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

Abstract

In this "Grand Challenges" paper, we review how the carbon isotopic composition of atmospheric CO 2 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 2 from fossil fuel combustion and land use change reduce the ratio of 13 C/ 12 C in atmospheric CO 2 (δ 13 CO 2). 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 CO 2. Emissions of CO 2 from fossil fuel combustion also reduce the ratio of 14 C/C in atmospheric CO 2 (Δ 14 CO 2) 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 CO 2 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 CO 2 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 CO 2 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 CO 2. For δ 13 CO 2 , 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 CO 2 trend slightly. A new compilation of ice core and flask δ 13 CO 2 observations indicates that the decline in δ 13 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 from fossil fuel combustion using Δ 14 CO 2 observations and models. In the future, δ 13 CO 2 and Δ 14 CO 2 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 CO 2 and Δ 14 CO 2 for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model...

Radiocarbon signature reveals that most springtails depend on carbon from living plants

“…Local variation in atmospheric 14 CO 2 has been widely reported and ascribed to local CO 2 emissions derived from fossil fuel (Δ 14 C = −1000‰), which decreases the Δ 14 C values in the curve [36,37]. If we assume the lowest Δ 14 C signature of −34.8‰ for Entomobryidae to be the current photosynthate C at our site in 2019, 17.2‰ of Δ 14 C value in the tree fine roots is equivalent to the value of 12–13 year-aged C on the simulation curve [35]. The results are consistent with the findings that C in fine roots was aged 10 ± 1 years on average because fine roots are produced from stored non-structural carbohydrates [38].…”

Insights from Time Series of Atmospheric Carbon Dioxide and Related Tracers

“…This article provides an overview of the observed long-term variability in atmospheric CO 2 and the related species, focusing on features that are characteristic of hemispheric or global scales. This article draws on previous reviews on CO 2 growth (9)(10)(11), on trends in carbon isotopes of CO 2 (12), and on trends and other variability in atmospheric O 2 and COS (13,14). We discuss trends, gradients, and cycles in these data, which provide insights into the major processes controlling the buildup of atmospheric CO 2 and the changing biogeochemistry of the land and oceans.…”

“…Atmospheric radiocarbon measurements are conventionally reported in 14 C notation in permil, reflecting deviations in 14 C/C from a standard ratio. 14 C is notionally equal to (r sample − r standard )/r standard × 1,000, where r is the 14 C/C ratio, but it includes small corrections for sample age and 13 C/ 12 C ratio. The primary standard is based on samples of oxalic acid and defined to be similar to atmospheric 14 C in 1890 (57).…”

“…Sustained measurements of atmospheric CO 2 concentration at remote observatories now span more than 60 years. Measurements of related atmospheric tracers such as the isotopes of CO 2 ( 14 C/C, 13 C/ 12 C, and 18 O/ 16 O), atmospheric O 2 , and carbonyl sulfide (COS) now also span many decades. Together with records from ice cores, these measurements have assumed a central role in the study of both the causes and consequences of climate change, thus providing critical insights into the functioning of the Earth in a time of rapid global change.…”

Application of the ECT9 protocol for radiocarbon-based source apportionment of carbonaceous aerosols

“…Materials containing carbon older than about 50 000 years ( 14 C 12 C) are described as "fossil" carbon. Over the past centuries, the 14 C content of the atmosphere has undergone distinct changes (Graven, 2015;Graven et al, 2020;Levin et al, 2010): anthropogenic combustion of fossil fuels emits 14 C-depleted carbon into the atmosphere (i.e., dilutes the proportion of 14 C relative to 12 C). In contrast, nuclear weapons testing doubled the 14 C content of CO 2 in the Northern Hemisphere in the mid-20th century, followed by mixing of this bomb-derived 14 C-enriched carbon into the ocean and biosphere.…”