Nuclear Test Fallout in Southern Hemisphere Soils Now Has a Map

A European research team has published what it calls the first hemisphere-scale maps of the radioactive fallout that twentieth-century nuclear weapons tests left in Southern Hemisphere soils, along with maps of how uncertain each estimate is. The work appeared Nov. 7, 2026, in Communications Earth & Environment. Aydoğan Avcıoğlu of BRGM in Orléans, France, led it with senior author Katrin Meusburger of the Swiss Federal Institute for Forest, Snow and Landscape Research in Switzerland.

Fallout cesium and plutonium are the standard tracers of soil movement: researchers compare how much of them a field holds today with how much should have landed there, and the gap says how much soil has washed away or piled up. The authors report that the method has been held back worldwide by weak baselines in the Southern Hemisphere, where far fewer soils have been sampled. The amounts they map are reference levels for that measurement; the paper does not take up any question of risk to health.

The authors report that predicted fallout changes sharply with latitude and with the shape of the land, and that it is highest in tropical and mountainous regions. Their cesium-137 predictions vary less from place to place and carry less relative uncertainty than those for plutonium-239 and -240, which came out patchier and less certain.

The team also tested what the remaining uncertainty costs. Under a standardized hypothetical case in which half of a site's stock of fallout had been lost, the authors report that uncertainty in regions that received little fallout to begin with does not carry through evenly: it can swell as it passes into erosion and carbon calculations, skewing soil carbon budgets by as much as 19.8 metric tons of carbon per hectare.

The authors describe the maps as a new benchmark for fallout-tracer work in the Southern Hemisphere and as a framework for carrying spatial uncertainty into sediment tracing, soil degradation assessment and carbon-cycle studies. The study is open access and was funded by the Swiss National Science Foundation and the French National Research Agency through a project called AVATAR. The journal has released it as a peer-reviewed accepted version, which a final edited version will replace.