this post was submitted on 18 Aug 2023
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Collapse
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i'm looking at figure 3 in the academic source.
delta carbon-13 is a measure of the ratio of carbon-12 vs. carbon-13 in a sample, in this case in samples of atmospheric methane gas. the lower the value, the fewer carbon-13 isotopes there are relative to carbon-12 isotopes. carbon-13 is stable, it doesn't radioactively decay.
https://en.wikipedia.org/wiki/%CE%9413C
so if you can date samples of atmosphere trapped in ice cores and measure the amount and isotopic composition of the methane then you can construct a graph like that figure 3. different processes result in distinct isotopic signatures for the methane released into the atmosphere. what their chart shows is that from about 1100 c.e. to 1900 c.e. the delta carbon-13 ratio dropped by about 2 per-mil while atmospheric methane concentration rose, which they attribute to a decrease in biomass burning sources of methane and an increase in agricultural sources of methane.
then in 1900 c.e. there's a big increase in atmospheric methane along with a big increase in delta carbon-13. fossil fuel sources of methane are much heavier than natural sources, so as trapped methane was released into the atmosphere it produced a sharp change in the isotopic ratio.
but then about 15 years ago the isotopic signature sharply reversed course even as methane concentration started suddenly rising more rapidly. so something weird is going on these past two decades where we've possibly already started feedback loops that result in releasing trapped permafrost methane, for example. the paper suggests it's mostly from tropical wetlands, but the focus of the paper is on the isotopic observations.