Is Greenland Melt “Off the Chart?”
That’s what the second author said about a new paper on Greenland’s ice, which arrived just in time for the annual meeting of the signatories of the UN’s 1992 treaty on climate change, this time in Katowice, Poland. Appearing in Nature, Rowan University Geologist Luke Trusel and several coauthors claimed ice-core data from Central-Western Greenland revealed melting in the recent two decades that has been “exceptional over at least the last 350 years.” The paper appeared in the December 6 issue of Nature.
How exceptional?
“Our results show a pronounced 250% to 575% increase in melt intensity over the last 20 years” as measured in four ice cores in west-central Greenland. Three of the cores were in the Jakobshavn Glacier, the largest-discharging glacier in the entire Northern Hemisphere. The Ilulissat icefjord, created by the glacier, some 25 miles in length, has historically calved nearly 50 cubic kilometers of ice per year into Disko Bay, near the town of Ilulissat.
They then correlated their ice-core data with a model for ice behavior in all of Greenland. The correlations, while significant, were modest, with the explained variance of the island-wide melting maxing at around 36%. The melt reached its maximum in the very strange summer of 2012, where the amount at the Summit site, near Greenland’s highest elevation, was the largest since the summer of 1889—worth noting because that was well over 100 years ago.
There’s a long-standing quality weather station at Ilulissat, and it certainly shows summer warming of about 2⁰C from its beginning around 1850 to the 1920s.
For a broader comparison, we looked at the summer temperature anomalies for the 5 X 5 degree gridcell that includes Disko Bay and the icefjord. Because it is relatively hospitable and settled, there are a number of stations within the cell so the data is quite reliable. The data we show is from the Climate Research Unit at the University of East Anglia, version HadCRUT4.
There’s very little to see in this temperature record. The authors are well-aware of this and offer a rather unsatisfactory explanation:
The non-linear melt-temperature sensitivity also helps explain why episodes of mid-twentieth-century warmth resulted in less intense and less sustained melting compared to the last two decades, despite being only marginally cooler…Additional factors, such as recent sea-ice losses, as well as regional and teleconnected general circulation changes may also play a part in amplifying the melt response.
“Teleconnected” means things that happen in different locations but occur at the same time, without the reason for the connection being necessarily known.
What is known is that the correlation between summer temperature and melt in the cores on the glacier is relatively low (but statistically significant) with local summer temperatures only explaining 11% of interannual variance in melt. The melt data is noisy, but shows a spectacular increase in the last two decades for the area around Disko Bay.
Figure 1. Regional temperature history for Disko Bay, which includes the Ilulissat icefjord.
The “250% to 525%” increase actually refers to the two somewhat separated locations, one on the Jakobshavn Glacier and the other on the peninsula that forms the northern boundary of Disko Bay
The actual data show the baseline summer melt is very close to zero (and in some cases is below zero, i.e. a gain), for the vast majority of years back into the 18th century. Five times a number that is very close to zero is still a small number. It’s “off the chart” if the chart stops at very near zero and ends at number five times very near zero, as it does in the paper.
The sudden change in melt is nonetheless impressive. And it was certainly published at a time that would have maximum impact on the partygoers in Katowice.
It has some competition, though, in the other direction. In 2013, the Danish “NEEM” team successfully drilled a core through the previous (penultimate) interglacial and found that summer temperatures were 6-8⁰C warmer than the 20th century average for 6000 years. They estimated Greenland only lost 30% of its ice. That anomaly is known as the Eemian period and was around 118,000 years ago.
Humans can’t induce a summer warming of that magnitude and length simply because there’s not enough fossil fuel handy. And the math there is somewhat reassuring. If all the ice came off Greenland, sea level would ultimately rise 23 feet. So 30% of that is 6.9 feet, spread out over 6000 years. That works out to a Greenland-induced sea level rise of 1.1 feet per millennium when it was much warmer.
Thanks to Ryan Maue for the Disko Bay analysis
