Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future. Issue 11 (15th November 2020)

• Record Type: Journal Article
• Authors: Graven, Heather; Keeling, Ralph F.; Rogelj, Joeri
• Is Part Of: Global biogeochemical cycles. Volume 34:Issue 11(2020:Nov.)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-15
• Subjects: carbon dioxide -- radiocarbon -- carbon‐13 -- fossil fuels -- nuclear bombs -- carbon cycle; Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1029/2019GB006170
• Languages: English
• ISSNs: 0886-6236

Abstract

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