Ohio State University researchers said the missing radioactivity -- originating as fallout from 1950s and 1960s nuclear tests -- might be an ominous sign for the Indian subcontinent.
The radioactivity is used by researchers as a benchmark against which they can gauge how much new ice has accumulated on a glacier or ice field.
The absence of that marker from ice cores drilled from a large glacier 19,849 feet high on the Tibetan Plateau means the ice field has been shrinking at least since the atomic bomb tests conducted half a century ago.
Ohio State University Professor Lonnie Thompson said the finding might foreshadow a future when the stockpiles of freshwater will vanish, affecting more than 500 million people on the Indian subcontinent.
"There's about 2,879 cubic miles of fresh water stored in the glaciers throughout the Himalayas -- more freshwater than in Lake Superior," said Thompson, who is also a researcher with the university's Byrd Polar Research Center.
Thompson presented his findings Tuesday in San Francisco during the annual meeting of the American Geophysical Union.