How do isotopic signatures (carbon-13/carbon-12) show fossil fuel contributions to atmospheric CO2?
Executive summary
Isotopic ratios of carbon—principally the stable 13C/12C ratio expressed as δ13C—act as a fingerprint that reveals the addition of CO2 from fossil fuels because fossil carbon is consistently depleted in 13C relative to the atmosphere; burning those fuels lowers atmospheric δ13C in a pattern known as the 13C Suess effect [1] [2]. Radiocarbon (14C) provides a complementary, even cleaner tracer because fossil fuels contain essentially no 14C, so mixing fossil-CO2 into the atmosphere drives Δ14C toward more negative values [3] [4].
1. How the isotopic fingerprints work: δ13C and Δ14C explained
Carbon comes in three naturally occurring isotopes—12C, ~1% 13C, and trace 14C—and scientists report 13C/12C as δ13C and 14C relative abundances as Δ14C, so changes in those numbers reveal shifts in source mixtures [1] [5]. Plants preferentially take up 12C during photosynthesis, so plant-derived carbon (and therefore most fossil fuels made from ancient biomass) is depleted in 13C relative to atmospheric CO2; typical fossil-fuel δ13C values range roughly −44‰ to −19‰ depending on fuel type and origin [3] [6]. Because fossil fuels are millions of years old, their 14C has decayed away and they carry essentially no measurable 14C (Δ14C ≈ −1000‰), so added fossil CO2 dilutes atmospheric 14C very clearly [3] [4].
2. The Suess effect: the observed isotopic signal of fossil emissions
The progressive drop in atmospheric 13C/12C and 14C/12C since the Industrial Revolution is called the Suess effect and is explained by large additions of 13C- and 14C‑depleted fossil carbon into the atmosphere; observational records and model reconstructions link that decline primarily to fossil-fuel combustion and cement manufacture [7] [1] [6]. Long-term monitoring shows the atmosphere’s δ13C has declined from preindustrial values (around −6.5‰) toward more negative values today as CO2 concentrations rose, a pattern consistent with addition of plant‑derived fossil carbon [2] [1].
3. Practical attribution: mixing models and end‑member fingerprints
Attribution uses mixing equations: measured δ13C (and Δ14C where available) in an air sample is treated as a weighted average of source end‑members, so knowing the δ13C signatures of fossil fuels, biosphere fluxes and ocean exchange allows estimation of fossil CO2 fractions [8] [9]. Urban and regional studies combine δ13C and Δ14C (or CO and other tracers) to separate petroleum vs. natural gas vs. biogenic CO2 and quantify local fossil contributions, but the approach depends on accurate local end‑member values because fuel isotopic distributions vary by geography and type [10] [11].
4. Complementary use of 14C and limits of 13C alone
Radiocarbon is a powerful binary test—fossil = no 14C—so Δ14C measurements can directly quantify fossil fractions, but 14C assays are lab‑intensive and expensive, limiting continuous monitoring [9] [8]. δ13C is more practical for routine sampling but is less unambiguous because biospheric CO2 is also 13C‑depleted and terrestrial photosynthetic discrimination has changed over time; recent work shows that changes in plant discrimination can influence the atmospheric δ13C trend and must be accounted for when attributing the full signal to fossil fuel inputs [12].
5. Uncertainties, alternative interpretations, and how science tests them
Some authors have argued that biosphere changes could explain δ13C declines without fossil inputs, but critics and broader analyses show such claims often ignore exchanges with oceans and the known isotopic fingerprint and magnitude of fossil emissions; independent lines of evidence (Δ14C, observed fossil emission inventories, mixing models, and global monitoring) reinforce the fossil attribution [13] [4] [1]. Remaining uncertainties are mostly quantitative—how much different regions’ biospheres and oceans offset or amplify the signal, and how spatial variation in fuel δ13C affects local source apportionment—so studies combine 13C, 14C, other gases (CO), and atmospheric transport models to constrain contributions [11] [14] [10].