My original post was incorrect. I originally compared April 18 vs. the second April 16 graph, not the first one. The new NSIDC graph lines up correctly with the old April 16 version, but the time stamp is two days later. My apologies for the error.
NSIDC says they have fixed the earlier error, but today’s graph still appears incorrect. The image above overlays the April 18 graph on the original April 16 graph, with 2012 data normalized. There is still a shift in the climatology relative to the 2012 data. This should not be happening.
Pixel counting the extent map shows 12% more area of excess ice (green) than missing ice (red.) So we should expect to see NSIDC cross above the mean in two days.
N_daily_extent_hires.png (1460×1740)
My pixel counts agree with Cryosphere Today numbers, which show more excess ice in the Bering Sea, than missing ice in the Barents Sea.
Polar Sea Ice Cap and Snow – Cryosphere Today
Keep on ‘em, Steve, until they get it right. It’s public data and it should be as right as humanly possible.
Came across this. A new “Poll” from Yale and George Mason. Comedy.
http://environment.yale.edu/climate/files/Extreme-Weather-Climate-Preparedness.pdf
Steve – didn’t Dr. Meier say that they were shifting the ‘climatology’ baseline from a 9 day centered average, to a 5 day trailing average, so to be coordinated with the data plot itself? Would that not explain the shift?
Any change which creates a relative offset shift between current and climatology is incorrect. Don’t fall for old Jedi mind tricks.
The old system used a centered mean for both, which kept everything properly lined up. If the new system were correct, the relative offset would be identical – but the time stamp would be two days later.
Steve,
Switching from a 9-day centered mean to a 5-day trailing mean does result in the slight change to the climatology line. However, to go to a trailing average, which is better, we need to make this change so that climatology is consistent with the daily values.
walt
Dr. Meier, thanks for providing the explanation to Anthony and for posting here. It sounds like the changes you’ve made were well thought out, if frustratingly timed. The animation you provided illustrates well the problem with using an extrapolated centered mean on live data.
It is an extrapolated mean on extrapolated data. It is not live data by any definition. What you are seeing is counting pixels created by a computer generated image that extrapolated the information from a group of satellite images.
Counting angels on the head of a pin would be more productive.
Mike Davis said:
April 21, 2012 at 12:46 am
“It is an extrapolated mean on extrapolated data. It is not live data by any definition. What you are seeing is counting pixels created by a computer generated image that extrapolated the information from a group of satellite images.
Counting angels on the head of a pin would be more productive.”
I see what Mike Davis is saying now!
He’s saying all this is guesswork from the NSIDC so they might as well pack their bags and go home.
Unless the ice is melting away that is… Then its not extrapolated guesswork.
You count does not agree with Cryosphere Today numbers, since they show a current overall area anomaly of -0.103 million sq km. You can’t just pick two regions and ignore the rest. As far as your pixel count goes, even by eye I can see you missed some pixels in the Greenland sea. But that omits a larger issue: the pixels that comprise the orange line itself. These (being the border of the ice) presumably are ice-covered in the historical mean data. Since we cannot see through the line, we do not know how many of them are ice-covered today. The orange line is several pixels wide and thousands of pixels long. Your count therefore has a minimum error of several thousand, which makes your entire exercise moot.
Regarding the NSIDC figure, the historical average line is not strictly shifted, it’s actually a subtly different shape, since it’s now based on a 5-day average rather than a 9-day average. If you look carefully you’ll see the line is now slightly less smooth.
What does that do to the overall shape? Well, after normalising and taking the current date as day 0, the April 16 graph shows the average of days (-6,-5,-4,-3,-2,-1-0,1,2) , while the April 18th graph shows the average of days (-4,-3,-2,-1,0). That is, there’s no “shift” in that both are centred on day -2, but there is a difference in the size of the smoothing window.
How does the inclusion of days -6,-5,1 & 2 affect the average? Not at all during parts of the year when the rate of gain or loss is ~ linear. However, during this part of the spring, melt is accelerating, and so days 1&2 are further below day -2 than days -6&-5 are above day -2. So, during this part of the year, the 9-day average will be fractionally lower than the 5-day average.
Taken together, that means that going from the 9-day average to the 5-day average, even when correctly centred, will cause the “average” line to rise fractionally. It’s how smoothing works. The reverse will hold true as the melt slows down in August/September, so as we approach the summer minimum the “new” line for the historical average will be fractionally lower than it was on last year’s graphs.
Heh, you corrected your post while I was replying. My logic still holds – moving to a 5-day window will indeed fractionally increase the historical average during parts of the year when melt is accelerating (or gain is decelerating). If the graphs line up OK, I presume the effect is too small to see
CT measures area , not extent. The small negative overall CT value is due to a few areas of less than 100% concentration in the Kara Sea and elsewhere. Those would not affect NSIDC’s extent numbers. The critical areas are Bering and Barents.
Chances are CT will hit close to zero today anyway.
Good analysis Peter. I hope you have no objection but I have quoted it in a blog post with full credit given;
http://reallysciency.blogspot.co.uk/2012/04/ruffled-feathers-but-no-lessons-learned.html
But the graph lines do not line up any more.
It’s bad science to mix datasets from disparate methods.
Still attempting readership Laz. Does your handler want results for his money?
And this (The post) shows the difference between scientists & activists.
The scientists put their hands up straight away & say “Sorry, made a mistake”.
The activists ignore their mistakes & deny that there even are any!
They who manipulate data only put their hands up when backed to a wall and cornered with a gun pointed at them.
So why have they not done the honest thing and pulled the line forward to match existing data? You can’t simply move one line and call it correct. This is what happened with Manns graph. They tacked a rapid climb at the end of his poor tree ring data to create the hockey stick… That’s not science.
Well done for making the correction Steve.
Prompt and open corrections are a way to foster trust. I am sure many climate scientists will take this good example on board.
Jokes are nice in this type of situation and that one brought a smile.
I thought your last one was hilarious.