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

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Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future UC San Diego UC San Diego Previously Published Works Title Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future. Permalink https://escholarship.org/uc/item/3jr069wc Journal Global biogeochemical cycles, 34(11) ISSN 0886-6236 Authors Graven, Heather Keeling, Ralph F Rogelj, Joeri Publication Date 2020-11-15 DOI 10.1029/2019gb006170 Peer reviewed eScholarship.org Powered by the California Digital Library University of California COMMISSIONED Changes to Carbon Isotopes in Atmospheric CO2 Over MANUSCRIPT the Industrial Era and Into the Future 10.1029/2019GB006170 Heather Graven1,2 , Ralph F. Keeling3 , and Joeri Rogelj2,4 Key Points: 1 2 • Carbon isotopes, 14C and 13C, in Department of Physics, Imperial College London, London, UK, Grantham Institute for Climate Change and the 3 atmospheric CO2 are changing in Environment, Imperial College London, London, UK, Scripps Institution of Oceanography, University of California, San response to fossil fuel emissions and Diego, La Jolla, CA, USA, 4ENE Program, International Institute for Applied Systems Analysis, Laxenburg, Austria other human activities • Future simulations using different SSPs show continued changes in “ ” isotopic ratios that depend on fossil Abstract In this Grand Challenges paper, we review how the carbon isotopic composition of 13 fuel emissions and, for C, atmospheric CO2 has changed since the Industrial Revolution due to human activities and their influence on BECCS 14 the natural carbon cycle, and we provide new estimates of possible future changes for a range of • Applications using atmospheric C 13 12 and 13C in studies of the carbon scenarios. Emissions of CO2 from fossil fuel combustion and land use change reduce the ratio of C/ Cin 13 12 cycle or other fields will be affected atmospheric CO2 (δ CO2). This is because C is preferentially assimilated during photosynthesis and by future changes 13 13 δ C in plant‐derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric δ CO2. 14 14 Emissions of CO2 from fossil fuel combustion also reduce the ratio of C/C in atmospheric CO2 (Δ CO2) Supporting Information: 14 ‐ ‐ • Supporting Information S1 because C is absent in million year old fossil fuels, which have been stored for much longer than the 14 14 14 • Table S1 radioactive decay time of C. Atmospheric Δ CO2 rapidly increased in the 1950s to 1960s because of C • Table S2 13 14 produced during nuclear bomb testing. The resulting trends in δ C and Δ C in atmospheric CO2 are influenced not only by these human emissions but also by natural carbon exchanges that mix carbon 14 between the atmosphere and ocean and terrestrial ecosystems. This mixing caused Δ CO2 to return toward Correspondence to: fi H. Graven, preindustrial levels in the rst few decades after the spike from nuclear testing. More recently, as the 14 [email protected] bomb C excess is now mostly well mixed with the decadally overturning carbon reservoirs, fossil fuel 14 13 emissions have become the main factor driving further decreases in atmospheric Δ CO2. For δ CO2,in 12 Citation: addition to exchanges between reservoirs, the extent to which C is preferentially assimilated during 13 Graven, H., Keeling, R. F., & Rogelj, J. photosynthesis appears to have increased, slowing down the recent δ CO2 trend slightly. A new (2020). Changes to carbon isotopes in 13 13 compilation of ice core and flask δ CO2 observations indicates that the decline in δ CO2 since the atmospheric CO2 over the industrial era and into the future. Global preindustrial period is less than some prior estimates, which may have incorporated artifacts owing to offsets 13 Biogeochemical Cycles, 34, from different laboratories' measurements. Atmospheric observations of δ CO2 have been used to e2019GB006170. https://doi.org/ investigate carbon fluxes and the functioning of plants, and they are used for comparison with δ13C in other 10.1029/2019GB006170 14 materials such as tree rings. Atmospheric observations of Δ CO2 have been used to quantify the rate of ‐ Received 7 FEB 2020 air sea gas exchange and ocean circulation, and the rate of net primary production and the turnover time of 14 Accepted 16 OCT 2020 carbon in plant material and soils. Atmospheric observations of Δ CO2 are also used for comparison Accepted article online 23 OCT 2020 with Δ14C in other materials in many fields such as archaeology, forensics, and physiology. Another major 14 application is the assessment of regional emissions of CO2 from fossil fuel combustion using Δ CO2 13 14 observations and models. In the future, δ CO2 and Δ CO2 will continue to change. The sign and magnitude of the changes are mainly determined by global fossil fuel emissions. We present here 13 14 simulations of future δ CO2 and Δ CO2 for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model Intercomparison Project (CMIP6). Applications using atmospheric 13 14 δ CO2 and Δ 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 δ13C and Δ14C across the Earth System and for further development of isotopic modeling and model‐data analysis tools. 1. Introduction ©2020. The Authors. This is an open access article under the Carbon isotopes are present in the atmosphere, ocean, and terrestrial biosphere in ratios of approximately 12 13 −12 14 12 13 14 terms of the Creative Commons 99% C/C, 1% C/C, and 1 × 10 C/C. C and C are stable isotopes while C is a radioactive isotope Attribution License, which permits use, called radiocarbon. Radiocarbon is formed naturally in the upper atmosphere from cosmogenic radiation, distribution and reproduction in any medium, provided the original work is which produces neutrons that react with atmospheric nitrogen. Because the isotopic composition of carbon properly cited. is affected by physical, chemical, and biological processes, these ratios are not constant, and they vary GRAVEN ET AL. 1of21 Global Biogeochemical Cycles 10.1029/2019GB006170 across different carbon pools and over time and space. Precise measurements of small differences in these ratios, together with theoretical or empirical models of isotopic fractionation and mixing, enable the inves- tigation of various aspects of the carbon cycle. Observing and analyzing the changes in carbon isotopic com- position of atmospheric CO2 can help to understand the natural carbon cycle's response to human activities. The notation δ13C refers to the deviation of the ratio 13C/12C in a sample from a standard ratio 13C/12C, referred to as Vienna Pee Dee Belemnite (VPDB). Typical measurement precision is ±0.01–0.03‰ for atmo- 13 spheric CO2. The primary international reference material for δ C is calcite (IAEA‐603 and, formerly, NBS19). Calcite must be converted to CO2 to implement the VPDB scale at individual laboratories, which has been shown to result in significant laboratory offsets (WMO/IAEA, 2003). Current activities to address measurement compatibility include the distribution of pure CO2 or CO2 in whole air reference materials (Brand et al., 2009; Wendeberg et al., 2013; WMO/IAEA, 2018), but achieving long‐term compatibility of 13 δ C measurements in atmospheric CO2 made at different laboratories remains a challenge, and laboratory offsets must be considered when compiling data (see section 5). The notation used for 14CisΔ14C, which is similar to the definition of δ13C in that it refers to deviations from a standard ratio termed “Modern.” The notation Δ14C includes a correction for radioactive decay in samples of known age and a correction for mass‐dependent fractionation, defined as Δ in Stuiver and Polach (1977). Assuming that any process discriminating against 13C will discriminate approximately twice as strongly against 14C, measurements of δ13C in a sample can be used to correct for mass‐dependent fractionation. This enables Δ14C to uncover effects that are unrelated to simple fractionation processes. Typical measure- 14 ment precision is ±2–3‰ for atmospheric CO2. Reference material used for Δ C measurements is typically oxalic acid (Stuiver, 1983), but whole air reference materials have also been used for atmospheric measure- ments (Graven et al., 2012b). Whole air and CO2 have been used in intercomparisons between radiocarbon laboratories making atmospheric measurements and generally showed compatibility of 2‰ or better (Hammer et al., 2017; Miller et al., 2013), in addition to intercomparison activities using wood cellulose and other materials (e.g., Scott et al., 2010). 13 14 In this paper, we review the observed changes in the C and C isotopic composition of atmospheric CO2 13 14 (δ CO2 and Δ CO2) over the Industrial Period and the processes driving these changes. We review key 13 14 applications for atmospheric δ CO2 and Δ CO2 observations from the literature, with an emphasis on glo- 13 bal or large‐scale processes. Then we present new simulations of future changes in atmospheric δ CO2 and 14 Δ CO2 corresponding to future emission scenarios through 2100. We discuss the impacts of these future 13 14 changes on applications for atmospheric δ CO2 and Δ CO2 observations and make recommendations for observational and modeling activities for δ13C and Δ14C. 2. The 14C and 13C Suess Effects The onset of the Industrial Revolution initiated extensive fossil fuel burning that introduced carbon pre- viously stored in geological reservoirs into the atmosphere. Fossil fuels are completely devoid of 14C because they have been stored in geological reservoirs for millions of years, much longer than the 14C half‐life of 5,700 years. This gives fossil fuels a Δ14C signature of −1,000‰. For 13C, the carbon in fossil fuels has an isotopic signature (δ13C) that ranges from −44‰ to −19‰ (Andres et al., 2000).
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