Changes in NSIDC Arctic ice since this morning. At 8:00 this morning, Arctic ice was about to cross the mean for the first time in at least six years. Now it is almost 4X as far away from the mean as it was this morning.
Anthony Watts got a response back from Walt Meier at NSIDC, which simply isn’t going to fly.
Thanks for letting us know. I have a guess at what this might be.
We’re starting to make some changes to our processing to update/improve things, including some you’ve suggested. One thing that we’ve decided to do is to change the way we calculate our 5-day average values. We’ve been doing it as a centered average – i.e., a given day’s value in the plot is actually an average of that day + 2 days before and 2 days after. This caused an issue at the end point because we’d extrapolate to get a 5-day average on the last day, which resulted in wiggles at the end that.
We’re now changing it to be a trailing 5-day average, i.e., a given day’s value in the plot is the average of that day and the 4 preceding days. This will take out the wiggle in the end of the plot (or most of it – there may be some change as sometimes we don’t get complete data and need to interpolate, and later (a day or two) we do get the data and process it.
A key point is that this change doesn’t actually change the data at all; in effect it simply shifts values two days later. In other words, the centered value for Day X is the same as the trailing value for Day X+2.
This change has been implemented in our test environment and we were going to roll it out some time in near future after we tested it for a bit we planned to announce the change. I think that by accident the test code got put into production. I’d need to confirm this, but from the plot differences, this looks like what likely happened.
We’ll look into this and get back to you. I’m traveling tomorrow, but will send a note to people and I or others will get back to you as soon as we can.
No, the key point is that it does change the data. That is why we are having this discussion.
If what Dr. Meier is saying was correct, the only change we would see is that everything (current data and mean) would be shifted equally two days to the right – and I probably never would have noticed there was a change. The current data would remain the same distance from the mean.
That is definitely not what we are seeing. The new data is 4X as far from the mean as the old data was - because there are two gross errors in the new graph.
- They removed the April 16 data
- They moved the mean data four days to the right, instead of two days to the right
I will demonstrate this with two images, The first image below is shifted two days to the left, so that the 2012 (and 2007) data line up between the old (blue) and new (pink) graphs. There are two obvious problems. The first is that the April 16 data is missing in the new graph, and the second is that the mean data is not lined up between the old and new graphs.
The next image is shifted left by four days. Now the means line up, but the 2012 and 2007 data don’t.
So basically they created a phony three day gap. NSIDC shifted the mean data by four days, but only shifted the current data by two days. This creates a two day gap between the mean and current data. The third day of the phony gap was created by leaving April 16 off the new graph.
Will they fix it?