Acid rain helped preserve glaciers.

• Position: Ecology

A chance discovery of 80-year-old photo plates in a Danish basement is providing new insight into how Greenland's glaciers are melting today. Researchers at the National Survey and Cadastre of Denmark--that country's federal agency responsible for surveys and mapping--had been storing the glass plates since explorer Knud Rasmussen's expedition to the southeast coast of Greenland in the early 1930s.

In Nature Geoscience, researchers describe how they analyzed 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. The imagery shows that glaciers in the region were melting taster in the 1930s than they are today, indicates Jason Box of the Byrd Polar Research Center, Columbus, Ohio.

A brief cooling period starting 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, and we've confirmed that glaciers are very sensitive indicators of climate."

Over the 80 years, two events stand out: glacial retreats from 1933-34 and 2000-10. In the 1930s, fewer glaciers were melting than are today, and most of those that were melting were land-terminating glaciers, meaning that they did not contact the sea. Those that were melting retreated an average of 20 meters per year--the fastest at 374 meters per year. Fifty-five percent of the glaciers in the study had similar or higher retreat rates during the 1930s than they do today.

However, more glaciers in southeast Greenland are retreating today, and...