Radionuclide tracers reveal climate-driven changes in the Pacific Ocean
Studies of anthropogenic radionuclides in the Pacific Ocean and marginal seas provide new evidence of changes associated with climate change impacts
Global warming is affecting fundamental processes throughout the Earth system. Changes in precipitation patterns, more frequent flash floods, droughts and wildfires, and the melting of glaciers are accompanied by rising sea levels and changes in the oceanic carbon cycle. At the same time, marine heatwaves are becoming increasingly important, with consequences for ocean biogeochemistry, surface ocean acidification, biodiversity, and the poleward migration of marine organisms. Anthropogenic radionuclides such as 3H, 14C, 90Sr, 129I, 137Cs, and plutonium (Pu) isotopes provide particularly valuable tools for investigating these changes. Released into the marine environment mainly as a result of atmospheric nuclear weapons testing since 1945 (global fallout), these radionuclides can serve as sensitive, time-resolved tracers of processes occurring in the ocean. Their well-characterized input histories and, for some radionuclides, largely conservative behavior in seawater make them effective time markers for studying water mass formation, transport, and vertical exchange. Measurements of their distribution in seawater, bottom sediments, and marine organisms can therefore provide information on climate-driven modifications in ocean circulation and stratification.
The Pacific Ocean, the largest ocean basin on Earth, has been extensively sampled for anthropogenic radionuclides over several decades. This unique observational record makes it an important natural laboratory for investigating the impacts of climate change on the marine environment. For example, analyses of 3H and 137Cs in seawater have documented multidecadal changes in upper-ocean ventilation and transport pathways, including changes associated with a slowing of ocean circulation. Plutonium isotopes provided complementary information on particle-mediated export and regeneration processes in the water column. Such observations helped to assess changes in marine biogeochemical processes and their potential implications for the efficiency of carbon storage through the ocean's biological pump. Plutonium isotope ratios measured in sediments from the Indonesian seas have provided evidence of a weakening of the transport of surface waters from the western tropical North Pacific through the Indonesian seas to the Indian Ocean. Taken together, radionuclide observations, circulation studies, and scavenging models present a picture broadly consistent with an ocean that is warming and becoming increasingly stratified in its upper layers. Stronger stratification can reduce vertical exchange and influence overturning circulation and connectivity between ocean basins, with potential consequences for the Global Meridional Overturning Circulation and the global redistribution of heat, carbon, nutrients, and other substances. The Japan Sea provides a particularly interesting example. Rising surface seawater temperatures observed in recent decades have been accompanied by reduced winter convection, subsurface temperature anomalies, and episodic marine heatwaves. Changes in cyclonic and anticyclonic eddy activity have also affected the recirculation of bottom waters toward the surface. These processes may have important consequences for marine ecosystems and fisheries. Radionuclide records from the South China Sea reflect variability associated with the Asian monsoon systems and El Niño–Southern Oscillation (ENSO) events, demonstrating the value of radionuclide tracers for studying interactions between the ocean and the climate system.
Long-term investigations of anthropogenic radionuclides have considerable potential to improve our understanding of present and future climate change impacts on the world ocean and adjacent seas. They provide independent, observation-based constraints on circulation changes that can be difficult to identify using physical measurements alone. Marine radionuclide datasets, including those stored in the International Atomic Energy Agency's Marine Radioactivity Information System (MARIS; https://maris.iaea.org), also provide valuable reference data for calibrating and validating Ocean General Circulation Models and Earth System Models used to investigate future climate scenarios. Further progress will require coordinated international high-resolution sampling programs covering the world's oceans, combined with advanced ultrasensitive analytical methods such as accelerator mass spectrometry and low-background gamma spectrometry. These techniques make it possible to analyze smaller samples of seawater, sediments, and corals while achieving higher-resolution radionuclide profiles. Particular attention should be given to the Southern Ocean because of its critical role in the global climate system and the relative scarcity of comprehensive radionuclide observations in this region.
The results of these investigations have recently been published in Communications Earth & Environment (Nature Portfolio) and the Journal of Environmental Radioactivity (Elsevier):
Pavel P. Povinec, Katsumi Hirose, Gi-Hoon Hong, Xiaolin Hou, Yayoi Inomata, Jakub Kaizer, and Daisuke Tsumune. Anthropogenic radionuclides as tracers of climate change in the Pacific Ocean. Communications Earth & Environment 7 (2026), 427.
https://doi.org/10.1038/s43247-026-03639-0
P. P. Povinec, K. Hirose, G. H. Hong, X. Hou, Y. Inomata, J. Kaizer, D. Tsumune, and X. Zhao. Marine radionuclides in climate change studies: Pacific Ocean and marginal seas. Journal of Environmental Radioactivity 298 (2026), 108022.
https://doi.org/10.1016/j.jenvrad.2026.108022
Figure descriptions:
Figure 1: The Global Meridional Overturning Circulation, sometimes referred to as the global ocean conveyor belt, is a large-scale circulation system connecting surface and deep waters across the world's ocean basins. Through the Southern Ocean, it links circulation in the Atlantic, Indian, Pacific, and Arctic Oceans and redistributes heat, carbon, freshwater, and nutrients. The circulation operates over timescales ranging from decades to millennia. Because its flow is slow and varies on multidecadal timescales, independent tracers such as anthropogenic radionuclides are valuable for constraining changes in its pathways and strength. Typical flow velocities are small (~1 cm/s), but the integrated volume of transported water is enormous and strongly influences regional climate, sea level, and marine ecosystems. Northward heat transport in the Atlantic, including the Gulf Stream system, contributes substantially to the relatively mild climate of northwestern Europe.
Figure 2: Distribution of 137Cs across the South Pacific (30°S), Indian (20°S), and Atlantic (30°S) Oceans were obtained during the BEAGLE (Blue Ocean Global Expedition), providing the first mapping across the World Ocean. The Indian Ocean shows pronounced subsurface maxima at depths of approximately 100–200 m, particularly near 100°E and 55°E, which can be explained by transport of surface waters from the North Pacific through the Indonesian seas into the South Indian Ocean, where the subtropical gyre acts as a long-term reservoir of anthropogenic pollutants. The observations provide further information on connections between the Pacific, Indian, and Atlantic Oceans within the broader Global Meridional Overturning Circulation.