Hostname: page-component-59f8fd8595-gzxxv Total loading time: 0 Render date: 2023-03-22T01:07:12.345Z Has data issue: true Feature Flags: { "useRatesEcommerce": false } hasContentIssue true


Published online by Cambridge University Press:  23 December 2021

Ingeborg Levin*
Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany
Samuel Hammer
Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany ICOS Central Radiocarbon Laboratory, Heidelberg University, Heidelberg, Germany
Bernd Kromer
Institut für Umweltphysik, Heidelberg University, Heidelberg, Germany
Susanne Preunkert
ICOS Central Radiocarbon Laboratory, Heidelberg University, Heidelberg, Germany Université Grenoble Alpes, CNRS, Institut des Géosciences de l’Environnement (IGE), Grenoble, France
Rolf Weller
Alfred-Wegener-Institut Helmholtz Zentrum für Polar- und Meeresforschung, Bremerhaven, Germany
Douglas E Worthy
Environment and Climate Change Canada, Climate Research Division, Toronto, Ontario, Canada
*Corresponding author. Email:
Rights & Permissions[Opens in a new window]


Since the 1950s, observations of radiocarbon (14C) in tropospheric carbon dioxide (CO2) have been conducted in both hemispheres, documenting the so-called nuclear “bomb spike” and its transfer into the oceans and the terrestrial biosphere, the two compartments permanently exchanging carbon with the atmosphere. Results from the Heidelberg global network of Δ14C-CO2 observations are revisited here with respect to the insights and quantitative constraints they provided on these carbon exchange fluxes. The recent development of global and hemispheric trends of Δ14C-CO2 are further discussed in regard to their suitability to continue providing constraints for 14C-free fossil CO2 emission changes on the global and regional scale.

Research Article
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (, which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
© The Author(s), 2021. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona


Atmospheric nuclear weapon testing in the 1950s and 1960s in the Northern Hemisphere was a period of great anxiety, however, it had significant side effects for environmental sciences in many aspects. The artificial production of more than 6 × 1028 atoms or about 1.4 tons of radiocarbon (14C) (Naegler and Levin Reference Naegler and Levin2006), lead to a doubling of the 14C/C ratio in tropospheric CO2 of the Northern Hemisphere with a prominent spike in 1963 (Nydal and Lövseth Reference Nydal and Lövseth1983; Levin et al. Reference Levin, Kromer, Schoch-Fischer, Bruns, Münnich, Berdau, Vogel and Münnich1985). This “bomb spike” that reached the Southern Hemisphere with some delay (Manning et al. Reference Manning, Lowe, Melhuish, Sparks, Wallace, Brenninkmeijer and McGill1990), has been used as a transient tracer in all compartments of the fast carbon cycle (e.g., Broecker et al. Reference Broecker, Peng, Östlund and Stuiver1985; Levin and Hesshaimer Reference Levin and Hesshaimer2000; Trumbore Reference Trumbore2009), but also to study atmospheric dynamics, such as inter-hemispheric (e.g., Münnich and Vogel Reference Münnich and Vogel1958, Reference Münnich and Vogel1963; Czeplak and Junge Reference Czeplak and Junge1974) and stratosphere-troposphere air mass exchange (e.g., Telegadas Reference Telegadas1971; Hesshaimer and Levin Reference Hesshaimer and Levin2000; Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010). As an indirect outcome of the weapon testing, a wealth of new insights into atmospheric and carbon cycle dynamics could be achieved in the decades following the nuclear test ban treaty in 1963.

Today, the transient bomb-radiocarbon signal has levelled off, and the anthropogenic input of radiocarbon-free fossil CO2 into the atmosphere has become the dominant driver of the decrease of the 14C/C ratio in global atmospheric CO2 (Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010; Graven Reference Graven2015). This 14CO2-free anthropogenic CO2 flux from the burning of fossil fuels and cement production has increased globally by more than fourfold compared to the 1960s, and, together with ongoing land-use changes, has resulted in an increase of the atmospheric CO2 burden by more than 5 PgC (1 PgC = 1015 gC) or >0.5% per year in the last decade (Friedlingstein et al. Reference Friedlingstein, O’Sullivan, Jones, Andrew, Hauck, Olsen, Peters, Peters, Pongratz and Sitch2020). The fossil CO2 emissions are not evenly distributed over the globe and because the biosphere and oceans today, in most regions, are sources of bomb 14CO2, the distribution of tropospheric Δ14C-CO2 has undergone significant changes since the 2000s.

After a brief overview on the development in the last 60+ years of 14C in tropospheric CO2, this paper reviews the insights from earlier studies that used bomb 14C for carbon cycle budgeting. Further, we present our extended high-precision Δ14C-CO2 observations in background air at Alert, Jungfraujoch, Izaña, Cape Grim, Macquarie Island and the German Antarctic Neumayer station (Figure 1), and discuss these data in view of their potential to serve as additional constraints for global carbon fluxes. Finally, we provide a perspective on how ongoing changes of the distribution and trends of Δ14C-CO2 in the background troposphere could potentially be used to constrain major components of the carbon cycle, including monitoring of anthropogenic CO2 emission and their reductions in view of the Paris agreement from the regional to the global scale.

Figure 1 Cooperative CO2 background air sampling network for high-precision 14C analysis in the Heidelberg Radiocarbon laboratory.


Shortly after Anderson and Libby (Reference Anderson and Libby1951) published their first measurements of natural radiocarbon in various compartments of the Earth system, a number of Radiocarbon Laboratories have been established world-wide. First measurements of Δ14C-CO2 in background air were conducted in 1954 in the Southern Hemisphere with sampling on the south-western coast of the Northern Island of New Zealand (NZ) close to Wellington (41.25°S, 174.69°E, 300 m asl; Rafter and Fergusson Reference Rafter and Fergusson1957). Münnich and Vogel (Reference Münnich and Vogel1963) collected their first atmospheric CO2 samples for 14C analysis in 1959 at a number of stations in the Northern and Southern Hemisphere. Quasi-continuous sampling was finally established at Vermunt in the Austrian Alps (47.07°N, 9.57°E, 1800 m asl; Levin et al. Reference Levin, Kromer, Schoch-Fischer, Bruns, Münnich, Berdau, Vogel and Münnich1985). In the early years, all samples were collected by passive absorption of atmospheric CO2 in sodium hydroxide solution. CO2 was then extracted in the laboratory from this basic solution by acidification. After purification of the extracted CO2 the samples were analyzed by gas proportional counting (Kromer and Münnich Reference Kromer and Münnich1992; Turnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017).

In the 1970s, advancements in both the sampling and analysis systems occurred, e.g. the Heidelberg sampling system at Vermunt station was changed to actively flushing air through a rotating absorption column, now allowing quantitative (less fractionated) sampling of CO2 over a well-defined time interval (Levin et al. Reference Levin, Münnich and Weiss1980). At the NZ station, in addition to passive sampling, whole air flask samples started to be collected, and 14C analysis changed from gas counting to Accelerator Mass Spectrometric analysis (for details, see Turnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017, supplementary material). While in Heidelberg measurement of background air samples was and still is today by gas proportional counting, we changed our European background monitoring site in the Alps from Vermunt in Austria to Jungfraujoch in Switzerland (46.55°N, 7.98°E, 3450 m asl) in 1986. This seemed advantageous as the high elevation of Jungfraujoch provides better access to free tropospheric air. In addition, several continuous measurements of other atmospheric trace substances, including CO2, are conducted at this site (Forrer et al. Reference Forrer, Rüttimann, Schneiter, Fischer, Buchmann and Hofer2000).

The two pioneer stations in New Zealand and in the Austrian Alps, which document the largest excursions of bomb 14CO2 in the Southern and the Northern Hemisphere troposphere, were supplemented by an increasing number of Δ14C-CO2 observations world-wide, including the tropics and sub-tropics. The most comprehensive records were published by Nydal and Lövseth (Reference Nydal and Lövseth1983, Reference Nydal and Lövseth1996) and by Meijer et al. (Reference Meijer, van der Plicht, Gislefoss and Nydal1995). These globally distributed data sets provided the opportunity to track the bomb 14CO2 spike throughout the entire troposphere. Unfortunately, most of these sampling efforts were discontinued in the 1980s after bomb 14CO2 had become well-mixed in both hemispheres.

Starting in 1983, when the German Antarctic Neumayer station became operational in Dronning Maud Land at the Antarctic coast (70.65°S, 8.25°E, 17 m asl), the Heidelberg Radiocarbon laboratory was granted the opportunity to start CO2 sampling for 14C analysis at this remote Antarctic site. Soon after the first data became available, it was realized that the Δ14C-CO2 level at Neumayer station was significantly influenced by a 14CO2 disequilibrium flux with the Antarctic circumpolar surface water, originating from upwelling of about 800-year-old intermediate water of the Pacific Ocean, which, due to its age, is strongly depleted in (natural) 14C (Levin et al. Reference Levin, Kromer, Wagenbach and Münnich1987). This exciting feature of lower Δ14C-CO2 levels in atmospheric CO2 of Southern Hemisphere air when compared to the Northern Hemisphere had already been observed by Lerman et al. (Reference Lerman, Mook and Vogel1970) on tree rings. Their results were now confirmed by direct atmospheric observations. For us, this finding was the catalyst to re-start global monitoring of Δ14C-CO2 in the atmosphere, i.e. during a period when other research agencies were terminating their 14CO2 monitoring efforts. The potential of Δ14C-CO2 observations as an ongoing constraint of gross carbon exchange fluxes between atmosphere, ocean and biosphere seemed obvious. However, unlike the situation immediately after the test ban treaty in 1963, being 20 years later, the spatial gradients and temporal variations of Δ14C-CO2 in the atmosphere were more than two orders of magnitude smaller. To detect such small signals required increased precision in Δ14C-CO2 analysis (about 2‰), and could only be achieved by gas proportional counting at that time. In the 1980s, we begun establishing a new network of Δ14C-CO2 observations at a number of globally distributed stations, in cooperation with colleagues operating the Global Atmosphere Watch stations for continuous greenhouse gases observations (see map in Figure 1). Other investigations (Hesshaimer et al. Reference Hesshaimer, Heimann and Levin1994; Levin and Hesshaimer Reference Levin and Hesshaimer2000), were also carried out using these data to close the atmospheric bomb 14C budget, and, in turn, determine the total input of anthropogenic 14C into the global carbon system (Naegler and Levin Reference Naegler and Levin2006).

In the 2000s, the Scripps Institution of Oceanography (SIO) started analyzing 14C on CO2 extracted from flask samples collected from their global network of stations along the Pacific Ocean, stretching from Barrow, Alaska, down to the South Pole (Graven et al. Reference Graven, Guilderson and Keeling2012). These samples were analyzed with high precision Accelerator Mass Spectrometry. The data were included in a global atmospheric transport model (TM3, Heimann and Korner Reference Heimann and Korner2003) to investigate the spatial Δ14C-CO2 distribution and document its changes since the 1990s. Unfortunately, Δ14C-CO2 results from the SIO network since 2007 have not been published. In the 2000s, the Institute of Arctic and Alpine Research (INSTAAR) began measuring Δ14C-CO2 on flask samples from the Global Monitoring Laboratory (GML) of the National Oceanic and Atmospheric Administration (NOAA/GML), primarily from aircraft flights and ground-based locations over North America (Turnbull et al. Reference Turnbull, Miller, Lehman, Tans, Sparks and Southon2006; Miller et al. Reference Miller, Lehman, Montzka, Sweeney, Miller, Karion, Wolak, Dlugokencky, Southon, Turnbull and Tans2012). The primary focus of the INSTAAR program was to use these data to partition the 14C-free fossil from the biogenic CO2 component over the continent (Miller et al. Reference Miller, Lehman, Montzka, Sweeney, Miller, Karion, Wolak, Dlugokencky, Southon, Turnbull and Tans2012; Basu et al. Reference Basu, Lehman, Miller, Andrews, Sweeney, Gurney, Xue, Southon and Tans2020). Δ14C-CO2 observations over continents have become the main focus of the carbon cycle community today, as 14C is the most direct tracer to quantify the regional fossil CO2 component (Levin et al. Reference Levin, Kromer, Schmidt and Sartorius2003; Turnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017; Graven et al. Reference Graven, Stephens, Guilderson, Campos, Schimel, Campbell and Keeling2009). Also, in Europe, a new network of Δ14C-CO2 monitoring stations was established as part of the atmospheric observational network of the Integrated Carbon Observation System (ICOS) Research Infrastructure (Levin et al. Reference Levin, Karstens, Eritt, Maier, Arnold, Rzesanke, Hammer, Ramonet, Vítková, Conil, Heliasz, Kubistin and Lindauer2020). Since 2018 Jungfraujoch station has officially become part of this network, and the Δ14C-CO2 analyses are conducted by the ICOS Central Radiocarbon Laboratory in Heidelberg (


The long-term development of Δ14C in tropospheric CO2 in both hemispheres is displayed in Figure 2. The increase in the Southern Hemisphere (SH) since 1954 is documented by data from the New Zealand site close to Wellington (Turnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017). From 1983 onwards, we extended this record with data from the Neumayer station, Antarctica (Levin and Hammer Reference Levin and Hammer2021). In the Northern Hemisphere (NH) atmospheric, background observations are only available from 1959 onwards. In Figure 2 we extend the NH record back to the 1940s using annual mean values for 30°–90°N from the compilation by Graven et al. (Reference Graven, Allison, Etheridge, Hammer, Keeling, Levin, Meijer, Rubino, Tans, Trudinger, Vaughn and White2017), based on tree ring analyses. The Vermunt record ended in 1985 when our Alpine background sampling site was moved to the Jungfraujoch station. For all stations, the results from individual atmospheric samples are shown in Figure 2. The data are available at the ICOS-ERIC Carbon Portal (Levin and Hammer Reference Levin and Hammer2021).

Figure 2 Development of tropospheric Δ1 4C-CO2 in both hemispheres since the start of the atmospheric nuclear bomb tests. New Zealand data are from Turnbull et al. (Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017). The inlay shows the derivative of annual mean Δ1 4C-CO2.

In the NH record, regular seasonal variations are clearly visible in the years of 1963–1968. This seasonality is caused by spring-time intrusion of stratospheric air into the troposphere (Telegadas Reference Telegadas1971). During these initial years after the nuclear test ban treaty in 1963, a large share of bomb-produced 14CO2 continued to reside in the stratosphere, which, in the course of the following years, was transported into the troposphere, mainly during spring (Tans Reference Tans1981; Hesshaimer and Levin Reference Hesshaimer and Levin2000). The subsequent transport of bomb 14CO2 from the NH troposphere into the SH then caused a decrease of Δ14C-CO2 in late summer. However, the approximately 1.5 years shift between Northern and Southern Hemisphere Δ14C-CO2 curves provides the most direct measure of the interhemispheric exchange time. This atmospheric Δ14C-CO2 dynamic during and shortly after the bomb tests has been used to calibrate or evaluate the validity of transport in atmospheric circulation models (e.g., Johnston Reference Johnston1989; Kjellström et al. Reference Kjellström, Feichter and Hoffmann2000; Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010).

The inlay of Figure 2 shows the year-to-year change of global mean Δ14C-CO2 values, calculated from the global compilation of Graven et al. (Reference Graven, Allison, Etheridge, Hammer, Keeling, Levin, Meijer, Rubino, Tans, Trudinger, Vaughn and White2017), extended with the most recent Heidelberg measurements from the NH and the SH. Unfortunately, no recent data from the tropics are available to calculate annual global means. We therefore extrapolated the tropical record with the annual mean trends observed in 2015–2020 for the extra-tropical latitudes. After the steep increase of tropospheric Δ14C-CO2, reaching a maximum in the NH in 1963 and in the SH about 1.5 years later, we observe a fast decrease in both hemispheres. This decrease is caused by equilibration of the atmospheric bomb 14C disturbance with reservoirs exchanging CO2 with the atmosphere, namely the ocean and the terrestrial biosphere. According to Levin et al. (Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010: Figure 7) the largest net uptake of bomb 14C by the world oceans occurred in the 1970s and was about twice as high when compared to the net uptake by the biosphere. While the terrestrial biosphere acted as a net sink of anthropogenic 14C only until the 1980s (Naegler and Levin Reference Naegler and Levin2009), the oceans continued to be a sink of bomb 14C until around 2010.

Starting in the mid-1990s, fossil CO2 emissions into the global atmosphere became the dominant contribution to the decreasing global Δ14C-CO2 trend. Levin et al. (Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010) attempted to use the global Δ14C-CO2 trend of the last decades as an independent constraint for the global fossil CO2 emissions into the atmosphere. However, the uncertainty estimates were determined to be too large (25–30%) to be useful, due to the large uncertainties in the partitioned contributions from ocean and biosphere exchange fluxes to the Δ14C-CO2 trend, compared to the global bottom-up fossil emission estimates, which were estimated to better than ±10% (Gilfillan and Marland Reference Gilfillan and Marland2021). Today though, with improved information on ocean and biosphere 14CO2 fluxes, it may be a useful endeavor to repeat this exercise. However, for this purpose we would first need more representative high precision Δ14C-CO2 observations at background stations that are compatible to better than ±0.5‰, in order to determine reliable global trends and hemispheric gradients, which today are only a few ‰ (see Figure 3). Better knowledge on compatibility can only be achieved through frequent intercomparisons between those labs that contribute to this global Δ14C-CO2 background monitoring network. An earlier intercomparison between our low-level counting laboratory in Heidelberg (ICOS CRL) and 8 AMS laboratories yielded an overall agreement of better that ±0.5‰, but it was not conclusive concerning the compatibility within the AMS laboratories themselves (Hammer et al. Reference Hammer, Friedrich, Kromer, Cherkinsky, Lehman, Meijer, Nakamura, Palonen, Reimer, Smith, Southon, Szidat, Turnbull and Uchida2017). More frequent regular intercomparisons are therefore urgently needed to achieve an overall compatibility of better than ±0.5‰. Then 14C-based ffCO2 estimates could provide an excellent independent top-down check on the global stocktake of the Paris Climate Accord (UNFCCC 2015).

Figure 3 a: Long-term trend of Δ1 4C-CO2 at the two polar stations Alert (Arctic) and Neumayer (Antarctica) (Levin and Hammer Reference Levin and Hammer2021). The solid lines are de-seasonalised fitted curves through the individual data. Panel b displays the difference between both fitted Δ1 4C-CO2 curves, while c shows the differences of corresponding fitted curves through CO2 concentration data from flask samples collected at the two stations (Weller et al. Reference Weller, Levin, Wagenbach and Minikin2007; Worthy et al. Reference Worthy2021). d: Meridional distribution of three-year averages of CO2 (Dlugokencky et al. Reference Dlugokencky, Thoning, Lang and Tans2019), and Δ1 4C-CO2 (e) for 1993–1995 in comparison to 2008–2010.


Because the bulk of the fossil CO2 emissions are released in the NH, observed CO2 concentrations in the Northern are higher than in the Southern Hemisphere (Dlugokencky et al. Reference Dlugokencky, Thoning, Lang and Tans2019). However, until the end of the last century, the north-south difference of Δ14C-CO2 was counterintuitively positive, with higher Δ14C-CO2 being observed in the North than in the South (Levin and Hesshaimer Reference Levin and Hesshaimer2000). This latter north-south difference was caused by a strong 14CO2 disequilibrium flux with the Δ14C-depleted surface water of the Southern Ocean around Antarctica (Levin et al. Reference Levin, Kromer, Wagenbach and Münnich1987). The sign of the observed north-south difference, however, changed in the mid-2000s, with lower Δ14C-CO2 being observed at high northern latitudes such as at Alert in the Arctic (82.45°N, 62.52°W, 185 m asl) when compared to Neumayer, Antarctica. We calculated smoothed fitted curves (Nakazawa et al. Reference Nakazawa, Ishizawa, Higuchi and Trivett1997) through the individual data from the two polar stations and plot the long-term trends together with the individual data in Figure 3a. The difference between the two curves, fitting Alert and Neumayer Δ14C-CO2 data, is shown in Figure 3b. It can be clearly seen that while the Δ14C-CO2 difference was positive until the end of the 1990s, it decreased to about zero at the turn of the century and around 2003, it changed to negative values and continues in this manner until today. At the same time, the CO2 concentration difference between the two polar stations increased from ca. 3.5 ppm in the 1990s to ca. 4–5 ppm in the last decade (Figure 3c).

The observed change in the sign in the north-south Δ14C-CO2 difference has been previously reported by Graven et al. (Reference Graven, Guilderson and Keeling2012). There are two reasons that may have caused this reversal: (1) The increase of the north-south gradient of CO2 due to increasing fossil CO2 emissions being more dominant in the NH, and in turn, causing an increasing dilution of the 14C/C ratio in NH CO2, and (2) the decrease of the Δ14C-CO2 disequilibrium between the atmosphere and surface waters in the circum Antarctic ocean (Graven et al. Reference Graven, Guilderson and Keeling2012). While the difference between atmospheric and surface ocean Δ14C-CO2 in the late 1980 and 1990s was still about 200–300‰, it decreased by about 150‰ in the following decade, because atmospheric Δ14C-CO2 decreased by this amount (Figure 2). If we assume that the increase of CO2 difference from about 1994 to 2009 between Northern and Southern Hemisphere of about 1.5 ppm was caused by 14C-free fossil CO2 alone, this would have caused a change in the Δ14C-CO2 north-south difference of approximately 3‰. The remaining difference of more than 2‰ must therefore be due to other reasons. The right panels of Figure 3 show the mean meridional distributions of CO2 concentration (d) and Δ14C-CO2 (e) in the period of 1993–1995 compared to the 15 years later period of 2008–2010. The change in Δ14C-CO2 relative to Neumayer (NMY) station and CO2 concentration relative to South Pole is observed throughout the Northern Hemispheric sites. CO2 and Δ14C-CO2 are rather homogeneously distributed from polar regions (Alert) to the subtropics at Izaña (Tenerife Island, 28.3°N, 16.48°W, 2373 m asl) with variations smaller than 1.5‰ in Δ14C-CO2 and 1 ppm in CO2. In contrast, corresponding latitudes of the Southern Hemisphere show a significant Δ14C-CO2 dip at Macquarie Island (54.5°S, 158.97°E, 6 m asl), which is not accompanied by a significant variation in CO2 concentration. In this region of the Southern Ocean around Macquarie Island (MQA) with depleted Δ14C the 14CO2 disequilibrium flux is largest, also because large wind velocities enhance air-sea gas exchange.

As is the case for the long-term trend, changes in the north-south Δ14C-CO2 difference can also provide a constraint on the predominantly Northern Hemispheric fossil CO2 emissions. But keeping in mind that there are uncertainties in the other components contributing to the meridional Δ14C-CO2 difference as well as uncertainties associated in atmospheric model transport limits the uncertainty of the fossil emissions constraint to around 25–30% (Levin et al. Reference Levin, Naegler, Kromer, Diehl, Francey, Gomez-Pelaez, Steele, Wagenbach, Weller and Worthy2010).

A special feature can be seen in the data of the tropical station Llano del Hato (Venezuela, 8.78°N 70.87°W, 3600 m asl). Here we observed a regional Δ14C-CO2 maximum in the 1990s, which we attributed to a signal of net bomb 14CO2 released by heterotrophic respiration from the tropical biosphere. Naegler and Levin (Reference Naegler and Levin2009) calculated from their global bomb-14C budget that the global biosphere switched from a net sink of anthropogenic 14CO2 to a net source to the atmosphere around the 1980s. Small fossil CO2 emissions or atmosphere ocean disequilibrium fluxes could be the reason why this bomb-14C signal from the biosphere is visible at these latitudes. Unfortunately, due to logistics problems, sampling at this important tropical site was terminated in 1997. Graven et al. (Reference Graven, Guilderson and Keeling2012) also found about 3–6‰ higher Δ14C-CO2 in 2005–2007 at two tropical stations in the Pacific Ocean when compared to mid-latitude sites in the NH. Continuing observations in the tropics could possibly provide independent constraints on the turnover times of carbon in the terrestrial biosphere, an important parameter to assess the sustainability of the currently observed net uptake of anthropogenic CO2 emissions by the terrestrial biosphere.


Δ14C-CO2 in the troposphere over the last 60+ years serves as an excellent transient tracer to study atmospheric and global carbon cycle dynamics. However, despite the wealth of information that is buried in the observed global distribution and trend of Δ14C-CO2, only the first few decades of the existing records have yet been used to constrain global and regional exchange fluxes between the main compartments of the carbon cycle. This may be due to the sparsity of representative observations, for example, in the very heterogeneous terrestrial biosphere. But also for the much more homogenous global oceans, representative continuous monitoring of 14C is incomplete. Further, only in the last decade, has 14C been fully implemented in global models such as in the National Center for Atmospheric Research Community Earth System model (Koven et al. Reference Koven, Riley, Subin, Tang, Torn, Collins, Bonan, Lawrence and Swenson2013; Jahn et al. Reference Jahn, Lindsay, Giraud, Gruber, Otto-Bliesner, Liu and Brady2015). Concerning atmospheric tracer transport modeling of CO2, including 14C in global and regional inversions is still at its infancy (e.g., Turnbull et al. Reference Turnbull, Rayner, Miller, Naegler, Ciais and Cozic2009; Basu et al. Reference Basu, Lehman, Miller, Andrews, Sweeney, Gurney, Xue, Southon and Tans2020). Here, not only do the ocean and biosphere “boundary conditions” need to be well defined, but reliable atmospheric modeling of 14CO2 also requires stratosphere-troposphere air mass exchange to be represented correctly in the model. Contrary to the stable isotopes in CO2, where the tropospheric variability is governed by sources and sinks located at the Earth surface, the 14CO2 cycle has a natural production source in the upper troposphere and lower stratosphere. All these challenges need to be accounted for in order to disentangle the small gradients and trends, particularly those observed in the last decade. However, most important are long-term high-precision measurements at key stations themselves. Models can always be improved in the future, but the atmosphere at present can only be sampled today.


This work would not have been possible without the dedicated efforts and long-term support by the staff at the globally distributed sampling sites, as well as the technical personnel in the Heidelberg Radiocarbon laboratories. We wish to thank Ed Dlugokencky and Pieter Tans for providing CO2 meridional profile data from their global monitoring network. Macquarie Island sampling was supported by the Australian Antarctic Program, Cape Grim sampling by the Australian Bureau of Meteorology, Izaña sampling by the Agencia Estatal de Meteorología, Spain, and Jungfraujoch sampling by the Internationale Stiftung Hochalpine Forschungsstationen Jungfraujoch und Gornergrat. Our long-term monitoring was financially supported by a number of agencies in Germany and Europe, namely the Heidelberg Academy of Sciences, the Ministry of Education and Science, Baden-Württemberg, Germany, the German Science Foundation, the German Federal Ministries for the Environment, Nature Conservation and Nuclear Safety, for Education and Research, and for Transport and Digital Infrastructure as well as by the European Commission, Brussels, and the ICOS Research Infrastructure.


Individual Δ14C-CO2 data from Alert, Jungfraujoch, and Neumayer stations are available at the ICOS-ERC Carbon Portal (Levin and Hammer Reference Levin and Hammer2021).



Anderson, EC, Libby, WF. 1951. World-wide distribution of natural radiocarbon. The Physical Review 81(1):6469.CrossRefGoogle Scholar
Basu, S, Lehman, SJ, Miller, JB, Andrews, AE, Sweeney, C, Gurney, KR, Xue, X, Southon, J, Tans, PP. 2020. Estimating US fossil fuel CO2 emissions from measurements of 14C in atmospheric CO2 , PNAS 117(24):1330013307. doi: 10.1073/pnas.1919032117.CrossRefGoogle ScholarPubMed
Broecker, WS, Peng, TH, Östlund, G, Stuiver, M. 1985. The distribution of bomb radiocarbon in the ocean. J. Geophys. Res. 90(C4):69536970. doi: 10.1029/JC090iC04p06953.CrossRefGoogle Scholar
Czeplak, G, Junge, C. 1974. Studies of interhemispheric exchange in the troposphere by a diffusion model. Adv. Geophys. 18:5772.CrossRefGoogle Scholar
Dlugokencky, EJ, Thoning, KW, Lang, PM, Tans, PP. 2019. NOAA Greenhouse gas reference from atmospheric carbon dioxide dry air mole fractions from the NOAA ESRL Carbon Cycle Cooperative Global Air Sampling Data Last accessed 8.5.2021.Google Scholar
Forrer, J, Rüttimann, R, Schneiter, D, Fischer, A, Buchmann, B, Hofer, P. 2000. Variability of trace gases at the high-Alpine site Jungfraujoch caused by meteorological transport processes, J. Geophys. Res. 105:1224112251. https://doi:10.1029/1999JD901178.CrossRefGoogle Scholar
Friedlingstein, P, O’Sullivan, M, Jones, MW, Andrew, RM, Hauck, J, Olsen, A, Peters, GP, Peters, W, Pongratz, J, Sitch, S, et al. 2020. Global carbon budget 2020. Earth Syst. Sci. Data 12:32693340. doi: 10.5194/essd-12-3269-2020.CrossRefGoogle Scholar
Gilfillan, D, Marland, G. 2021. CDIAC-FF: global and national CO2 emissions from fossil fuel combustion and cement manufacture: 1751–2017. Earth Syst. Sci. Data 13:16671680. doi: 10.5194/essd-13-1667-2021.CrossRefGoogle Scholar
Graven, HD. 2015. Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century. Proceedings of the National Academy of Sciences 112:95429545. doi: 10.1073/pnas.1504467112.CrossRefGoogle ScholarPubMed
Graven, HD, Allison, CE, Etheridge, DM, Hammer, S, Keeling, RF, Levin, I, Meijer, HAJ, Rubino, M, Tans, PP, Trudinger, CM, Vaughn, BH, White, JWC. 2017. Compiled records of carbon isotopes in atmospheric CO2 for historical simulations in CMIP6. Geoscientific Model Development 10(12):44054417. doi: 10.5194/gmd-10-4405-2017.CrossRefGoogle Scholar
Graven, HD, Guilderson, TP, Keeling, RF. 2012. Observations of radiocarbon in CO2 at seven global sampling sites in the Scripps flask network: Analysis of spatial gradients and seasonal cycles. J. Geophys. Res. 117(D02303). doi: 10.1029/2011JD016535.Google Scholar
Graven, HD, Stephens, BB, Guilderson, TB, Campos, TL, Schimel, DS, Campbell, JE, Keeling, RF. 2009. Vertical profiles of biospheric and fossil fuel-derived CO2 and fossil fuel CO2:CO ratios from airborne measurements of 14C, CO2 and CO above Colorado, USA. Tellus B 61:536546. doi: 10.1111/j.1600-0889.2009.00421.x.CrossRefGoogle Scholar
Hammer, S, Friedrich, R, Kromer, B, Cherkinsky, A, Lehman, SJ, Meijer, HA, Nakamura, T, Palonen, V, Reimer, RW, Smith, AM, Southon, JR, Szidat, S, Turnbull, J, Uchida, M. 2017. Compatibility of atmospheric 14CO2 measurements: comparing the Heidelberg low-level counting facility to international accelerator mass spectrometry (AMS) laboratories. Radiocarbon 59(3):875883. doi: 10.1017/RDC.2016.62.CrossRefGoogle Scholar
Heimann, M, Korner, S. 2003. The Global Atmospheric Tracer Model TM3. Model description and users manual release 3.8a. Tech. Rep. 5. Max Planck Inst. for Biogeochem., Jena, Germany.Google Scholar
Hesshaimer, V, Heimann, M, Levin, I. 1994. Radiocarbon evidence for a smaller oceanic carbon dioxide sink than previously believed. Nature 370:201203. doi: 10.1038/370201a0.CrossRefGoogle Scholar
Hesshaimer, V, Levin, I. 2000. Revision of the stratospheric bomb 14CO2 inventory. J. Geophys. Res. 105(D9):11,641–11,658.Google Scholar
Jahn, A, Lindsay, K, Giraud, X, Gruber, N, Otto-Bliesner, BL, Liu, Z, Brady, EC. 2015. Carbon isotopes in the ocean model of the Community Earth System Model (CESM1). Geoscientific Model Development 985 8:24192434. doi: 10.5194/gmd-8-2419-2015.CrossRefGoogle Scholar
Johnston, HS. 1989. Evaluation of excess carbon-14 and strontium-90 data for suitability to test two-dimensional stratospheric models. J. Geophys. Res. 94:18 485–18 493.Google Scholar
Kjellström, E, Feichter, J, Hoffmann, G. 2000. Transport of SF6 and 14CO2 in the atmospheric general circulation model ECHAM. Tellus 52B:118. doi: 10.1034/j.1600-0889.2000.00882.x.Google Scholar
Koven, CD, Riley, WJ, Subin, ZM, Tang, JY, Torn, MS, Collins, WD, Bonan, GB, Lawrence, DM, Swenson, SC. 2013. The effect of vertically resolved soil biogeochemistry and alternate soil C and N models on C dynamics of CLM4. Biogeosciences 10(11):71097131. doi: 10.5194/bg-10-7109-2013.CrossRefGoogle Scholar
Kromer, B, Münnich, KO. 1992. CO2 gas proportional counting in radiocarbon dating—review and perspective. In: Taylor RE, Long A, Kra RS, editors. Radiocarbon after four decades. New York: Springer. p. 184–197.CrossRefGoogle Scholar
Lerman, J, Mook, W, Vogel, J. 1970. C14 in tree rings from different localities. In: Olsson IU, editor. Radiocarbon variations and absolute chronology. Proceedings, XII Nobel Symposium. New York: Wiley. p. 275–301.Google Scholar
Levin, I, Hammer, S. 2021. Supplementary data to Levin et al. (2021), Radiocarbon in global tropospheric carbon dioxide, ICOS-ERIC Carbon Portal. Scholar
Levin, I, Hesshaimer, V. 2000. Radiocarbon–a unique tracer of global carbon cycle dynamics. Radiocarbon 42:6980. doi: 10.1017/S0033822200053066.CrossRefGoogle Scholar
Levin, I, Karstens, U, Eritt, M, Maier, F, Arnold, S, Rzesanke, D, Hammer, S, Ramonet, M, Vítková, G, Conil, S, Heliasz, M, Kubistin, D, Lindauer, M. 2020. A dedicated flask sampling strategy developed for ICOS stations based on CO2 and CO measurements and STILT footprint modelling, Atmos. Chem. Phys. 20:1116111180. doi: 10.5194/acp-20-11161-2020.CrossRefGoogle Scholar
Levin, I, Kromer, B, Schmidt, M, Sartorius, H. 2003. A novel approach for independent budgeting of fossil fuels CO2 over Europe by 14CO2 observations. Geophys. Res. Lett. 30(23):2194. doi: 10.1029/2003GL018477.CrossRefGoogle Scholar
Levin, I, Kromer, B, Schoch-Fischer, H, Bruns, M, Münnich, M, Berdau, D, Vogel, JC, Münnich, KO. 1985. 25 years of tropospheric 14C observations in central Europe. Radiocarbon 27:119. doi: 10.1017/S0033822200006895.CrossRefGoogle Scholar
Levin, I, Kromer, B, Wagenbach, D, Münnich, KO. 1987. Carbon isotope measurements of atmospheric CO2 at a coastal station in Antarctica. Tellus B 39B(1–2):8995. doi: 10.1111/j.1600-0889.1987.tb00273.x.CrossRefGoogle Scholar
Levin, I, Münnich, KO, Weiss, W. 1980. The effect of anthropogenic CO2 and 14C sources on the distribution of 14CO2 in the atmosphere. Radiocarbon 22:379391. doi: 10.1017/S003382220000967X.CrossRefGoogle Scholar
Levin, I, Naegler, T, Kromer, B, Diehl, M, Francey, RJ, Gomez-Pelaez, AJ, Steele, LP, Wagenbach, D, Weller, R, Worthy, DE. 2010. Observations and modelling of the global distribution and long-term trend of atmospheric 14CO2 . Tellus B 62(1):2646. doi: 10.1111/j.1600-0889.2009.00446.x.CrossRefGoogle Scholar
Manning, MR, Lowe, DC, Melhuish, WH, Sparks, RJ, Wallace, G, Brenninkmeijer, CAM, McGill, RC. 1990. The use of radiocarbon measurements in atmospheric studies. Radiocarbon 32(1): 3758. doi: 10.1017/S0033822200039941.CrossRefGoogle Scholar
Meijer, HAJ, van der Plicht, J, Gislefoss, JS, Nydal, R. 1995. Comparing long term atmospheric 14C and 3H records near Groningen, the Netherlands with Fruholmen, Norway and Izana, Canary Islands 14C stations. Radiocarbon 37(1):3950. doi: 10.1017/S0033822200014776.CrossRefGoogle Scholar
Miller, JB, Lehman, SJ, Montzka, SA, Sweeney, C, Miller, BR, Karion, A, Wolak, C, Dlugokencky, EJ, Southon, J, Turnbull, JC, Tans, PP. 2012. Linking emissions of fossil fuel CO2 and other anthropogenic trace gases using atmospheric 14CO2. J. Geophys. Res. 117:D08302. doi: 10.1029/2011JD017048.Google Scholar
Münnich, KO. 1963. Der Kreislauf des Radiokohlenstoffs in der Natur. Naturwissenschaften 50(6):212218.CrossRefGoogle Scholar
Münnich, KO, Vogel, J. 1958. Durch Atomexplosionen erzeugter Radiokohlenstoff in der Atmosphäre. Naturwissenschaften 45(14):327329.CrossRefGoogle Scholar
Münnich, KO, Vogel, J. 1963. Investigations of meridional transport in the troposphere by means of radiocarbon measurements. In: Symposium on radioactive dating. Proc.: IAEA Vienna.Google Scholar
Naegler, T, Levin, I. 2006. Closing the global radiocarbon budget 1945–2005. J. Geophys. Res. 111:D12311. doi: 10.1029/2005JD006758.CrossRefGoogle Scholar
Naegler, T, Levin, I. 2009. Biosphere-atmosphere gross carbon exchange flux and the δ13CO2 and Δ14CO2 disequilibria constrained by the biospheric excess radiocarbon inventory. J. Geophys. Res. 114:D17303. doi: 10.1029/2008JD011116.CrossRefGoogle Scholar
Nakazawa, T, Ishizawa, M, Higuchi, K, Trivett, NBA. 1997. Two curve fitting methods applied to CO2 flask data. Environmetrics: The official journal of the International Environmetrics Society 8.3:197218.3.0.CO;2-C>CrossRefGoogle Scholar
Nydal, R, Lövseth, K. 1983. Tracing bomb 14C in the atmosphere 1962–1980. J. Geophys. Res. 88:36213642.CrossRefGoogle Scholar
Nydal, R, Lövseth, K. 1996. Carbon-14 measurements in atmospheric CO2 from Northern and Southern Hemisphere sites, 1962–1993. ORNL/CDIAC-93, NDP-057. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee. 67 p. doi: 10.3334/CDIAC/atg.ndp057.CrossRefGoogle Scholar
Rafter, TA, Fergusson, GJ. 1957. “Atom Bomb Effect”—recent increase of carbon-14 content of the atmosphere and biosphere. Science 126(3273):557558.CrossRefGoogle ScholarPubMed
Tans, PP. 1981. A compilation of bomb 14C data for use in global carbon model calculations. In: Bolin B, editor. SCOPE 16, carbon cycle modelling. Chichester, New York, Brisbane, Toronto: Wiley. p 131–157.Google Scholar
Telegadas, K. 1971. The seasonal atmospheric distribution and inventories of excess carbon-14 from March 1955 to July 1969. HASL report 243, Health and Safety Lab., U.S. At. Energy Comm., New York.Google Scholar
Turnbull, JC, Miller, JB, Lehman, SJ, Tans, PP, Sparks, RJ, Southon, J. 2006. Comparison of 14CO2, CO, and SF6 as tracers for recently added fossil fuel CO2 in the atmosphere and implications for biological CO2 exchange. Geophys. Res. Lett. 33:L01817. doi: 10.1029/2005GL024213.CrossRefGoogle Scholar
Turnbull, JC, Lehman, SJ, Miller, JB, Sparks, RJ, Southon, JR, Tans, PP. 2007. A new high precision 14CO2 time series for North American continental air. J. Geophys. Res. Atmos. 112(D11). doi: 10.1029/2006JD008184.CrossRefGoogle Scholar
Turnbull, J, Rayner, P, Miller, JB, Naegler, T, Ciais, P, Cozic, A. 2009. On the use of 14CO2 as a tracer for fossil fuel CO2: quantifying uncertainties using an atmospheric transport model. J. Geophys. Res. 114(D22). doi: 10.1029/2009JD012308.Google Scholar
Turnbull, JC, Mikaloff Fletcher, SE, Ansell, I, Brailsford, GW, Moss, RC, Norris, MW, Steinkamp, K. 2017. Sixty years of radiocarbon dioxide measurements at Wellington, New Zealand: 1954–2014. Atmos. Chem. Phys. 17:1477114784. doi : 10.5194/acp-17-14771-2017.CrossRefGoogle Scholar
Trumbore, SE. 2009. Radiocarbon and soil carbon dynamics. Annual Review of Earth and Planetary Sciences 37:4766. doi: 10.1146/ Scholar
Weller, RI, Levin, I, Wagenbach, D, Minikin, A. 2007. The air chemistry observatory at Neumayer stations (GvN and NH-II) Antarctica. Polarforschung 76(1–2):3946.Google Scholar
Worthy, DE et al. 2021. Results of a long-term international comparison of greenhouse gas and isotope measurements at the Global Atmosphere Watch (GAW) Station in Alert, Nunavut, Canada. In preparation.Google Scholar
Figure 0

Figure 1 Cooperative CO2 background air sampling network for high-precision 14C analysis in the Heidelberg Radiocarbon laboratory.

Figure 1

Figure 2 Development of tropospheric Δ14C-CO2 in both hemispheres since the start of the atmospheric nuclear bomb tests. New Zealand data are from Turnbull et al. (2017). The inlay shows the derivative of annual mean Δ14C-CO2.

Figure 2

Figure 3 a: Long-term trend of Δ14C-CO2 at the two polar stations Alert (Arctic) and Neumayer (Antarctica) (Levin and Hammer 2021). The solid lines are de-seasonalised fitted curves through the individual data. Panel b displays the difference between both fitted Δ14C-CO2 curves, while c shows the differences of corresponding fitted curves through CO2 concentration data from flask samples collected at the two stations (Weller et al. 2007; Worthy et al. 2021). d: Meridional distribution of three-year averages of CO2 (Dlugokencky et al. 2019), and Δ14C-CO2 (e) for 1993–1995 in comparison to 2008–2010.