Sorry, glacial thinning does not equal glacial growth
On Sunday, Bjørn Lomborg wrote:
And while the delegations first fly into Kangerlussuaq, about 100 miles to the south, they all change planes to go straight to Ilulissat — perhaps because the Kangerlussuaq glacier is inconveniently growing.
But is it? I questioned this claim — and asked readers for the relevant citation, which they provided. The key article from which he is drawing this claim is “Rapid Changes in Ice Discharge from Greenland Outlet Glaciers” from Science in March of this year. The article begins by noting ominously:
The recent, marked increase in ice discharge from many of Greenland’s large outlet glaciers has upended the conventional view that variations in ice-sheet mass balance are dominated on short time scales by variations in surface balance, rather than ice dynamics. Beginning in the late 1990s and continuing through the past several years, the ice-flow speed of many tidewater outlet glaciers south of 72° North increased by up to 100%, increasing the ice sheet’s contribution to sea-level rise by more than 0.25 mm/year. The synchronous and multiregional scale of this change and the recent increase in Arctic air and ocean temperatures suggest that these changes are linked to climate warming.
The article’s key conclusion about Kangerlussuaq (KL) — based on a study of ice dynamics from 2000 to 2006 — hardly justifies labeling it an “inconveniently growing” glacier:
From summer 2004 to spring 2005, KL retreated by 5 km, and its speed increased by 80% near the front and by 20% at 30 km inland. Between April and July 2005, the increase in speed migrated rapidly inland with a 5% decrease in speed close to the front and a 7% increase in speed in areas farther inland (up-glacier). This upstream propagation continued from July 2005 through July 2006, with the near-front deceleration of 15% and up-glacier acceleration of 25%, with the transition between speedup and slowdown at 15 km. The glacier thinned rapidly during acceleration, with 80 m of thinning near the front and thinning of at least 40 m extending 40 km inland by summer 2005. Thinning moved inland between 2005 and 2006, with a peak thinning of 68 m at about 26 km, but with virtually no thinning at the front. Average thinning over the glacier during the summer of 2006 declined to near zero, with some apparent thickening in areas on the main trunk.
The authors conclude: “Integrating the time series of discharge anomaly from 2000 to 2006 gives totals of 52 Gt at KL.” That is, 52 billion tons of ice loss! And apparently most of the loss from KL and another glacier came “in the interval from summer 2004 to summer 2006.”
This is not a growing glacier, inconveniently or otherwise.
The authors also note that their estimates cannot account for the huge ice loss measured by satellite observations and gravitational measurements from the GRACE satellite. So there may well be other ice dynamics occurring that these authors aren’t seeing.
The authors conclude ominously (again):
[O]ur results are notable in that they show that Greenland mass balance can fluctuate rapidly. If these changes are the result of recent warm summers, continued warming may cause a long-term drawdown of the ice sheet through a series of such discharge anomalies, perhaps with a similar degree of variability.
Are you reassured? Greenland can discharge ice very rapidly, and is likely to do so in the future, especially if we take no action to reduce global warming.
If Lomborg actually read this article, he should be ashamed of citing its research to convince people that they don’t have to worry about sea-level rise.
Finally, Lomborg believes in polling (economic) experts to help guide the public — that is the basis of his Copenhagen Consensus. Well, the experts are very worried about one-meter sea-level rise over the next century or so. As the AP recently reported:
Few of the more than two dozen climate experts interviewed disagree with the one-meter projection. Some believe it could happen in 50 years, others say 100, and still others say 150.
Lomborg is very familiar with the climate literature, as his writing makes clear — and I suppose that’s what bugs me the most. He cherry-picks and selectively cites, when any objective review of the literature and discussion with the experts would be lead one to be more worried about the climate, not less.