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 C-13/C-12 in atmospheric CO2 (delta(CO2)-C-13). This is because C-12 is preferentially assimilated during photosynthesis and delta C-13 in plant-derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric delta(CO2)-C-13. Emissions of CO2 from fossil fuel combustion also reduce the ratio of C-14/C in atmospheric CO2 (Delta(CO2)-C-14) because C-14 is absent in million-year-old fossil fuels, which have been stored for much longer than the radioactive decay time of C-14. Atmospheric Delta(CO2)-C-14 rapidly increased in the 1950s to 1960s because of C-14 produced during nuclear bomb testing. The resulting trends in delta C-13 and Delta C-14 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 Delta(CO2)-C-14 to return toward preindustrial levels in the first few decades after the spike from nuclear testing. More recently, as the bomb C-14 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 Delta(CO2)-C-14. For delta(CO2)-C-13, in addition to exchanges between reservoirs, the extent to which C-12 is preferentially assimilated during photosynthesis appears to have increased, slowing down the recent delta(CO2)-C-13 trend slightly. A new compilation of ice core and flask delta(CO2)-C-13 observations indicates that the decline in delta(CO2)-C-13 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 delta(CO2)-C-13 have been used to investigate carbon fluxes and the functioning of plants, and they are used for comparison with delta C-13 in other materials such as tree rings. Atmospheric observations of Delta(CO2)-C-14 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 Delta(CO2)-C-14 are also used for comparison with Delta C-14 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 Delta(CO2)-C-14 observations and models. In the future, delta(CO2)-C-13 and Delta(CO2)-C-14 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 delta(CO2)-C-13 and Delta(CO2)-C-14 for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model Intercomparison Project (CMIP6). Applications using atmospheric delta(CO2)-C-13 and Delta(CO2)-C-14 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 delta C-13 and Delta C-14 across the Earth System and for further development of isotopic modeling and model-data analysis tools.