Here, icebergs discharged from Allison Glacier float near Kullorsuaq, western Greenland © Margie Turrin/Lamont-Doherty Earth Observatory
The Greenland ice sheet is the second largest ice sheet in the world, with an area four times the size of California, and a depth of over three kilometres at points. That’s a whole lot of ice, and were it to melt completely sea levels would rise by more than seven metres, a consequence that would be disastrous for humankind. Understanding the dynamics of the ice sheet is therefore essential, yet two apparently conflicting papers published in Nature prove how limited our current knowledge is.
Both groups of scientists analysed bedrock associated with the ice sheet using a revolutionary technique, in which the ratio of two radioactive isotopes are determined by accelerated mass spectrometry. The isotopes, beryllium-10 and aluminium-26, only appear in bedrock that has been exposed to the sky, and therefore indicates if an area of Greenland was covered in ice, and for how long. Lower levels of isotope suggest that the ice sheet had melted at some point, while higher levels suggest the bedrock remained covered in ice.
The first study, led by Paul Bierman, a geologist at the University of Vermont, obtained samples of seafloor mud that contained bedrock from the east side of Greenland. Analysis of the bedrock revealed relatively low levels of radioactive isotopes, suggesting that area of Greenland had been covered by ice for considerable time. The team estimated that glacial ice had covered this part of Greenland for the past 7.5 million years pretty much consistently. They acknowledged that although some melting may have occurred this would just have been a result of the ice sheet responding to global climate fluctuations. Their findings provide an insight into the history of a large area of the ice sheet, and agree with current models.
The second opposing study used a core of bedrock retrieved in 1993, which has been the basis for a multitude of paleoclimate studies. Led by Joerg Schaefer of Columbia University, the researchers determined the isotope ratio of the bedrock in a similar fashion to Bierman’s group, and found a high abundance of beryllium-10 and aluminium-26. From this, Schaefer concludes that this area of bedrock had been exposed to the atmosphere for a considerable amount of time, and was ice-free for at least 280,000 years in the last 1.4 million years. “We just asked the surface, 'Have you been ice free or not?'” says Schaefer to Nature. “It clearly told us, 'I have been ice free.'”
Rather confusingly, Schaefer’s study seems to contradict the results of Bierman’s, however both results may be correct for the two different areas of the vast ice sheet. Bierman is keen to highlight that both studies reveal the importance of furthering our understanding of the ice sheet.
"These two apparently conflicting - but not necessarily conflicting - studies in Nature really force the issue that we don't know enough about how ice sheets work over deep time. We must recognize the importance of advancing polar science to understand how our world works,” says Bierman. “And, right now, because we're pumping huge plumes of greenhouse gases into the atmosphere, we really need to know how our world works."
Although all the ice in Greenland isn’t likely to melt tomorrow, if the sheet is unstable there is a greater risk of sea levels rising due to climate change. Sea levels are expected to increase by nearly a metre by the end of the century, but current climate models make the assumption that Greenland had continuous ice cover during the last millennium, and may therefore be underestimates.
Perhaps most importantly, both studies demonstrate the urgency at which we need to learn about our planet, before human intervention on the environment means it is too late.