A radiation warning sign in the Chernobyl Exclusion Zone, Ukraine. ArticCynda, CC BY-SA 4.0, via Wikimedia Commons
On 1st March 1954, a Japanese fishing boat called the Lucky Dragon No. 5 was navigating waters in the middle of the Pacific. Just before dawn, as the crew went through the routine of a last catch before returning home, a multicoloured flash lit up the sky in the west. Minutes later, the crew heard an explosion, and within two hours they were covered in a gritty dust that they would come to know as “death ash”.
Earlier that morning, a 10.7 tonne bomb was dropped on Bikini Atoll in the Marshall Islands, some 80 miles to the west. This was Castle Bravo, one of the United States’ many thermonuclear weapon design tests. Despite attempts to calculate the impact, this particular test turned out to be three times as powerful as anticipated. Though outside the danger zone the US military had advised vessels to avoid, the Lucky Dragon No. 5 was nonetheless hit by the bomb’s fallout. All the crew suffered acute radiation syndrome, and one member died.
With a yield of 15 megatons, Castle Bravo was the US’ largest bomb ever detonated—and the fifth largest in the world. It represents the peak of an era of unprecedented military testing, the results of which are still lamentably apparent across the globe. Though the tragic human impact is well-documented, including (among many other instances) the inhabitants of Bikini Atoll who were evacuated from the Marshall Islands too late to avoid the bomb’s radiation, the ecological impact has been more gradual in coming to light.
Nuclear fallout, whether from nuclear detonations or unintentional explosions like in Chernobyl in 1986, falls to Earth from the atmosphere as dust and other materials. Though much of the radioactive portion will often decay quickly, some hangs around for longer. Indeed, atmospheric weapons tests were responsible for a near doubling of the concentration of carbon-14 in the Northern Hemisphere, until their ban under the Partial Nuclear Test Ban Treaty in 1963. The subsequent peak in 1964 has been proposed by some as the beginning of the Anthropocene—the geological epoch defined by human impact.
Some persistent radioisotopes—radioactive variants of elements—enter the food chain and accumulate in the tissues of living organisms. While animals close to the blasts get first-hand exposure to radioactive isotopes, contaminants are spread far beyond the immediate area of the blast due to the interconnectedness of the world’s ecosystems. Circulation of matter through the oceans and atmosphere inadvertently enables these hugely destructive human experiments to ricochet throughout the natural world.
In the late 20th century, bomb radiocarbon began to be acknowledged as a valuable resource for understanding environmental systems. Scientists recognised that the global distribution of carbon-14 could be used as an indicator of oceanic and atmospheric carbon flux. By analysing where it was introduced into the oceans and atmosphere in the nuclear era, they could better describe how in general carbon moves between different regions.
For example, up on the Altiplano plateau in the central Andes, bomb signals in tree rings have changed our understanding of circulation dynamics in the Southern hemisphere. They suggest, for example, that the carbon turnover rate in the Amazon may be faster than previously thought (which would have important implications for the climate). Similarly, measures of radiocarbon build-up in coral in the South Pacific have revealed how ocean currents influenced its spread, demonstrating in particular the complex role of climate events in determining circulation patterns.
However, beyond ecosystem effects, the story of the spread of bomb radiocarbon has much to tell us about the lives of individual animals. A number of studies are showing us that radioactive carbon, released into the environment during the atomic testing era of the 1940s–60s, is detectable in many wild animals alive today.
In 2020, for instance, biologist Dr Joyce Ong and colleagues reported testing the carbon radioisotopes present in the vertebrae of whale sharks from Taiwan and Pakistan. This technique is applicable to many carbon-containing tissues, such as bones, teeth, and horns. Using radiocarbon dating, the researchers found that the accumulation of carbon-14 during the bomb testing period allowed them to date the annual bands in the sharks’ vertebrae. This revealed one shark to be 50 years old, the oldest confirmed age for the species.
Recovery of radiocarbon signals from all corners of the natural world continues to expand our understanding of its history. Bomb radiocarbon has now been used to validate other methods of ageing individual animals of many species. This is a valuable tool in assessing the age of wild animals, which, among other things, is important for their conservation. How long the average animal lives for is crucial information for predicting population trajectories—the new upper limit for whale sharks suggests it may be more vulnerable to the threat of extinction, for example. It also reveals just how long-lasting our impacts on the creatures around us are.
But, as well as revising our estimates of species’ longevity, bomb radiocarbon dating is also finding more practical applications. In commercial fish species—such as mackerel and triggerfish—it has often been done via sampling of the otolith, a calcium carbonate-containing structure of the inner ear. Some argue this provides a valuable tool for fisheries management, where ageing of fish stocks is useful for guiding sustainable fishing activity.
Elsewhere, radiocarbon-based ageing might help in the fight against the illegal wildlife trade, as part of what is known as wildlife forensics. This is essentially the principles of forensic science—scientific assessment of criminal evidence—applied to solving wildlife-related crime. Dating of elephant ivory and rhino horn, for instance, can identify illegal products since tissue age is the basis of many trade restrictions imposed by the Convention on the International Trade of Endangered Species.
Ong’s research on whale sharks is a great example of how individual animals can bear the signatures of events from our history, but it is not the first. Reports of individual animals showing unmistakable signs of past interaction with humans are not uncommon. In 2007, an Inuit whaling crew found a harpoon fragment in a bowhead whale dating from 1879, suggesting the animal was between 115–130 years old. Almost 60 years earlier, a white stork was found near a village in northern Germany pierced through the neck with an 80 cm arrow (see image). That the arrow was unmistakably from central Africa provided the first evidence of long-distance migration in birds, catalysing a revolution in the understanding of animal migration.
But among the countless human activities that have influenced wildlife, the effects of the nuclear era are of course uniquely pervasive, and radioactive fallout has impacted wildlife in ways that reveal more than just their age and origin. In many places it has showcased the nightmarish potential of radiation to wreak havoc on organisms’ physiology. Many mammals and birds in these regions have cataracts and unusually small brains, echoing patterns seen in human survivors of Hiroshima and Nagasaki. Malformed sperm is a common consequence of radiation exposure, and in some radioactive areas, up to 40% of male birds are completely sterile.
While horrifying, these observations are a boon to researchers trying to understand the impact of genetic mutations. One analysis of bird species around the Chernobyl disaster site revealed that those with higher natural mutation rates, such as barn swallows and blackcaps, are doing worse. Their populations have on average declined more quickly, presumably due to the relative impact of radiation-induced mutations. Another study at Chernobyl confirmed the natural variation in mutation rate, even finding it to be influenced more strongly among plants than animal species. (It’s not obvious whether humans’ “intermediate sensitivity” compared to other species should be reassuring or not.)
Despite all the deeply destructive impacts of historic detonations and accidents on the natural world, part of their legacy is, counterintuitively, to have left us with a greater understanding of many organisms’ development, evolution, and genetics. As more information comes to light—whether about mutation rates, ageing, or the longevity of carbon isotopes in animal bodies—we are learning ever more about the effects of radiation on biological systems.
The impact of nuclear fallout on organisms’ physiology is presumably far greater than we know. It is felt particularly uniquely, perhaps, by long-lived animals alive during humans’ nuclear heyday, who still carry heightened concentrations of radioactive isotopes in their tissues. For many species, however, the legacy of nuclear accidents is a destroyed or contaminated habitat (a fate disproportionately affecting those living around coral reefs.) For inhabitants of areas that remain highly radioactive today, the mutagenic effect may be even more insidious.
Others have managed to carve out space in otherwise uninhabitable locations. For example, the forbidden zone around the Chernobyl disaster site has proved an oasis for wolves, who are flourishing in the absence of humans. And, over 70 years after Castle Bravo, the ecosystem of Bikini Atoll seems to have bounced back, despite abundant radioactive contamination.
Across the world, highly radioactive zones form natural experiments in the biological effects of radiation. The human price of these experiments has been unimaginably vast. We may also consider the huge toll they have taken on the natural world. Whether they have felt the consequences or not, it is certainly a strange thought that those elderly animals, of unanticipated value to modern science, roam the planet with a “bomb pulse” of radiocarbon still detectable inside them.