Ice cores collected from central-north Greenland provide a fine-grained record of climatic conditions in the area, and it’s not good news. Among other things they reveal the decade from 2001-2011 was 1.5°C (2.7°F) warmer than the 20th century average, as well as being the area’s hottest since at least 1000 CE.
Greenland’s mighty ice sheet shapes the climate of the North Atlantic, and also provides an unmatched record of regional conditions before the invention of the thermometer. A new paper takes advantage of this fact by establishing the temperatures at which the ice was deposited.
“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 Dr Maria Hörhold of the Alfred Wegener Institute in a statement.
Astonishing amounts of Greenland’s ice have melted in recent years, both during extreme hot spells and on an annual basis measured by net ice loss. However, records of this type only go back a few decades, so more information is needed to fill in the long-term picture.
Ice cores are a different matter. Not only do they show the amount of snow deposited in a year, the oxygen ratio of oxygen-16 to oxygen-18 varies depending on the temperatures at which that molecule evaporated and condensed to fall as snow.
At some locations, so little snow falls each year that temperature readings need to be averaged over long periods to gain sufficient data, but the cores collected as part of the North Greenland Ice Core Project allow for higher resolution.
Cores previously taken in the 1990s in the North Greenland Traverse had shown global warming trends were hard to distinguish from local variation at that point. On the other hand, 2001-2011 stands out from the noise. The authors conclude it would have been almost impossible for a decade to be that hot without human activity.
The team also calculated how much meltwater Greenland had produced each year from 1871 to 2011 by combining satellite ice-mass measurements in recent years and earlier weather station data. “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,” Hörhold said.
Using this relationship, the authors extrapolated their ice core data to reveal melting for the last 1,000 years.
A surprising feature of the study’s results is the disconnect between central Greenland’s climate and that of the rest of the Arctic. The authors attribute this to their cores being collected at a point where the ice sheet is kilometers thick. The altitude at the top of the ice increases exposure to atmospheric circulation patterns that have less influence at sea level.
At one time, the climate change denier myth of the month was to claim that Greenland’s name proved it was once covered in verdant fields. Historians instead think Erik the Red was just a canny marketer. Based on this dubious evidence it has been concluded by some that Greenland, and by implication the whole world, was warmer in the late Middle Ages than today, thereby discrediting anthropogenic global warming. Hundreds of scientific papers have disproven these statements, and this one shows the error goes all the way to the icy heart.
The paper is published in Nature.