decreasing. There is no other possible explanation for this than that the burning of fossil fuels has caused the rise in carbon dioxide in the atmosphere over the past two centuries.