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

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