RADIOCARBON IN GLOBAL TROPOSPHERIC CARBON DIOXIDE

Abstract

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.

INTRODUCTION

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 2006), 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 1983; Levin et al. 1985). This “bomb spike” that reached the Southern Hemisphere with some delay (Manning et al. 1990), has been used as a transient tracer in all compartments of the fast carbon cycle (e.g., Broecker et al. 1985; Levin and Hesshaimer 2000; Trumbore 2009), but also to study atmospheric dynamics, such as inter-hemispheric (e.g., Münnich and Vogel 1958, 1963; Czeplak and Junge 1974) and stratosphere-troposphere air mass exchange (e.g., Telegadas 1971; Hesshaimer and Levin 2000; Levin et al. 2010). 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. 2010; Graven 2015). 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. 2020). 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.

THE HISTORY OF ATMOSPHERIC Δ¹⁴C-CO₂ OBSERVATIONS

Shortly after Anderson and Libby (1951) 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 1957). Münnich and Vogel (1963) 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. 1985). 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 1992; Turnbull et al. 2017).

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. 1980). 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. 2017, 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. 2000).

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 (1983, 1996) and by Meijer et al. (1995). 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. 1987). 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. (1970) 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. 1994; Levin and Hesshaimer 2000), 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 2006).

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. 2012). These samples were analyzed with high precision Accelerator Mass Spectrometry. The data were included in a global atmospheric transport model (TM3, Heimann and Korner 2003) 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. 2006; Miller et al. 2012). 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. 2012; Basu et al. 2020). 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. 2003; Turnbull et al. 2017; Graven et al. 2009). 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. 2020). 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 (https://www.iup.uni-heidelberg.de/research/kk/icos).

TEMPORAL DEVELOPMENT OF Δ¹⁴C-CO₂ IN THE GLOBAL TROPOSPHERE

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. 2017). From 1983 onwards, we extended this record with data from the Neumayer station, Antarctica (Levin and Hammer 2021). 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. (2017), 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 2021).

Figure 2 Development of tropospheric Δ^{1 4}C-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 Δ^{1 4}C-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 1971). 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 1981; Hesshaimer and Levin 2000). 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 1989; Kjellström et al. 2000; Levin et al. 2010).

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. (2017), 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. (2010: 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 2009), 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. (2010) 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 2021). 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. 2017). 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 Δ^{1 4}C-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 Δ^{1 4}C-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 Δ^{1 4}C-CO₂ (e) for 1993–1995 in comparison to 2008–2010.

SPATIAL VARIATION OF Δ¹⁴C-CO₂ IN THE TROPOSPHERE

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. 2019). 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 2000). 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. 1987). 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. 1997) 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. (2012). 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. 2012). 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. 2010).

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 (2009) 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. (2012) 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.

CONCLUSIONS AND PERSPECTIVES

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. 2013; Jahn et al. 2015). Concerning atmospheric tracer transport modeling of CO₂, including ¹⁴C in global and regional inversions is still at its infancy (e.g., Turnbull et al. 2009; Basu et al. 2020). 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.