Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century

Abstract

Radiocarbon analyses are commonly used in a broad range of fields, including earth science, archaeology, forgery detection, isotope forensics, and physiology. Many applications are sensitive to the radiocarbon ((14)C) content of atmospheric CO2, which has varied since 1890 as a result of nuclear weapons testing, fossil fuel emissions, and CO2 cycling between atmospheric, oceanic, and terrestrial carbon reservoirs. Over this century, the ratio (14)C/C in atmospheric CO2 (Δ(14)CO2) will be determined by the amount of fossil fuel combustion, which decreases Δ(14)CO2 because fossil fuels have lost all (14)C from radioactive decay. Simulations of Δ(14)CO2 using the emission scenarios from the Intergovernmental Panel on Climate Change Fifth Assessment Report, the Representative Concentration Pathways, indicate that ambitious emission reductions could sustain Δ(14)CO2 near the preindustrial level of 0‰ through 2100, whereas "business-as-usual" emissions will reduce Δ(14)CO2 to -250‰, equivalent to the depletion expected from over 2,000 y of radioactive decay. Given current emissions trends, fossil fuel emission-driven artificial "aging" of the atmosphere is likely to occur much faster and with a larger magnitude than previously expected. This finding has strong and as yet unrecognized implications for many applications of radiocarbon in various fields, and it implies that radiocarbon dating may no longer provide definitive ages for samples up to 2,000 y old.

Keywords: 14C dating; atmospheric CO2; fossil fuel emissions; isotope forensics; radiocarbon.

Fig. 1.

Model predictions of atmospheric radiocarbon for the RCPs. (Left) Atmospheric CO₂ concentration (Top), CO₂ emissions from fossil fuel combustion (Middle), and CO₂ emissions from land use change (Bottom) in the RCP scenarios (11, 34). (Middle) Fossil fuel CO₂ emitted to the atmosphere is shown with solid lines; dashed lines show net fossil fuel CO₂ emitted, including “negative emissions” from biomass energy with carbon capture and storage. (Right) Observed (7, 10, 18, 35, 36) (1940–2012) and projected (2005–2100) radiocarbon content of atmospheric CO₂ (Δ¹⁴CO₂) (Table S2). The right axis shows the conventional radiocarbon age of a carbon-containing specimen with the same radiocarbon content, calculated by 8033 * ln (Δ¹⁴C/1,000 + 1). Filled areas indicate the range simulated for different sets of model parameters, each consistent with 20th-century atmospheric and oceanic Δ¹⁴C and CO₂ observations, within their uncertainties (10) (SI Text).

Fig. 2.

Simulation of radiocarbon in various carbon reservoirs for the low-emission and business-as-usual RCPs. Simulated Δ¹⁴C in biospheric and four ocean carbon reservoirs, surface mixed layer, 300, 600, and 3,500 m in RCP2.6 (Left) and RCP8.5 (Right). The black line shows the midrange value of simulated atmospheric Δ¹⁴CO₂ (full simulated range of Δ¹⁴CO₂ is shown in Fig. 1).

Fig. S1.

Simulated radiocarbon inventories, relative to 1950, in atmospheric, oceanic, and biospheric reservoirs for the simulations shown in Figs. 1 and 2. The atmospheric radiocarbon inventory over 1950–2005 is shown in each panel.