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 graphThe 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 snapshotsThe 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
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Patrick Henry
“They described the new ice as being The worst kind to a small boat. Tricky concept for someone who likes to shoot their mouth off before they think.”
And it is, however they appear to have misread the ‘eggs’, there’s no new ice shown on the charts, in fact the ice shown off Resolute (M) is indicated as 2/10 coverage, old ice, predominantly of ‘Land origin’!
The purple ice is either ‘big ice floes’ or ‘vast ice floes’ of ‘multi year ice’ or ‘thick first year ice’.
You can find the codes for the eggs at: http://www.natice.noaa.gov/egg_code/index4.htm
Paul K check your email
For anyone still interested in pixel-counting, there’s a very clear-looking series of polar ice maps available from the University of Bremen:
http://www.iup.uni-bremen.de:8084/amsr/amsre.html
Image archive pages and folders here:
http://www.iup.uni-bremen.de:8084/amsredata/asi_daygrid_swath/l1a/n6250/
It does appear that Steven Goddard’s Register piece was rather hastily and carelessly put together.
Anthony –
Your explanation of methodology is basically accurate. First, the software removes all non-ice pixels from the UIUC image. It then removes any pixels outside the radius of the earth. Then it converts all remaining pixels to a single color. Finally, it counts all the pixels of that color. Using a single color to represent all >15% concentrations is the same technique which NSIDC uses, and makes the generated videos easier to visualize.
I generated a graph showing the daily percentage gain in ice extent during August vs. last year, and was surprised to see that the trend has a positive slope and is thus increasing. The average over the month is 30% gain.
http://spreadsheets.google.com/pub?key=pj0h2MODqj3jAfVXaaKHbCQ&oid=5&output=image
This graph uses only conservative, unadjusted pixel counts – which tend to underestimate the actual gain. I will try to keep it updated through the remainder of the season.
REPLY: Steve, perhaps to add strength to your argument, you can run the same method on the maps at the URL in the previous comment? Replication is always good.
Phil –
Are you suggesting that the crew of the Bermilla imagined the new ice they were struggling to get through?
dash –
Why not count pixels using the same map sets which I did, and then make your judgment? Several people here keep making the same mistake of comparing vs. different data sets. One of the primary points of my article was to show that the NSIDC graph is significantly inconsistent with the UIUC maps. All you are doing is repeating my assertion.
No one has made any attempt to demonstrate that I measured the UIUC maps incorrectly, or that the UIUC maps are incorrect. Until you do so, your criticisms are illogical and empty.
Steven Goddard: I believe you have counted the pixels correctly, even if you didn’t eyeball the ‘10% difference’ correctly (more like 13.5-14%).
But according to the NSIDC’s website, sea ice extent is not simply derived from a pixel count, but from the number of pixels multiplied by their ice concentration:
“The values for ice area are obtained by summing the concentration of ice within each pixel over the entire ice extent. For example, if a pixel’s area was 600 km² and its ice concentration was 75%, then the ice area for that pixel would be 450 km²”
http://nsidc.org/data/seaice_index/derivation.html
Have you done this? And may I also draw your attention to this sea ice extent graph from Japanese researchers – it looks very similar to that of the NSIDC, no?
http://www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm
Patrick Henry,
I posted a response to you but it appears to have been eaten by the spam filter.
Please figure out where the boat was, Lincoln Sound, and its direction. It was a great distance away from where you suggest the NSIDC image shows the NW Passage to be blocked, and heading away. Yes, it was experiencing some ice (“shortlived”), as they stated, but no, this was not an eyewitness account of the passage being blocked where you, or Steven Goddard, had suggested.
Your attempts to sneer at my intelligence are purposeless. Try to address the points rather than resorting to the ad hominem.
You have posted a ink to NSIDC which you suggest shows the passage blocked. Here is a link to a much more detailed image, which suggests otherwise –
http://www.iup.uni-bremen.de:8084/amsr/arctic_AMSRE_visual.png
So what? – either way. It’s of no consequence to me at all whether or not the passage is navigable, it was only of interest to me whether or not the yacht’s report was truly eye-witness evidence of the passage being blocked where claimed.
Anthony,
Good suggestion. The Bremen maps, NSIDC maps and NSIDC graphs are in close agreement. I’m not disagreeing with their internal consistency. The NSIDC/Bremen maps and area calculations use an adjusted projection (an attempt at equal area,) which involves a fair amount of math to generate.
By contrast, the UIUC maps use a natural, unadjusted projection – essentially what an astronaut flying over the North Pole on a cloudless day would see. The published UIUC maps also have much higher resolution of the Arctic Basin than either NSIDC or Bremen maps.
I can measure the UIUC maps directly, and make reliable estimates of relative areas from map to map. There is a radial distortion in these maps, but as I have explained before, this distortion causes my estimates of larger ice areas to be conservative – on the low side.
Because the Bremen/NSIDC maps are intentionally distorted using a more complex algorithm, there is no simple way to determine why they appear to disagree with the UIUC maps. I will assert that if the UIUC maps are correct, and if I counted the pixels properly, there is the possibility of a mathematical error in the NSIDC calculations. There is an inconsistency.
Either NSIDC is in error, UIUC is in error, or I have measured/interpreted the UIUC maps incorrectly. Like I said, no one has made any attempt to demonstrate any one of those three possibilities. I am fully aware that my numbers are inconsistent with NSIDC. That is exactly the point!
Do we trust our own eyes? The area growth on August 11vs. last year was clearly greater than the 10/12/14% (whatever) which the August 11 NSIDC graph indicated.
If someone wants to prove me wrong, they need to demonstrate that either the UIUC maps are incorrect, or that I am not interpreting them correctly. I would greatly appreciate it if people would stop parroting that my numbers are inconsistent with NSIDC. I know that, and that was one of my primary reasons for writing the article.
Steven Goddard (07:03:07) :
Phil –
Are you suggesting that the crew of the Bermilla imagined the new ice they were struggling to get through?
No I’m saying you imagined it yourself, they never said they were ‘struggling to get through’ new ice, in fact they never mentioned new ice at all! That was their support guy, Pat in Nome, who as I pointed out apparently misread the egg charts.
dash –
Why not count pixels using the same map sets which I did, and then make your judgment? Several people here keep making the same mistake of comparing vs. different data sets. One of the primary points of my article was to show that the NSIDC graph is significantly inconsistent with the UIUC maps. All you are doing is repeating my assertion.
No one has made any attempt to demonstrate that I measured the UIUC maps incorrectly, or that the UIUC maps are incorrect. Until you do so, your criticisms are illogical and empty.
Actually they have, you’ve been told you don’t know what projection the map is using and that by measuring directly measuring from the map you’re making an error. Those who’ve measured directly from the data and used the actual area/pixel get results that agree with the published data, JAXA and NSIDC. That you choose to ignore the criticisms doesn’t make them illogical or empty.
Have you done this?
I am looking at today’s comparison. Estimating by eyeball, I see a much greater area of high-percentage ice coverage area this year than last year.
Phil,
Thanks for clarifying that the blog entry was not an eye-witness account but rather the backup’s misinterpretation of the ice map.
Patrick Henry – given that has now been demonstrated, do you have the courtesy to reconsider your remarks towards me made in your post at (21:51:37) ?
Steven Goddard,
If you are concerned to understand what you think is a discrepancy (clearly there are some others who do not agree with you), may I suggest that you contact UIUC and/or NSIDC to ask for clarification? Surely that we be good practice in researching the matter? You’re not obliged to agree with anything they might say, but at least you would give them the opportunity to explain whether or not there is, in fact, any such discrepancy.
phil,
You are again confusing “area” and “extent.” Read the paragraph you quoted from NSIDC more closely. “Area” is derived from both “extent” and “concentration.”
The NSIDC map for today again shows that all routes through the Northwest Passage are blocked. This was not true a few days ago. Obviously this has to be due to new ice, as described in the Bermilla blog.
They got out just in time. Resolute is freezing in behind them
What is it they “got out” of? What is freezing “behind them?” The answers seem quite obvious, que no?
Evan ones
“I am looking at today’s comparison. Estimating by eyeball, I see a much greater area of high-percentage ice coverage area this year than last year.”
I suspect that the “eyeballers” are concentrating on the area off of the eastern half of Siberia where there is ice in 2008 but not 2007 and not noticing the areas where there was ice in 2007 but not 2008. The choice of colors might play a role in this. Many people do not like red because it tends to dominate other colors.
Steven Goddard
“f someone wants to prove me wrong, they need to demonstrate that either the UIUC maps are incorrect, or that I am not interpreting them correctly.”
You’re the one who claimed that NSIDC was fudging data. It’s up to you to prove that NSIDC’s data are wrong. This is just a standard [snip – stop with the ad-homs, last warning] tactic: burden of proof shifting.
Mr Goddard:
Under our “sea ice news and analysis” site, you will see a link to frequently asked questions. It is a useful educational resource and as such I suggest that you read it. Furthermore, you might want to analyze the raw data themselves which are freely available at NSIDC.
Mark C. Serreze
NSIDC
REPLY: Mr. Serreze, thank you. It is nice of you to comment here.
Would you be willing to answer questions here on this forum? Thank you for your consideration – Anthony Watts
Steven Goddard
“The NSIDC/Bremen maps and area calculations use an adjusted projection (an attempt at equal area,) which involves a fair amount of math to generate.”
The maps are not an attempt at equal area projections. They are Polar Stereographic projections, which have been used in meteorology for decades.
“The Bremen maps, NSIDC maps and NSIDC graphs are in close agreement.”
As are the NCEP maps. In fact, everything is in close agreement EXCEPT your alleged calculations.
http://polar.ncep.noaa.gov/seaice/hires/nh.xml
“Because the Bremen/NSIDC maps are intentionally distorted using a more complex algorithm, there is no simple way to determine why they appear to disagree with the UIUC maps.”
It’s actually easy to do get a qualitative idea of the difference. And what you find is that there isn’t any real difference.
Jeff says
“I suspect that the “eyeballers” are concentrating on the area off of the eastern half of Siberia where there is ice in 2008 but not 2007 and not noticing the areas where there was ice in 2007 but not 2008. ”
LOL. Where would those be, Jeff?
http://igloo.atmos.uiuc.edu/cgi-bin/test/print.sh?fm=08&fd=11&fy=2007&sm=08&sd=11&sy=2008
This isn’t much different or difficult than looking at a piechart (which is also data) and seeing a 30% slice marked as 10%. There is an obvious inconsistency in either the UIUC maps, or the NSIDC chart for Aug 11, or the data.
Patrick Henry,
You appear to be someone who likes to call others stupid but who is not prepared to look at presented evidence yourself –
The NSIDC map for today again shows that all routes through the Northwest Passage are blocked. This was not true a few days ago. Obviously this has to be due to new ice, as described in the Bermilla blog.
The NSDIC shows low resolution cells. The far higher-res Bremen image, to which I have already linked you, shows the Amundsen route as open:
http://www.iup.uni-bremen.de:8084/amsr/arctic_AMSRE_visual.png
Your comment please?
“They got out just in time. Resolute is freezing in behind them”
What is it they “got out” of? What is freezing “behind them?” The answers seem quite obvious, que no?
This comment is written by the back-up, who is sitting somewhere warm misinterpreting an ice map, as has been carefully explained to you! To use your own words, “someone of at least moderate intelligence might be clever enough to realize” what is being said, but I guess it’s a “tricky concept for someone who likes to shoot their mouth off before they think.”
“By contrast, the UIUC maps use a natural, unadjusted projection”
Please explain this. If that projection was “natural” and “unadjusted”, why isn’t it widely used? Since it’s possible to compute the actual Earth area of the ice from NSIDC’s data, why isn’t is possible to compute the Earth area of the ice from UIUC’s maps?
Incidentally, Patrick Henry, to help you commenting on the difference between the Bremen image and the NSIDC image, see this from the BSIDC FAQ:
Why do other sources indicate there is no ice where passive-microwave data shows ice?
The passive-microwave sensor records ice in 25-by-25-kilometer (16-by-16-mile) areas, which is lower resolution than other types of satellite sensors. This means that the ice edge could be off by as much as 25 to 50 kilometers (16 to 31 miles) in passive-microwave data compared to higher-resolution satellite systems. We define ice extent as anywhere with at least 15% ice. Regions with 15% ice will look quite open in higher-resolution satellite data, but will still count as ice in the passive-microwave extent fields.
Other reasons that passive-microwave data may show ice where none actually exists on the ground include signal variation along coastlines between land and water, the shift in albedo of actively melting ice, and atmospheric interference from rain or high winds over the ice-free ocean. In the daily extent data images, gaps (shown in dark gray in the extent map) are replaced with values interpolated from surrounding days, but temporary spurious results may occur.
http://nsidc.org/arcticseaicenews/faq.html#data_sea_ice_index
Your comments?
I recommend looking at the NSIDC FAQ, as Mark Serreze has suggested. Doing some basic research is usually a more profitable process than leaping to a conclusion that suits you.
Glenn
“Where would those be, Jeff?”
ftp://sidads.colorado.edu/pub/DATASETS/seaice/polar-stereo/nasateam/near-real-time/browse/north
“This isn’t much different or difficult than looking at a piechart (which is also data) and seeing a 30% slice marked as 10%. There is an obvious inconsistency in either the UIUC maps, or the NSIDC chart for Aug 11, or the data.”
Wrong.
If you read my 12:27:53 post, you will see what I believe is the same data that UIUC uses mapped to the same projection that NSIDC uses. How does it compare?
Patrick Henry (11:16:20) :
phil,
You are again confusing “area” and “extent.” Read the paragraph you quoted from NSIDC more closely. “Area” is derived from both “extent” and “concentration.”
I’m not confusing area and extent not now nor earlier, it’s more accurate to say that extent and area are both derived from the same source, total area being the sum of the product of area and concentration whereas extent is the sum of area alone (for concentration greater than 15%).
The NSIDC map for today again shows that all routes through the Northwest Passage are blocked. This was not true a few days ago. Obviously this has to be due to new ice, as described in the Bermilla blog.
The NSIDC is rather low resolution you’be better off looking at the uni bergen map instead:
http://iup.physik.uni-bremen.de:8084/amsr/arctic_AMSRE_nic.png
It is not obvious that it is new ice, in fact it is old ice emerging from the strait to the north. http://ice-glaces.ec.gc.ca/prods/WIS35CT/20080819180000_WIS35CT_0003924087.gif
That map shows today’s detail of the area around Resolute, the region marked W is the ice referred to in this comment by the Berrimilla blogger based in Nome who misread this chart.
“They got out just in time. Resolute is freezing in behind them”
What is it they “got out” of? What is freezing “behind them?” The answers seem quite obvious, que no?
Yes, nothing’s freezing behind them, as I told you above he misread the chart! The ice at W is 2/10 coverage made up of 50/50 old ice and thick first year ice predominantly of land origin (icebergs).
The color code used is intended to assist navigation decisions in ice infested water. It is loosely based on the concept of a traffic light, where green means proceed, yellow means caution and red signals danger. W is shown as green as I’m sure you can see! The presence of old ice (multi-year ice) is indicated by the colour purple (remember all that ‘new’ purple ice mentioned in the Berrimilla blog the other day?)
The NW Passage opened for navigation on Aug 21st 2007, looks like it will be close to that this year.
Glenn,
“Where would those be, Jeff?”
http://igloo.atmos.uiuc.edu/cgi-bin/test/print.sh?fm=08&fd=11&fy=2007&sm=08&sd=11&sy=2008
2007 extends further into the Beaufort Sea and towards the northern tip of Novaya Zemlya (my eyeballing’s as valid as yours! ;-)).
The passive-microwave sensor records ice in 25-by-25-kilometer (16-by-16-mile) areas, which is lower resolution than other types of satellite sensors.
This refers to the SSMI imager used by NSIDC and CT (at least in the old images used by Goddard), JAXA and Uni-Bremen and CT?(new) use AMSR-E which has 6×4 km resolution.
I see that Mr. Goddard’s article has gotten quite a discussion going. I’ve emailed The editors at The Register, but haven’t heard back, and just recently Mr. Goddard directly.
Mr. Goddard’s approach to counting pixels is simply not the correct approach. First, let me clarify a couple things.
1. The satellite data doesn’t directly measure sea ice area or extent. It measures brightness temperature – a measure of the amount of energy emitted by the ice. This is converted to area or extent using an algorithm.
2. There are several different algorithms and they can yield different results in terms of absolute numbers. However, their trends and change from year to year show similar magnitudes. The Bremen AMSR data is from a different algorithm – hence it looks different.
3. Both UIUC and NSIDC use the same brightness temperature data.
4. Both UIUC and NSIDC use the same algorithm, but with some differences in the specifics, so the numbers aren’t perfectly matched, but there is very good overall agreement and they yield the same conclusions about changes in Arctic sea ice.
5. People have talked a lot about “pixels”, but one needs to understand what one is talking about. There are two types of “pixels”. One is “data pixels”; this is a function of the spatial resolution of the sensor (i.e., how small of an area the sensor can resolve). For the data UIUC and NSIDC uses, the data pixels are about 25 x 25 km. The other is “image pixels”, which describes the qualities of the image.
6. The data has to be gridded onto a projection, which yields a gridded resolution, which is also about 25 x 25 km, but varies depending on the type of projection and where the grid cell within the projection. The input data for both UIUC and NSIDC is on a 25 x 25 km grid. The UIUC grid that Mr. Goddard analyzes has been interpolated onto a different grid. I do not know the specifics of that grid, but such interpolation will change how the data looks when viewed.
7. The data can then be conveyed in an image. The image has an “image pixel” resolution. This is generally given in dpi or dots per inch. Higher dpi means a sharper image. However it does NOT change the fundamental resolution of the data.
8. An image is simply a way to convey data; it is not data itself. Therefor it is not proper to do analysis on the image. You need to use the data.
9. The gridded data, when analyzed, must account for the projection in terms of the area of the grid cells. You have to sum the ice, weighted by the correct area for each grid cell. NSIDC uses a polar stereographic projection with a true latitude of 70 N. Other than at 70 N there will be distortion that needs to be corrected for, as NSIDC does.
10. NSIDC freely distributes all the data, tools to work with the data, and the grid cell area files. So anyone can do their own analysis.
11. NSIDC’s methods have been around for over 20 years, have been thoroughly vetted in peer-reviewed science journals, and confirmed numerous times over by independent scientists conducting the proper method.
12. Finally, Mr. Goddard need not have wasted his time doing his image pixel counting. He could’ve simply referred to the UIUC site, which actually counts the pixels properly and creates a timeseries plot:
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.365.jpg
If you look at that plot, you’ll that because it is area instead of extent, the raw numbers are lower. However, while it’s a bit hard to make out the values real accurately, you see that this year on Aug. 11, it was ~4 million sq km, while last year on Aug. 12, it was ~3.6 million sq km (actually, since it’s a one-year sliding window, Aug. 12, 2007 is no longer visible, but that’s what it was on that data and the current range shows a similar difference). That’s a bit more than an 11% difference.
Hopefully Mr. Goddard will have a correction posted prominently on The Register as soon as possible.
Walt Meier
Research Scientist
National Snow and Ice Data Center
University of Colorado at Boulder