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The 1950s atmospheric bomb-14C peak

Published online by Cambridge University Press:  19 October 2026

Pieter M. Grootes Open the ORCID record for Pieter M. Grootes [Opens in a new window] ,
Helene L. Svarva Open the ORCID record for Helene L. Svarva [Opens in a new window]  and
Marie-Josée Nadeau Open the ORCID record for Marie-Josée Nadeau [Opens in a new window]
Pieter M. Grootes*
Affiliation:
National Laboratory for Age Determination, Norwegian University of Science and Technology, Trondheim, Norway Institute for Ecosystem Research, Christian-Albrechts University, Kiel, Germany
Helene L. Svarva
Affiliation:
National Laboratory for Age Determination, Norwegian University of Science and Technology, Trondheim, Norway
Marie-Josée Nadeau
Affiliation:
Division of Geochronology and Environmental Isotopes, Institute of Physics, Silesian University of Technology, Gliwice, Poland National Laboratory for Age Determination, Norwegian University of Science and Technology, Trondheim, Norway
*
Corresponding author: Pieter M. Grootes; Email: pgrootes@ecology.uni-kiel.de
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Abstract

Comparison of cellulose 14C of the growth rings 1962–1965 of Norwegian Scots pine with Norwegian atmospheric 14CO2 over these bomb spike years indicates that tree rings can provide a good proxy for atmospheric 14CO2. The sub-seasonal cellulose 14C values of five trees from 63° to 69° N reveal local atmospheric 14CO2 values for the growing seasons 1950–1965. The 1950s are of special importance because there exist few direct measurements of atmospheric 14CO2 during this period of active atmospheric nuclear testing. First indications of bomb 14C are seen already in 1952. The temporary halt of atmospheric testing from November 1958 to September 1961 led to a temporary maximum in atmospheric 14C concentrations in 1959, creating the 1950s atmospheric bomb-14C peak. Longer atmospheric travel of 14C from low-latitude and Southern Hemisphere tests to Norway led to the highest tree 14C values in 1959 and 1963/’64 after testing stopped, respectively, in 1958 and 1962. The Scots pine data demonstrate that local atmospheric 14CO2 records with sub-annual resolution can generally be obtained. Combination with the SIPRI data base of nuclear testing, allows a focus on the production of bomb 14C and its transport through the atmosphere to the tree and promises a better understanding of the atmospheric mixing of CO2. Together with high-resolution local 14CO2 records this can better define the exchange between the atmosphere and the other reservoirs of the carbon cycle during the period of isotopic disequilibrium caused by atmospheric nuclear testing.

Keywords

1950s bomb spikeatmospheric nuclear testsbomb 14Ccellulose 14Csub-annual resolution

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Conference Paper
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Radiocarbon , First View , pp. 1 - 10
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This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
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© The Author(s), 2026. Published by Cambridge University Press on behalf of University of Arizona

Introduction

The extensive testing of nuclear weapons immediately preceding the Atmospheric Test Ban Treaty of 1963 produced the well-known atmospheric 14C “bomb spike” (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, de Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022). The atmospheric 14C consequences of the final testing have been directly documented because, by 1962, the 14C measurement technique had been well developed and monitoring programs had been set up to reveal the effect of the nuclear weapons testing on atmospheric and oceanic 14C concentrations. In Trondheim, Reidar Nydal developed, in 1962/63, a network of stations, mainly in the Northern Hemisphere but extending to a few stations in the Southern Hemisphere (Nydal and Løvseth Reference Nydal and Løvseth1996). In the Southern Hemisphere, atmospheric sampling started as early as 1950 in Pretoria, South Africa (Vogel and Marais Reference Vogel and Marais1971) in Wellington, New Zealand, in 1954 (Turnbull et al. Reference Turnbull, Mikaloff Fletcher, Ansell, Brailsford, Moss, Norris and Steinkamp2017) and on Suva, Fiji, in 1958 (Manning et al. Reference Manning, Lowe, Melhuish, Sparks, Wallace, Brenninkmeijer and McGrill1990). The effect of the 14C, added by the large tests of 1961/1962, to a carbon system that was in strong 14C disequilibrium due to the testing of the 1950s, has thus been directly documented. Continued monitoring has followed the decrease of the elevated atmospheric bomb 14C concentrations, the movement of the bomb 14C into various reservoirs, and its transport inside them.

There are, however, no detailed atmospheric 14C data of the effects of the nuclear weapon tests of the 1950s for the Northern Hemisphere and only few for the Southern Hemisphere. Monitoring at Vermunt, Austria, started in 1959 (Levin and Kromer Reference Levin and Kromer2004) during the Moratorium on atmospheric testing from September 1958 to April 1962. The earlier 1950s tests quickly increased the atmospheric 14C concentration through the pulsed local injections of bomb 14C, starting at a fossil-fuel-influenced near-equilibrium 14C concentration. Proxy records of atmospheric 14C concentrations during the 1950s will make it possible to use the atmospheric 14C injections of the 1950s bomb tests as tracers for the study of atmospheric mixing and of the global carbon cycle.

Detailed knowledge of the global carbon cycle is important for the projection of future greenhouse gas concentrations and global change scenarios. The transport of carbon between and within the reservoirs of the carbon cycle, atmosphere, ocean, biosphere, soils, and sediments, is difficult to quantify because transport and mixing are influenced by many environmental factors varying on time scales from hours to millennia, for instance day-night, the seasons, and orbital Milankovitch cycles. High-resolution data from instrumental records cover mainly the last century. Longer data sets from environmental archives, like tree rings, sediments, speleothems, and ice cores, often provide lower time resolution.

The Trondheim bomb-14C tree-ring project targets the environmental and climatic information preserved in the tree-ring archives. It uses the strong disequilibria in 14C content created by the pulsed 14C injection of atmospheric nuclear weapons testing. The rapid and large changes in atmospheric 14CO2 concentration of the “bomb spike” provide a “magnifying glass” to study carbon uptake in, and travel through different parts of the carbon cycle. Grootes et al. (Reference Grootes, Farwell, Schmidt, Leach and Stuiver1989) used the 1962–1964 “bomb spike” to demonstrate that tree-ring cellulose 14C concentrations closely follow atmospheric 14CO2. Svarva et al. (Reference Svarva, Grootes, Seiler and Nadeau2025) repeated and expanded this study to Norwegian Scots pine and confirmed that the tree-ring archive contains environmental information with sub seasonal to -weekly resolution (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming). We here use these same Scots pine data as proxy for atmospheric 14CO2 concentrations to explore the connection between changes in atmospheric 14C concentration in Norway and the atmospheric nuclear test series that caused them.

Data sets

We consider here the relation between data sets. The detailed Scots pine 14C data 1950–1965 are presented by Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming). The database of atmospheric nuclear tests is from the Stockholm International Peace Research Institute (SIPRI - Bergkvist and Ferm Reference Bergkvist and Ferm2000). These data are also available in the UNSCEAR 2000 Report (United Nations Scientific Committee on the Effects of Atomic Radiation). The global atmospheric circulation patterns and their link to atmospheric 14C distribution (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, de Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) are well known.

SIPRI database and global atmospheric circulation

The natural 14C distribution results from 14C production by cosmic radiation in the upper atmosphere and its incorporation in the global carbon cycle, especially the atmospheric and oceanic mixing and exchange. In addition, we have non-nuclear anthropogenic influences, especially through the use of fossil fuel.

The atmospheric nuclear tests, listed in the SIPRI report (Bergkvist and Ferm Reference Bergkvist and Ferm2000), provided instantaneous local 14C injections, mostly into the lower stratosphere, and were unique disturbances of the natural global atmospheric 14C distribution. Although this distribution was not uniform and changed dynamically over time, the differences in atmospheric 14C concentration and their rate of change were small relative to the bomb 14C related changes. This makes the local injection of bomb 14C a valuable tracer for studies of atmospheric mixing and the carbon cycle. Observed local, bomb-related 14C concentration changes are determined by the spreading of the test-produced, mostly stratospheric, bomb 14C through the atmosphere down to the local lower troposphere.

Of particular interest for the spreading are the location and timing of a test in connection with the seasonally shifting position of the Inter-Tropical Convergence Zone (ITCZ, also labelled the Tropical Low Pressure Belt, TLPB). The ITCZ is a several hundred kilometers wide low-pressure belt (“the thermal equator”) and its latitude follows the seasonally shifting maximum of solar heating. The ITCZ separates the northern from the southern hemispheric circulation. The latitude of a test and its time of the year are critical for the direction of the initial atmospheric spreading and the time of arrival at a specific location of the bomb 14C it produced in low-latitude tests. The distribution of the bomb 14C of most of the high-yield tests of the USA and the UK (Figure 1) was thus affected by the moving ITCZ and its arrival in Norway, the region of the data we use here, was delayed.

Figure 1.

Atmospheric radiocarbon zonation (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, de Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) with positions of the Inter-Tropical Convergence Zone (ITCZ) shaded in yellow and nuclear test sites indicated by the “atomic” symbol. The boundary between Northern Hemisphere zones 1 and 2 (NH 1/2) is data-based; the NH2/3 boundary follows the June-July-August position of the ITCZ. The equator defines the boundary between the Northern Hemisphere zone NH 3 and the Southern Hemisphere. SH zone 3 covers only the area of the ITCZ south of the equator.

A map showing atmospheric radiocarbon zonation with nuclear test sites and the Inter-Tropical Convergence Zone.

The start and the end of cellulose formation, the growing season, depend strongly on local temperature at the tree site, especially in colder regions such as Norway, and was estimated from local weather data for each of the Scots pine sites by Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming). Trees will only record ambient 14CO2 levels in their cellulose during the growing season. Table 1, summarizes the yearly nuclear test detonation yields per season and latitude and makes the a priori assumption that 14C from nuclear tests at mid- to high northern latitude during the first three seasons of the year (December to August) will be recorded in the same year’s cellulose in Norway. 14C from tests in fall (September, October, November) and tests at low northern and at southern latitudes will only be seen in next year’s cellulose.

Table 1.

Nuclear test detonation yields in kT (rounded to 1 kT) per season and first incorporation of their bomb 14C in Norwegian trees (right column). We assume bomb 14C from NH mid- to high-latitude tests during December through August reaches Norwegian trees the same year (first row; Sept-Nov blank)). Tests in September-November, at low Northern, and at Southern latitudes influence next year’s growth (second row). December yields are from the previous calendar year listed in the left column. (Bergkvist and Ferm Reference Bergkvist and Ferm2000). Dual values indicate the minimum/maximum estimates when the accurate yields were not known.

A table showing the timing of nuclear test detonation yields in kilotons per season, separated in mid-to-high Northern Hemisphere and ‘others’ plus the year of detection of the bomb 14C in Norwegian Scots pine cellulose.
Table 1. Long description

A table with 13 rows and 6 columns detailing atmospheric nuclear test detonation yields in kilotons (kT) per year and season and the corresponding year of bomb 14 cellulose incorporation. The columns are labeled as follows: Detonation year, four seasons: Dec - Feb, Mar - May, Jun - Aug, Sept - Nov, and Cellulose year. Each row lists the detonation year, followed by the detonation yields, split for each season in mid-to-high Northern Latitude tests detected in the same Norwegian cellulose year and tests incorporated in the next year. Notable entries include high yields such as 33200 kT in Mar - May 1954 and 32284 kT in Jun - Aug 1958, as well as heavy testing in Fal 1961 (83225/95965 kt) and Summer/Fall 1962 (52859/62899 kT and 64378/114538 kT). The table also includes dual values indicating minimum/maximum estimates when accurate yields were not known.

Scots pine cellulose 14C data as proxy of atmospheric 14CO2 and their uncertainty

The Scots pine tree-ring cellulose 14C data are presented and discussed by Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming). The overlap of the Scots pine cellulose 14C data with the results of the direct measurements of Norwegian atmospheric 14CO2 concentrations reported by Nydal and Løvseth (Reference Nydal and Løvseth1996) for the bomb spike years 1962 to 1965 is crucial. The agreement between atmospheric and cellulose data (Figure 2) indicates that the tree rings provide a suitable proxy for local atmospheric 14C concentrations during the growing season. The Scots pine 14C data from five Norwegian sites, from 63°15’ to 69°24’ N, sampled sub-seasonally (2–10 samples per season) from 1950 to 1965, thus provide hitherto unavailable details of the ground-level atmospheric 14CO2 increase in Norway during the atmospheric nuclear testing period 1950–1962 preceding the classical “bomb spike”. The systematic differences in 14CO2 concentration at different latitudes in Norway are discussed by Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming).

Figure 2.

Scots pine cellulose Δ14C values and atmospheric 14C concentrations of Northern Hemisphere zone 1 for the years 1950–1965. light grey band from Hua et al. (Reference Hua, Barbetti and Rakowski2013); darker grey band, a composite atmospheric curve, compiled for Norway from regional atmospheric measurements 1962–1965 (Svarva et al. Reference Svarva, Grootes, Seiler and Nadeau2025; Nydal and Løvseth Reference Nydal and Løvseth1996). The lower graphs show the sum of the maximum detonation yields (Bergkvist and Ferm Reference Bergkvist and Ferm2000) for the Southern and mid northern latitudes in pink, and for Nova Zemlya in cyan per season as defined in Table 1. The detonation yields for the Northern Hemisphere, except for Nova Zemlya, are too low to be seen on the graphs. Coordinates of the pine trees: Trondheim 63º16’N, 10º27’E at 134 m a.s.l.; Vassfjellet 63°15′N, 10°21′E, at ~495 m a.s.l.; Saltdal ca. 67°5′N, 15°21′E at 25 m a.s.l.; Målselv 69°1′N, 18°34′E at ~150 m a.s.l.; Pasvik 69°24′N, 29°43′E at 50 m a.s.l.

Three line graphs depict changes in atmospheric 14C levels of Northern Hemisphere zone 1 and those indicated by Scots pine cellulose together with detonation yields over the years 1950–1965.
Figure 2. Long description

Panel A: The line graph spans the years 1950 to 1956 on x-axis. The y-axis represents Δ 14C permil, ranging from -40 to 40. The graph includes data points from various locations: Målselv, Pasvik, Vassfjellet, Saltdal, and Trondheim, each represented by different colored symbols. The detonation yields are shown as bars at the bottom, with units in megatons (MT). The graph shows a gradual seasonal increase in Δ14C levels over time, picking up with the 1954 tests. Panel B: The line graph spans the years 1956 to 1962 on x-axis. The y-axis represents Δ14C in permil, ranging from -50 to 400. Similar to Panel A, it includes data points from the same locations and shows a significant increase in Δ14C levels in 1958, a spike in 1959 during the testing Moratorium with continued high 14C in 1961–1962. Detonation yields again are depicted as bars at the bottom. Panel C: The line graph spans the years from 1962 to 1966 on x-axis. The y-axis represents Δ14C in permil, ranging from 100 to 1300. This panel also includes tree cellulose data points from the same locations and shows a peak in Δ14C levels around 1963–1964 after the end of atmospheric testing in 1962, followed by a gradual decline. Detonation yields are depicted as bars at the bottom. The graph highlights the delayed impact of nuclear testing on atmospheric 14C levels in Norway over time.

Data uncertainty and observable differences

The statistical uncertainty of the Scots pine cellulose 14C data limits how large a local difference must be to be considered significant. Svarva et al. (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming) conclude that the effective uncertainty of the Scots pine data is close to the AMS statistical measurement uncertainty of ∼2‰ for a single data point. The yearly growing seasons have been estimated by Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming) based on meteorological data for each tree’s location. The increase in 14C values during the growing season is, in many cases, close to linear (Figure 2 and 3). Assuming a linear fit to the seasonal increase in 14C for each ring integrates the information from all increments of that ring and strengthens conclusions. This makes it possible to identify site differences of only a few permille and use these to explore both the differences in atmospheric 14C concentrations due to local conditions (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming) and those related to tests at different geographic locations and times.

Figure 3.

Single-year tree-cellulose Δ14C and atmospheric 14C concentrations of Northern Hemisphere zone 1. light gray band from Hua et al. (Reference Hua, Barbetti and Rakowski2013); dark gray band, a composite atmospheric curve compiled for Norway from regional atmospheric measurements (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming; Nydal and Løvseth Reference Nydal and Løvseth1996). The gray data points in the 1965 plot represent the atmospheric measurements done at Fruholmen by Nydal and Løvseth (Reference Nydal and Løvseth1996).

High resolution single-year data plots 1950–1965 of the cellulose 14C data of Figure 2 with their 1 sigma uncertainty and the Northern Hemisphere zone 1 Δ14C 1 sigma envelope.

Discussion

Figure 2: from a bird’s eye view

The Scots pine 14C data of Svarva et al. (Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming) are presented together with the Northern Hemisphere zone 1 (NH1) curve (Hua et al. Reference Hua, Barbetti and Rakowski2013, Figure 2), because Hua et al. (Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, de Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022) already include some of these Scots pine data (Svarva et al. Reference Svarva, Grootes, Seiler, Stene, Thun, Værnes and Nadeau2019). The values start slightly below those of NH1 with no clear seasonal trend in 1950 and 1951. A general increase in local atmospheric 14C concentration during the growing season is noticeable already in 1952. The 1951 Nevada (37°N) Spring testing of 40 kT was apparently too small to be seen in the Norwegian data. The testing of Fall 1951 in Nevada (71.9 kT) and Semipalatinsk (80 kT, 48ºN) plus the Nevada Spring testing of 1952 (104 kT) then caused this first bomb 14C increase during the 1952 growing season at 63º to 69ºN in Norway. Whether the Spring 1951 Enewetak tests (399 kT, 11.3ºN) already contributed is questionable. This early spreading of bomb-produced 14CO2 to Norway complements reports of the Yale Radiocarbon Laboratory that, starting in 1952, radioactive fallout made 14C measurements using Libby’s solid carbon method only possible when there was no nuclear testing (Blau et al. Reference Blau, Deevey and Gross1953; Preston et al. Reference Preston, Person and Deevey1955).

The increased testing in the following years is reflected in Scots pine 14C values that are higher for each subsequent season and increase ever more over each growing season until atmospheric testing paused November 1958. The 1958 Scots pine 14C values were probably determined by the high-yield tests at Nova Zemlya (73ºN) of September/October 1957 (4.5 MT) and February/March 1958 (3.3 MT). High-yield tests in Spring and Summer of 1958 were at low latitudes, at Bikini/Enewetak (35.6 MT, 11.3ºN) and at Christmas Island (3.0 MT, 2ºN), and probably contributed to the Norwegian peak in cellulose 14C values in 1959.

An informal suspension of atmospheric nuclear tests, agreed upon by the U.S., UK, and Soviet Union, started in October 31, 1958. The “gentleman’s agreement” proved fragile, ultimately breaking down in September 1961. Moderately lower 14C values in 1960 and 1961 indicate further atmospheric mixing of bomb 14C as well as carbon exchange of the highly 14C-enriched atmosphere with the ocean, biosphere, and soils. The 1959–1961 Scots pine 14C values at 63º to 69ºN agree in general with the early atmospheric 14C values at Vermunt (47°N; Levin and Kromer Reference Levin and Kromer2004). Then the extensive testing that preceded the 1963 Atmospheric Test Ban Treaty started and produced the familiar 1960s “bomb 14C” peak.

Figure 3: from a detailed view

The resolution of Figure 2, displaying the Scots pine 14C data for the 1950–1965 growing seasons, is too low to evaluate the differences in 14C concentration between the trees. This requires the sub-seasonal 14C data plotted year-by-year in Figure 3. The introduction of bomb 14C created an increase in atmospheric Δ14C values and a seasonal cellulose 14C increase for each tree across a growth-ring. We approach this increase in Δ14C by a straight line. The statistical uncertainty of this line is reduced relative to the uncertainty of a single data point, dependent on the number of sub-seasonal increments included. This allows the discussion of small differences in Δ14C values—between the years and the five trees—as statistically significant (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming).

A visual inspection of the seasonally evolving 14C concentrations of the five Scots pine over the years 1950–1965 yields the following observations that will be discussed pointwise below:

  • • No significant difference in 14C concentration between the trees in 1950 and 1951.

  • • Start of the seasonal 14C increase in the 1952 growing season.

  • • Roughly linear increase in 14C concentration during the growing season for each tree brings out differences between the trees.

  • • The rate of 14C increase during the 1963 growing season is quite similar for all five trees although they have different 14C concentrations (up to 70‰ difference).

  • • A non-linear rapid rise in 14C early in the 1958 growing season levels off later. This coincides with the start of tests at Nova Zemlya at 73°N in September–March 1957/58.

  • • Atmospheric 14C concentrations at the start of the growing seasons 1959 and 1963 are significantly higher than they were in the preceding Fall of 1958 and 1962.

  • • The peak in Scots pine 14C cellulose occurred in 1959, although atmospheric testing stopped early November 1958.

  • • 14C concentrations still increase in the first half of the 1964 growing season despite that testing ended December 1962.

A spread in the measured 14C concentrations in 1950 and 1951 that can be explained as the simple scatter of data with a statistical uncertainty of 2 to 3‰, and no statistically significant increase with time, is followed in 1952 by increased scatter and first indications of a bomb-Δ14C “slope”.

During the 1963 growing season, the advection of the large amounts of bomb 14C produced in 1961/1962 (232.8 MT) led to a rapid atmospheric 14C increase, recorded in cellulose 14C in all five trees, that highlights environmental differences between the trees. The linear regressions reflect the influence of different geography, topography, and growing conditions, on the cellulose 14C concentrations over the growing season. The rapid response of cellulose 14C to newly produced bomb 14C over the growing season is supported by 13C pulse-labeling experiments on an evergreen conifer branch by Kagawa et al. (Reference Kagawa, Sugimoto, Yamashita and Abe2005) that showed the first incorporation of photosynthesis carbon into cellulose happens within a few days both in earlywood and in latewood. The 14C differences between the trees themselves and between the trees and Norwegian atmospheric 14CO2 reflect that photosynthesis takes place in a local boundary layer that may have a lower 14CO2 concentration due to the admixture of older CO2 from biospheric decay and plant respiration (Svarva et al. Reference Svarva, Grootes, Seiler and Nadeau2025).

The rapid increase of 14C concentrations early in the 1958 growing season stands out and could reflect the high-yield tests of September 1957 to March 1958 at Nova Zemlya (7.8 MT, 73°N) injecting their bomb 14C just slightly north of the Norwegian Scots pine before the start of the growing season, mid-May to mid-June depending on site (Svarva et al. Reference Svarva, Grootes, Nadeau, Seiler, Philippsen and Rzadeczka-Jugaforthcoming).

The large difference in Scots pine 14C concentrations between the latewood/fall of 1958 (<180‰) and 1962 (<500‰) and the earlywood/spring of 1959 (>280‰) and 1963 (>600‰) documents a strong increase in local atmospheric 14C concentration during fall and winter. This demonstrates that the processes of the Brewer-Dobson atmospheric circulation (Butchart Reference Butchart2014) bring bomb 14C from various production sites to the local Scots pine troposphere throughout the year.

Although atmospheric testing temporarily paused on November 3rd, 1958, the Scots pine 14C concentrations of 1959 (280–300‰) are significantly higher than the 1958 maximum of ∼180‰. This most likely reflects when and where the 1958 testing took place. The 14C of USSR testing of Fall/Winter 1957/1958, at mid-to-high northern latitudes (Semipalatinsk and Nova Zemlya), reached Norway in Spring/Summer 1958. The USA and UK tests of late April to September 1958 were mainly at Enewetak/Bikini (11.3°N) and Christmas Island (2°N), at the time north of, yet close to the Inter Tropical Convergence Zone (ITCZ) over the Pacific Ocean. Their bomb 14C entered the Northern Hemisphere atmospheric circulation at low latitude and reached Norway in 1959 (before the start of the growing season – May-June). The same explanation of the time difference between the end of atmospheric testing and high Northern Hemisphere atmospheric 14C concentration holds for the end of atmospheric testing in December 1962 and the high atmospheric 14C concentrations of 1963/1964.

Outlook

In times of rapid 14C changes under natural or anthropogenic influences, sub-annual tree-ring 14C can provide detailed local atmospheric 14C values, indicate atmospheric transport processes, and for carbon cycle modeling, define the composition of the atmosphere during the strong 14C disequilibrium caused by atmospheric nuclear testing.

The 1962–1965 sub-seasonal cellulose 14C concentrations of the Norwegian Scots pine confirm the detailed tree-ring 14C record as a high-resolution proxy for local atmospheric 14C concentrations during the growing season. An archive of local atmospheric 14C concentrations during and after the atmospheric nuclear tests of the 1950s and 1960s has therefore, in principle, been preserved in the cellulose 14C concentrations of trees worldwide. Developments in accelerator mass spectrometry presently allow the measurement of small 14C samples (<1 mg C) at high precision (∼2‰) with a high sample throughput. This makes it feasible to obtain, at chosen locations, high resolution proxy records of atmospheric 14C concentrations during the growing seasons of the years of the atmospheric nuclear tests. The availability of meteorological data allows for a better estimate of local growing seasons and the timing of local atmospheric 14CO2 changes.

A first comparison of the detailed Scots pine 14C records with the SIPRI/UNSCEAR information about the location, date, and yield of atmospheric nuclear tests shows that details of the testing can be connected with some features of the Scots Pine data. Combination of the data of the atmospheric testing of nuclear weapons with sub-seasonal-resolution tree-ring 14C records from selected geographic regions will make it possible to trace the pathways of the distribution of 14C, produced in the 1950s tests, through the global atmosphere, up to the 1963/’64 “bomb spike”. The resulting detailed knowledge of the dynamics of global atmospheric mixing will be especially valuable for atmospheric 14C concentrations in the low-latitude zones that include the ITCZ (NH-3 and SH-3) where back-trajectory calculations indicate seasonally changing origin of local air masses (Ancapichún et al. Reference Ancapichún, de Pol-Holz, Christie, Santos, Collado-Fabbri, Garreaud, Lambert, Orfanoz-Cheuquelaf, Rojas, Southon, Turnbull and Creasman2021; Santos et al. Reference Santos, Albuquerque, Barros, Ancapichún, Oelkers Randreu-Hayles, de Faria, de Pol-Holz and das Neves Brandes2022, Reference Santos, Pereira, Barbosa, Ancapichún, van der Sleen, Zuidema and Groenendijk2025). The intra-annual results for the Cameroon tree (5ºN, 9ºE, Santos et al. Reference Santos, Pereira, Barbosa, Ancapichún, van der Sleen, Zuidema and Groenendijk2025) indicate that sub-seasonal resolution can also be attained for tropical trees. For carbon cycle modelling of the ocean and the biosphere, high-resolution atmospheric 14C data during the years of atmospheric nuclear testing and rapidly increasing 14C-system disequilibrium will be especially valuable and can expand the study of the Southern Ocean air-sea CO2 fluxes using back-trajectory calculations (e.g. Lewis et al. Reference Lewis, Corran, Mikaloff-Fletcher, Behrens, Moss, Brailsford, Lorrey, Norris and Turnbull2025).

Discussions of atmospheric bomb 14C generally focus on the high-yield tests (>1 Mt) that were mainly carried out at high- (Nova Zemlya) or quite low latitudes (Enewetak/Bikini, Christmas Island, Johnston Island, Malden Island). Low-yield testing continued, however, e.g. at the Nevada Test site, Semipalatinsk, and Nova Zemlya and may have contributed substantially to a local/regional atmospheric 14C increase (Hua et al. Reference Hua, Turnbull, Santos, Rakowski, Ancapichún, de Pol-Holz, Hammer, Lehman, Levin, Miller, Palmer and Turney2022; Li et al. Reference Li, Wei, Wang, Li, Sun, Feng, Cheng, Zhu, Wang, Chen, Zhao, Zhong, Zhou, Li and Zhang2025). Tree-ring 14C records taken “down-wind” from low-yield testing sites can make an important contribution to our knowledge of the local tropospheric effects of low-yield nuclear explosions and reveal the limitations of the use of a generalized bomb 14C record for the quantification of local fossil carbon contributions to atmospheric CO2.

Tales from the 14C bomb spike of the 1950s are waiting to be developed!

Acknowledgments

We thank the team of the NTNU National Laboratory for Age Determination for their support.

Footnotes

Selected Papers from the 25th International Radiocarbon Conference, Krakow, Poland, 29 June–4 July 2025

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Figure 0

Figure 1. Atmospheric radiocarbon zonation (Hua et al. 2022) with positions of the Inter-Tropical Convergence Zone (ITCZ) shaded in yellow and nuclear test sites indicated by the “atomic” symbol. The boundary between Northern Hemisphere zones 1 and 2 (NH 1/2) is data-based; the NH2/3 boundary follows the June-July-August position of the ITCZ. The equator defines the boundary between the Northern Hemisphere zone NH 3 and the Southern Hemisphere. SH zone 3 covers only the area of the ITCZ south of the equator.

Figure 1

Table 1. Nuclear test detonation yields in kT (rounded to 1 kT) per season and first incorporation of their bomb 14C in Norwegian trees (right column). We assume bomb 14C from NH mid- to high-latitude tests during December through August reaches Norwegian trees the same year (first row; Sept-Nov blank)). Tests in September-November, at low Northern, and at Southern latitudes influence next year’s growth (second row). December yields are from the previous calendar year listed in the left column. (Bergkvist and Ferm 2000). Dual values indicate the minimum/maximum estimates when the accurate yields were not known.Table 1 long description.

Figure 2

Figure 2. Figure 2 long description.Scots pine cellulose Δ14C values and atmospheric 14C concentrations of Northern Hemisphere zone 1 for the years 1950–1965. light grey band from Hua et al. (2013); darker grey band, a composite atmospheric curve, compiled for Norway from regional atmospheric measurements 1962–1965 (Svarva et al. 2025; Nydal and Løvseth 1996). The lower graphs show the sum of the maximum detonation yields (Bergkvist and Ferm 2000) for the Southern and mid northern latitudes in pink, and for Nova Zemlya in cyan per season as defined in Table 1. The detonation yields for the Northern Hemisphere, except for Nova Zemlya, are too low to be seen on the graphs. Coordinates of the pine trees: Trondheim 63º16’N, 10º27’E at 134 m a.s.l.; Vassfjellet 63°15′N, 10°21′E, at ~495 m a.s.l.; Saltdal ca. 67°5′N, 15°21′E at 25 m a.s.l.; Målselv 69°1′N, 18°34′E at ~150 m a.s.l.; Pasvik 69°24′N, 29°43′E at 50 m a.s.l.

Figure 3

Figure 3. Single-year tree-cellulose Δ14C and atmospheric 14C concentrations of Northern Hemisphere zone 1. light gray band from Hua et al. (2013); dark gray band, a composite atmospheric curve compiled for Norway from regional atmospheric measurements (Svarva et al. forthcoming; Nydal and Løvseth 1996). The gray data points in the 1965 plot represent the atmospheric measurements done at Fruholmen by Nydal and Løvseth (1996).