Changes to Carbon Isotopes in Atmospheric CO₂ Over the Industrial Era and Into the Future

In this "Grand Challenges" paper, we review how the carbon isotopic composition of atmospheric CO₂ has changed since the Industrial Revolution due to human activities and their influence on the natural carbon cycle, and we provide new estimates of possible future changes for a range of scenarios. Emissions of CO₂ from fossil fuel combustion and land use change reduce the ratio of ¹³C/¹²C in atmospheric CO₂ (δ¹³CO₂). This is because ¹²C is preferentially assimilated during photosynthesis and δ¹³C in plant-derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric δ¹³CO₂. Emissions of CO₂ from fossil fuel combustion also reduce the ratio of ¹⁴C/C in atmospheric CO₂ (Δ¹⁴CO₂) because ¹⁴C is absent in million-year-old fossil fuels, which have been stored for much longer than the radioactive decay time of ¹⁴C. Atmospheric Δ¹⁴CO₂ rapidly increased in the 1950s to 1960s because of ¹⁴C produced during nuclear bomb testing. The resulting trends in δ¹³C and Δ¹⁴C in atmospheric CO₂ are influenced not only by these human emissions but also by natural carbon exchanges that mix carbon between the atmosphere and ocean and terrestrial ecosystems. This mixing caused Δ¹⁴CO₂ to return toward preindustrial levels in the first few decades after the spike from nuclear testing. More recently, as the bomb ¹⁴C excess is now mostly well mixed with the decadally overturning carbon reservoirs, fossil fuel emissions have become the main factor driving further decreases in atmospheric Δ¹⁴CO₂. For δ¹³CO₂, in addition to exchanges between reservoirs, the extent to which ¹²C is preferentially assimilated during photosynthesis appears to have increased, slowing down the recent δ¹³CO₂ trend slightly. A new compilation of ice core and flask δ¹³CO₂ observations indicates that the decline in δ¹³CO₂ since the preindustrial period is less than some prior estimates, which may have incorporated artifacts owing to offsets from different laboratories' measurements. Atmospheric observations of δ¹³CO₂ have been used to investigate carbon fluxes and the functioning of plants, and they are used for comparison with δ¹³C in other materials such as tree rings. Atmospheric observations of Δ¹⁴CO₂ have been used to quantify the rate of air-sea gas exchange and ocean circulation, and the rate of net primary production and the turnover time of carbon in plant material and soils. Atmospheric observations of Δ¹⁴CO₂ are also used for comparison with Δ¹⁴C in other materials in many fields such as archaeology, forensics, and physiology. Another major application is the assessment of regional emissions of CO₂ from fossil fuel combustion using Δ¹⁴CO₂ observations and models. In the future, δ¹³CO₂ and Δ¹⁴CO₂ will continue to change. The sign and magnitude of the changes are mainly determined by global fossil fuel emissions. We present here simulations of future δ¹³CO₂ and Δ¹⁴CO₂ for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model Intercomparison Project (CMIP6). Applications using atmospheric δ¹³CO₂ and Δ¹⁴CO₂ observations in carbon cycle science and many other fields will be affected by these future changes. We recommend an increased effort toward making coordinated measurements of δ¹³C and Δ¹⁴C across the Earth System and for further development of isotopic modeling and model-data analysis tools.