Eighty-year-old photos discovered by chance in a Danish basement have thrown light on how Greenland glaciers are melting today.
Researchers at the National Survey and Cadastre of Denmark - the federal agency responsible for surveys and mapping - had held the glass plates since explorer Knud Rasmussen's 7uth Thule expedition to the southeast coast of Greenland in the early 1930s.
And, now, Ohio State University researchers and colleagues have compared the images on the plates with aerial photographs and satellite images taken from World War II to today - and found that glaciers in the region were melting even faster in the 1930s than they are now.
A brief cooling period starting in the mid-20th century allowed new ice to form, with melting starting to accelerate again in the 2000s, says Jason Box, associate professor of geography and researcher at the Byrd Polar Research Center at Ohio State.
"Because of this study, we now have a detailed historical analogue for more recent glacier loss," he says. "And we've confirmed that glaciers are very sensitive indicators of climate."
Pre-satellite observations of Greenland glaciers are rare, and when researchers at the National Survey found the plates they realised they might have some scientific value. They'd been discarded because, once they'd been used for mapping, they weren't considered to have much value.
The plates document Rasmussen's 7th Thule Expedition to Greenland and contain aerial photographs of land, sea and glaciers in the southeast region of the country, along with travel photos of Rasmussen's team.
The researchers digitized the images and looked for differences in the shape of the southeast Greenland coastline where the ice meets the Atlantic Ocean. They then calculated the distance the ice front moved in each time period.
Over the 80 years, two events stand out: glacial retreats from 1933 to 1934 and from 2000 to 2010. In the 1930s, fewer glaciers were melting than are today, and most of those that were melting were land-terminating glaciers. Fifty-five percent of the glaciers in the study had similar or higher retreat rates during the 1930s than they do today.
Still, more glaciers in southeast Greenland are retreating today, and the average ice loss is 50 meters per year. That's because a few glaciers with very fast melting rates - including one retreating at 887 meters per year - boost the overall average.
Between the two melting events - 1943 to 1972 - southeast Greenland cooled, probably because of sulfur pollution, which reflects sunlight away from the earth. Its presence in the atmosphere peaked just after the Clean Air Act was established in 1963 - and as it was removed from the atmosphere, the earlier warming resumed.