In the last couple of days, there has been much to-do in glacier world about an error in the Times Atlas on Greenland glaciers. See for example here here here.

Unlike the authors of 1000-year temperature reconstructions, glaciologists seem to be concerned about things like using data upside down. As of today, the Times Atlas has issued a somewhat contradictory press release, resiling from its press release, but standing by the atlas:

we issued a press release which unfortunately has been misleading with regard to the Greenland statistics. We came to these statistics by comparing the extent of the ice cap between the 10th and 13th editions (1999 vs 2011) of the atlas. The conclusion that was drawn from this, that 15% of Greenland’s once permanent ice cover has had to be erased, was highlighted in the press release not in the Atlas itself. This was done without consulting the scientific community and was incorrect. We apologize for this and will seek the advice of scientists on any future public statements. We stand by the accuracy of the maps in this and all other editions of The Times Atlas.

The error has occasioned considerable speculation as to the provenance of the error, which, like Hansen’s Y2K error, looks like it originated in a change of datasets between the 1999 edition and the 2011 edition.

This is evident from comments at Cryolist by Martin O’Leary, a graduate student at Cambridge, as follows:

As fun as it is to bash The Times, the images I’ve seen of the erroneous map do bear a striking resemblance to the following map which I was able to pull up on the NSIDC website by poking about looking for ice thickness information: http://bit.ly/o3iV6i Anyone from NSIDC care to comment on what the data source being used here is? I’m sure it’s just an issue of definitions over what’s an “ice sheet” and what’s an “outlet glacier”, but it’s pretty easy to see how an atlas-maker could use that as an authoritative source on what’s going on in Greenland.

The NSIDC Atlas of the Cryosphere is here. If you zoom into Greenland a few times and, for the Java button “Glacier Basemaps”, select “none”; for the Java button “Greenland Basemaps” select “ice sheet thickness”; and for the snow extent button and sea ice extent buttons, select none, you’ll get the map shown by O’Leary, which matches the Times Atlas version sufficiently closely that one can say with considerable confidence that this is the provenance. I’ve shown the NSIDC map and the Times 2011 map below (the latter rotated to match NSIDC):

Figure 1. Left – NSIDC with Glacier button set at “none”; right – Times 2011.

A later commenter, responding to O’Leary, observed:

Your map of ice thickness plots the thickness of the “Greenland ice sheet”. If you choose a map showing “glaciers”, you can cover most of East Greenland. The Times Atlas should have shown the area of glaciers. The precise distinction between Greenland ice sheet (Indlandsisen?) and (local) glaciers is a useful one for professional glaciologists but not something that the Times Atlas should bother itself with. The thickness map suggests zero thickness in the coastal regions while the surface elevation map suggests there is ice in the coastal regions. My guess is that the thickness map refers to the main ice sheet only whereas the elevation map includes all peripheral ice caps and glaciers as well. Nevertheless, I have to agree with Martin that this is confusing and begs misinterpretation by journalists and other non-experts.

If one returns to the NSIDC Atlas of the Cryosphere and changes the java button “Glacier Basemaps” from none to glaciers – a setting that seems more plausible to me if one is making a cryosphere map – then one gets a far more “realistic” map. If one compares this to the 1999 version of the Times Atlas – which was used as a starting point in the comparison, you can readily see that the 1999 version almost certainly showed glaciers as well as the icesheet simpliciter.

Figure 2. Left – NSIDC with Glacier button set at “glacier”; right – Times 1999.

Glaciologists have sent a critical letter to the Times Atlas for the error, but their letter did not precisely diagnose its provenance. It appears almost certain that the “15% decrease” is an artifact of comparing area of ice sheet plus glaciers (1999) to ice sheet without glaciers (2011).

Maybe the glaciologists will take a look at bristlecones, Yamal and Upside Down Mann.



