The Greenland Ice Sheet is the second largest body of ice in the world. It covers around 80 percent of Greenland, extending 2,900 km (1,800 mi) in a north–south direction, and has a maximum width of 1,100 km (680 mi). Scientists have been monitoring the thinning of the glaciers and ice sheet around the coast of Greenland for decades, as global warming has advanced, but the influence of warming on the elevated, central areas of the ice sheet has remained unknown, due to a lack of long-term observations.
In a new study published in the journal Nature, a team led by researchers from the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) has used a set of ice cores of unprecedented length and quality, to reconstruct temperatures in central-north Greenland over the past 1,000 years. Their new dataset has come from areas in the central-north that are elevated up to 3,000 m, which represents the highest and thickest parts of the ice sheet.
Previous ice cores obtained at co-located sites, starting in the 1990s, did not indicate clear warming patterns in central-north Greenland, despite rising global mean temperatures. Part of the reason is that there is substantial natural climate variability in the region and it is challenging to identify consistent patterns of warming against the background of natural variability.
However, in their new study, the AWI researchers have extended the previous datasets up to winter 2011/2012 by a dedicated redrilling effort. They used the concentrations of stable oxygen isotopes present in the water that formed the ice, to deduce the temperatures prevailing at the times of ice formation. This method was used consistently for the entire ice core record. Previous studies have used a range of different climate archives and datasets to reconstruct past temperatures, which has introduced much greater levels of uncertainty into the results.
“The time series we recovered from ice cores now continuously covers more than 1,000 years, from year 1000 to 2011. This data shows that the warming in 2001 to 2011 clearly differs from natural variations during the past 1,000 years. Although grimly expected in the light of global warming, we were surprised by how evident this difference really was,” said AWI glaciologist Dr. Maria Hörhold, lead author of the study. Together with colleagues from AWI and the University of Copenhagen’s Niels Bohr Institute, she analyzed the isotope composition in shallow ice cores gathered in central-north Greenland during dedicated AWI expeditions.
The researchers found that the most recent decade surveyed in the study, the years 2001 to 2011, was the warmest decade in the past 1,000 years. Their data also showed that the region is now 1.5 °C warmer than during the 20th century, indicating that global warming has most certainly begun to impact even the areas of the Greenland Ice Sheet that are at the highest altitudes.
In addition to assessing the temperatures, the team also reconstructed the drainage of melt water from the Ice Sheet and investigated the relationship between temperature changes at elevated points inland, and melting along the edges of the Ice Sheet. The Greenland Ice Sheet is crucial in the global climate system. With enormous amounts of water stored in the ice (about 3 million cubic kilometers), melt and resulting sea-level rise is considered a potential tipping point. If global emissions remain unchecked and its ‘business as usual’, the Greenland Ice Sheet is projected to contribute as much as 50 cm to global mean sea-level rise by 2100.
In order to assess the rate of melt, the researchers used data from a regional climate model for the years 1871 to 2011, along with satellite observations of ice-mass changes for the years 2002 to 2021 from the GRACE/GRACE-FO gravimetry missions. This allowed them to convert the temperature variations identified in the ice cores into melting rates, and provide estimates for the past 1,000 years.
The experts found that melting has increased substantially in Greenland since the 2000s and now significantly contributes to global sea-level rise. “We were amazed to see how closely temperatures inland are connected to Greenland-wide meltwater drainage – which, after all, occurs in low-elevation areas along the rim of the Ice Sheet, near the coast,” said Dr. Hörhold.
This represents an important dataset for climate research: better understanding of the past dynamics of the Ice Sheet melt improves projections of related future sea-level rise; reduced uncertainties in projections is one step to help optimize adaptation measures.
Another exciting finding from the study was that the climate of the Greenland Ice Sheet is largely decoupled from the rest of the Arctic. The Arctic-wide study of proxy temperatures during the past 2,000 years, known as ‘Arctic 2k’ and published in Nature in 2014, does not reflect the conditions obtained by Hörhold in the current study of the Greenland Ice Sheet.
“Our reconstruction now offers a robust representation of temperature evolution in central Greenland, which has proven to have a dynamic of its own,” explained study co-author Professor Thomas Laepple. “Actually, we had expected the time series to strongly covary with the warming of the Arctic region.”
The researchers have an explanation for these differences: Greenland is more affected by patterns of atmospheric circulation than other parts of the Arctic, due to the height of the ice cap above sea level. Temperature time series on the Arctic with regional resolution are needed, said Laepple, in order to describe climate change in the Arctic more reliably.
By Alison Bosman, Earth.com Staff Writer
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