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 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.

中文翻译：

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工业时代和未来大气中CO 2 碳同位素的变化

摘要 在这篇“重大挑战”论文中，我们回顾了自工业革命以来大气二氧化碳的碳同位素组成如何因人类活动而发生变化及其对自然碳循环的影响，并对一系列未来可能发生的变化提供了新的估计。场景。化石燃料燃烧和土地利用变化产生的 CO2 排放降低了大气 CO2 中 13C/12C 的比率 (δ13CO2)。这是因为 12C 在光合作用过程中优先被同化，并且陆地生态系统和化石燃料中植物源碳中的 δ13C 低于大气中的 δ13CO2。化石燃料燃烧排放的 CO2 也会降低大气 CO2 中 14C/C 的比率 (Δ14CO2)，因为百万年历史的化石燃料中不存在 14C，而这些化石燃料的储存时间远长于 14C 的放射性衰变时间。由于核弹试验过程中产生 14C，大气中的 Δ14CO2 在 20 世纪 50 年代至 1960 年代迅速增加。由此产生的大气二氧化碳 δ13C 和 Δ14C 趋势不仅受到这些人类排放的影响，还受到大气、海洋和陆地生态系统之间混合碳的自然碳交换的影响。这种混合导致 Δ14CO2 在核试验激增后的最初几十年内恢复到工业化前的水平。最近，由于炸弹 14C 过剩现在大部分与十年翻转的碳库充分混合，化石燃料排放已成为推动大气 Δ14CO2 进一步减少的主要因素。对于δ13CO2，除了储存库之间的交换之外，光合作用过程中优先同化12C的程度似乎有所增加，从而略微减缓了近期δ13CO2的趋势。冰芯和烧瓶 δ13CO2 观测结果的新汇编表明，自工业化前时期以来 δ13CO2 的下降量小于之前的一些估计，这可能包含了由于不同实验室测量的偏移而产生的人为因素。大气中 δ13CO2 的观测已用于研究碳通量和植物的功能，并用于与树木年轮等其他材料中的 δ13C 进行比较。Δ14CO2 的大气观测已用于量化海气交换速率和海洋环流速率、净初级生产力速率以及植物材料和土壤中碳的周转时间。Δ14CO2 的大气观测也用于与考古学、法医学和生理学等许多领域的其他材料中的 Δ14C 进行比较。另一个主要应用是使用 Δ14CO2 观测值和模型评估化石燃料燃烧产生的二氧化碳区域排放量。未来δ13CO2和Δ14CO2将不断变化。变化的迹象和幅度主要取决于全球化石燃料排放。我们在此展示基于第六耦合模型比对项目 (CMIP6) 的共享社会经济路径 (SSP) 的六种情景下未来 δ13CO2 和 Δ14CO2 的模拟。使用大气 δ13CO2 和 Δ14CO2 观测在碳循环科学和许多其他领域的应用将受到这些未来变化的影响。我们建议加大力度协调整个地球系统的 δ13C 和 Δ14C 测量，并进一步开发同位素建模和模型数据分析工具。