Recently unearthed photographs taken by Danish explorers in the 1930s show glaciers in Greenland retreating faster than they are today, according to researchers.

We're not worried about rising sea levels. Well, we are in a seaplane.

The photos in question were taken by the seventh Thule Expedition to Greenland led by Dr Knud Rasmussen in 1932. The explorers were equipped with a seaplane, which they used to take aerial snaps of glaciers along the Arctic island's coasts.

After the expedition returned the photographs were used to make maps and charts of the area, then placed in archives in Denmark where they lay forgotten for decades. Then, in recent years, international researchers trying to find information on the history of the Greenland glaciers stumbled across them.

Taken together the pictures show clearly that glaciers in the region were melting even faster in the 1930s than they are today, according to Professor Jason Box, who works at the Byrd Polar Research Center at Ohio State uni.

There's much scientific interest in the Greenland ice sheet, as unlike most of the Arctic ice cap it sits on land: thus if it were to melt, serious sea level rises could occur (though the latest research says that this doesn't appear to be on the cards).

It's difficult to know exactly what's happening to the Greenland ice in total and very different estimates have been produced in recent times. However Professor Box says that many glaciers along the coasts have started retreating in the past decade.

It now appears that the glaciers were retreating even faster eighty years ago: but nobody worried about it, and the ice subsequently came back again. Box theorises that this is likely to be because of sulphur pollution released into the atmosphere by humans, especially by burning coal and fuel oils. This is known to have a cooling effect.

Unfortunately atmospheric sulphur emissions also cause other things such as acid rain, and as a result rich Western nations cracked down on sulphates in the 1960s. Prof Box believes that this led to warming from the 1970s onward, which has now led to the glaciers retreating since around 2000.

Other scientists have said recently that late-20th-century temperature rises in the Arctic may result largely from clean-air legislation intended to deal with acid rain: some have even gone so far as to suggest that rapid coal- and diesel-fuelled industrialisation in China is serving to prevent further warming right now.

Still other scientists, differing with Prof Box, offer another picture altogether of Arctic temperatures, in which there were peaks both in the 1930s and 1950s and cooling until the 1990s: and in which the warming trend which resulted in the melting seen by Rasmussen's expedition actually started as early as 1840, before the industrial revolution and human-driven carbon emission had even got rolling. In that scenario, variations in the Sun seem to have much more weight than is generally accepted by today's climatologists.

At any rate, the new information from the old Danish pictures adds some more data to the subject. The new study by Box and his co-authors is published by Nature Geoscience, here. ®