The glass photo plates, taken on Danish explorer Knud Rasmussen's expedition to the southeast coast of Greenland in the early 1930s, were stored in the basement of the National Survey and Cadastre of Denmark, the country's federal agency for surveys and mapping.
Ohio State University researchers and colleagues in Denmark have been able to analyze ice loss in the region by comparing the images on the plates to aerial photographs and satellite images taken from World War II to today.
Their analysis shows glaciers in the region were melting even faster in the 1930s than they are today, before a brief cooling period in the mid-20th century allowed new ice to form, and then the melting began to accelerate again in the 2000s.
"Because of this study, we now have a detailed historical analogue for more recent glacier loss," geographer Jason Box said in an OSU release Tuesday. "And we've confirmed that glaciers are very sensitive indicators of climate."
Pre-satellite observations of Greenland's glaciers are rare, researchers said, making the discovery of the old aerial photos extremely important, lead study author Anders Anker Bjork at the Natural History Museum of Denmark said.
"They were cleaning up in the basement and had found some old glass plates with glaciers on them," he said. "The reason the plates were forgotten was that they were recorded for mapping, and once the map was produced they didn't have much value."