Isotopes – fingerprints in climate change, or how we know, what we know

By John Atcheson

Isotopes tell us a great deal about where carbon and methane are coming from.  They act as fingerprints that identify with a great deal of accuracy whether the source of increased greenhouse gasses in the atmosphere are caused by humans, or are by-products of natural processes.  

The first thing we need to do is define what an Isotope is.  Elements are classified by how many protons they have in their nucleus.  Thus, every carbon atom has six protons. But carbon atoms can have six, seven, or eight neutrons, and neutrons weigh essentially the same as a proton. This means that carbon has three naturally occurring isotopes: C12, C13, and C14.  The numeric designations are the atomic weight of an element and they are the sum of both the protons and the neutrons. Since neutrons don’t have a charge, the elements share the same chemical properties. In the case of carbon, C12 and C13 are stable, while C14 decays over time.

The source of the carbon or methane gives it a distinct ratio of Isotopes, and in the case of methane, we can get additional data from hydrogen and oxygen isotopes (the latter because methane reacts in the atmosphere). 

Emissions have a distinct ratio of C12 to C13, depending upon their source.  Carbon from fossil fuel combustion has a lower C-12 to C-13 ratio than the air does. Thus, if the observed increased carbon in the atmosphere comes from fossil fuels or combustion of plants, the ratio of C12 to C13 in the atmosphere should be going down in proportion to the increase in atmospheric carbon.  And it is.

Since methane contains carbon — its formula is CH4, or 1 carbon atom combined with 4 hydrogen atoms – the ratio of C12 to C13 in the emissions and the ratio in the atmosphere can also help us identify the source of those emissions. In addition, Hydrogen has several isotopes that are indicative of the source of the methane, and since methane is highly reactive in the atmosphere oxygen isotopes also tell us something about the source. Now, methane has more potential sources than carbon, and as it is highly reactive in the atmosphere, the fingerprinting gets complex, it can be done.  

What we know about methane is that  there are three main sources contributing to the increase: 1) the output of microbes living in anoxic environments such as wetlands, landfills, and the stomachs and butts of ruminants; 2) fossil methane in gas, coal, and other underground fuel reserves that are released as those reserves are exploited; and 3) the burning of vegetation like forests, bush, and crop residues. 

The shocking thing about methane is that all three sources are increasing, and that humans are contributing to that increase.

Isotopes tell us that microbial sources are the largest cause for an increase in atmospheric concentrations of methane, and human cultivation of rice crops, livestock, and disposal in landfills are contributing to that increase.  Fracking and increased use and transmission of fossil fuels are the second big contributor, and of course, humans are the source of it. Finally, the explosion in wildfires that now occur everywhere on Earth from the Arctic to the Amazon, are a result of anthropogenic warming and poor land-use management.

The next time someone tries to tell you we can’t be sure where the increased emissions are coming from – or whether there’s been an increase – tell them they’re wrong.  The evidence is in the fingerprints.

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