Greenland's ice sheet melted twice as fast between 2003 and 2010 as it did from 1900 to 1983, according to the first study of Greenland ice loss over the past century that is based on observations rather than models.

Between 1900 and 2010, meltwater from Greenland boosted the global sea level by 25 millimetres, making up 10 to 18 per cent of global sea level rise, reports the new study published today by an international team of researchers, including some in Canada.

Helheim Glacier in Southeast Greenland

The new study also shows exactly where ice has disappeared from. Generally, the places that were melting during the 20th century are the same ones that are still melting now. (Nicolaj Krog Larsen/Aarhus University)

Previous estimates have suggested that Greenland contains enough ice to raise world sea levels by about six metres if it all melted – a process that could take thousands of years.

The new study, published Wednesday in Nature, provides the first good estimate of how much Greenland has contributed to sea level rise so far, filling a gap in the last report by the United Nations Intergovernmental Panel on Climate Change, the researchers report.

That will help make models — and future predictions — of sea level rise more reliable, said Kristian Kjeldsen, lead author of the paper. Kjeldsen is a postdoctoral researcher at the University of Ottawa and at the Centre for GeoGenetics at Natural History Museum of Denmark at the University of Copenhagen.

The melting of Greenland's ice sheet has been measured since 1992 using satellites, but there wasn't any direct data about how much ice had melted off Greenland before that, the researchers say.

4,000 photographs

Kjeldsen and his collaborators wanted to use more direct observations to figure out both how much ice was lost during the 1900s and precisely which glaciers it came from.

To do that, they turned to 3D or stereophotogrammic aerial photos captured between 1978 and 1987. The images clearly show a mark called a trimline. Below it, the landscape is a different colour, where the glaciers scraped against the rock and eroded away the vegetation. The trimline is the maximum extent of glaciers before they started retreating around 1900, the end of a cold period called The Little Ice Age.

By using the trimline as a reference and combining the 4,000 aerial photos with later satellite data and "control points" of known elevation, the researchers were able to measure and reconstruct the extent of the ice in 1900, and how much it changed over time.

Kangiata Nunata Sermia

Kangiata Nunata Sermia in southwest Greenland shows different lines that marks the extent of the ice at different points in time. The upper trimline (transition) between the lighter and darker valley sides marks the extent of the glacier during the Little Ice Age, while the lower lines shows the extent at later points in time. (Hans Henrik Tholstrup/Natural History Museum of Denmark)

"What we have is an estimate that's really founded in observation," said Kjeldsen.

Previously, estimates of ice lost during the 20th century came from computer models based on recent satellite data.

The new study also shows exactly where ice has disappeared from. Generally, the places that were melting during the 20th century are the same ones that are still melting now, Kjeldsen said.

"The pattern agreed so much going back in time."

Overall, from 2003 to 2010, Greenland dumped 186 billion tonnes of meltwater a year into the world's oceans, the researchers estimated.

From 1900 to 1983, the rate averaged 75 billion tonnes a year – much lower, but still significant, Kjeldsen said.

"That was a little bit surprising, that it was so consistent and so high," he said.

Still, he said, "The mass loss has really increased quite a bit going into the 21st century."