To find how fast Greenland is melting, look at the past

Analyzing Greenland’s past temperatures will aid scientists in assessing how quickly the island’s vast ice sheet is melting, new research suggests.

The ice sheet has been shrinking since 1900 and the yearly loss of ice has doubled since 2003, other researchers have shown. The accelerated melting of the Greenland ice sheet is contributing to sea level rise.

The glaciers and ice sheet of Greenland cover a land area greater than the European countries of Germany, France, Spain, and Italy combined. If all Greenland’s ice melted, sea levels would rise by about 7 meters (23 feet).

Greenland warming shot by NASA
The redder the area on this map of Greenland, the more days that site was warmer in April 2016 than the average of April temperatures from 2001 to 2010. (Credit: Jesse Allen/NASA Earth Observatory)

Figuring out how fast the island’s ice has melted and will melt in the future requires knowing the past and the present surface air temperatures, according to researchers J. E. Jack Reeves Eyre and Xubin Zeng.

“Greenland is particularly important to global climate change because it has the potential to cause a big change in sea level,” says lead author Reeves Eyre, a doctoral student in the hydrology and atmospheric sciences department at the University of Arizona. “Knowing how it’s going to change over the next century is important.”

Calculating an average yearly surface temperature for the whole of Greenland is difficult. During most of the 20th century, the only weather stations were along the coast. There was no network of weather stations in Greenland’s interior until 1995.

Other groups of researchers have used combinations of weather station readings, satellite remote sensing data, statistical analyses, and climate models to calculate the island’s annual surface temperatures back to 1901. However, the results of those analyses disagree with one another substantially.

How Greenland’s massive ice sheet will respond to future warming is not well understood, says Zeng, a professor of hydrology and atmospheric sciences at the University of Arizona.

By combining the best two of the previous analyses, the study provides the most accurate estimates of Greenland’s 20th-century temperatures, says Reeves Eyre.

The finding will help improve climate models so they more accurately project future global climate change and its effects.

“That’s why we look at the historical period—it’s not about the history, it’s about the future,” says Zeng.

Knowing Greenland’s past temperatures is important for improving climate models, because scientists test regional and global climate models by seeing how well they predict what the climate was in the past.

Greenland was ice-free. Scientists worry it could be again

Previous analyses of the island’s past temperatures came up with contradictory results: Some says the 1930s were warmer than present, while other analyses say the opposite.

To find the best estimate of 20th-century temperatures, the scientists compared 16 different analyses. The team compared more datasets covering the period 1901 to 2014 and used more information from weather stations and field expeditions than previous studies.

“We are the first to bring all those datasets together,” Zeng says.

To avoid bias from lumping temperature data from different elevations, Reeves Eyre and Zeng divided the temperature data into three categories: data from coastal regions, data from lower than 1,500 meters (about 4,900 feet), and data from above 1,500 meters.

The coastal regions of Greenland are ice-free year-round, whereas the glaciers and ice sheet at the intermediate elevation melt some in the summer but refreeze in the winter, Reeves Eyre says. The ice sheet and glaciers at the intermediate elevations are shrinking a bit each year because temperatures are increasing.

Above 1,500 meters, the ice generally does not melt and may even gain mass, he says. However, the bit of ice gained at the highest elevations does not offset the loss of ice at the lower elevations.

The study resolves the discrepancies among the other analyses and provides the best estimates of Greenland’s past temperatures.

“The combination of the MERRA2 and GISTEMP (analyses) gives the most accurate results over the 20th century,” Reeves Eyre says. “Putting them together is more than the sum of the parts. Neither of them individually can do what both of them together can do.”

Although some previous analyses suggest the 1930s were warmer than it is now, the new analysis shows that current temperatures are warmer than the 1930s. The long-term trend for Greenland’s ice sheet appears to be for ever-higher surface temperatures, he says.

“By studying a wide range of available data and combining two of the best data sources, we’ve come up with a combination that best represents the whole distribution of temperatures over Greenland from 1880 to 2016,” Reeves Eyre says. “Using this dataset is the best way to evaluate climate models and their projection of temperature change over Greenland.”

Greenland’s ‘uplift’ makes ice loss hard to measure

The research paper appears online in the journal The Cryosphere.

NASA, the US Department of Energy, and the University of Arizona Agnese Nelms Haury Program in Environment and Social Justice funded the research.

Source: University of Arizona