Arctic Ice Extent Discrepancy: NSIDC versus Cryosphere Today

Foreword: I had originally planned to post a story on this, but Steven Goddard of the UK Register sends word that he has already done a comparison. It mirrors much of what I would have written. There is a clear discrepancy between the two data sources. What is unclear is the cause. Is it differing measurement and tabulation methods? Or, is it some post measurement adjustment being applied. With a 30 percent difference, it would seem that the public would have difficulty determining which dataset is the truly representative one.

UPDATE: The questions have been answered, see correction below – Anthony

Arctic ice refuses to melt as ordered

Published Friday 15th August 2008 10:02 GMT – source story is here

Just a few weeks ago, predictions of Arctic ice collapse were buzzing all over the internet. Some scientists were predicting that the “North Pole may be ice-free for first time this summer”. Others predicted that the entire “polar ice cap would disappear this summer”.

The Arctic melt season is nearly done for this year. The sun is now very low above the horizon and will set for the winter at the North Pole in five weeks. And none of these dire predictions have come to pass. Yet there is, however, something odd going on with the ice data.

The National Snow and Ice Data Center (NSIDC) in Boulder, Colorado released an alarming graph on August 11, showing that Arctic ice was rapidly disappearing, back towards last year’s record minimum. Their data shows Arctic sea ice extent only 10 per cent greater than this date in 2007, and the second lowest on record. Here’s a smaller version of the graph:

The National Snow and Ice Data Center (NSIDC)’s troublesome ice graph

The problem is that this graph does not appear to be correct. Other data sources show Arctic ice having made a nice recovery this summer. NASA Marshall Space Flight Center data shows 2008 ice nearly identical to 2002, 2005 and 2006. Maps of Arctic ice extent are readily available from several sources, including the University of Illinois, which keeps a daily archive for the last 30 years. A comparison of these maps (derived from NSIDC data) below shows that Arctic ice extent was 30 per cent greater on August 11, 2008 than it was on the August 12, 2007. (2008 is a leap year, so the dates are offset by one.)

Ice at the Arctic: 2007 and 2008 snapshots

The video below highlights the differences between those two dates. As you can see, ice has grown in nearly every direction since last summer – with a large increase in the area north of Siberia. Also note that the area around the Northwest Passage (west of Greenland) has seen a significant increase in ice. Some of the islands in the Canadian Archipelago are surrounded by more ice than they were during the summer of 1980.

The 30 per cent increase was calculated by counting pixels which contain colors representing ice. This is a conservative calculation, because of the map projection used. As the ice expands away from the pole, each new pixel represents a larger area – so the net effect is that the calculated 30 per cent increase is actually on the low side.

So how did NSIDC calculate a 10 per cent increase over 2007? Their graph appears to disagree with the maps by a factor of three (10 per cent vs. 30 per cent) – hardly a trivial discrepancy.

What melts the Arctic?

The Arctic did not experience the meltdowns forecast by NSIDC and the Norwegian Polar Year Secretariat. It didn’t even come close. Additionally, some current graphs and press releases from NSIDC seem less than conservative. There appears to be a consistent pattern of overstatement related to Arctic ice loss.

We know that Arctic summer ice extent is largely determined by variable oceanic and atmospheric currents such as the Arctic Oscillation. NASA claimed last summer that “not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming”. The media tendency to knee-jerkingly blame everything on “global warming” makes for an easy story – but it is not based on solid science. ®

Bootnote

And what of the Antarctic? Down south, ice extent is well ahead of the recent average. Why isn’t NSIDC making similarly high-profile press releases about the increase in Antarctic ice over the last 30 years?

The author, Steven Goddard, is not affiliated directly or indirectly with any energy industry, nor does he have any current affiliation with any university.

NOTE OF CORRECTION FROM STEVEN GODDARD:

The senior editor at the Register has added a footnote to the article with

excerpts from Dr. Meier’s letter, and a short explanation of why my analysis

was incorrect.

To expound further – after a lot of examination of UIUC maps, I discovered

that while their 2008 maps appear golden, their 2007 maps do not agree well

with either NSIDC maps or NASA satellite imagery.  NSIDC does not archive

their maps, but I found one map from August 19, 2007.  I overlaid the NSIDC

map on top of the UIUC map from the same date.  As you can see below, the

NSIDC ice map (white) shows considerably greater extent than the UIUC maps

(colors.)  The UIUC ice sits back much further from the Canadian coast than

does the NSIDC ice.  The land lines up perfectly between the maps, so it

appears possible that the UIUC ice is mapped using a different projection

than their land projection.

Click for larger image

Because the 2007 UIUC maps show less area, the increase in 2008 appears

greater.  This is the crux of the problem. I am convinced that the NSIDC

data is correct and that my analysis is flawed.  The technique is

theoretically correct, but the output is never better than the raw data.

Prior to writing the article, I had done quite a bit of comparison of UIUC

vs. NSIDC vs. NASA for this year.  The hole in my methodology was not

performing the same analysis for last year.  (The fact that NSIDC doesn’t

archive their maps of course contributed to the difficulty of that

exercise.)

My apologies to Dr. Meiers and Dr. Serreze, and NSIDC.  Their analysis,

graphs and conclusions were all absolutely correct.  Arctic ice is indeed

melting nearly as fast as last year, and this is indeed troubling.

– Steven Goddard