Hi Steve, Anthony,
I think you’ve probably heard from Julienne and seen the posts we’ve made. But now that I have a chance to respond, I’ll add a few words of explanation and some thoughts. If you want to post these, you’re welcome to.
Thank you to both of you for noticing the issue and bringing it to our attention. Let me clarify (in case it’s not already clear) and provide some context. We are well aware that the daily timeseries plot, as we call it, is closely watched, particularly during the summer melt season. We’ve received various critiques of the plot, which we have taken under consideration to change when we got resources to do it. One them was the “wiggle” in the last two days of the plot. The plot was initially, and by and large still is, meant to provide a simplified glimpse of sea ice extent. The focus was on creating a clean, clear, easy to read, easy to understand graphic. As seen in other plots, the extent is often fairly noisy from day to day. Some of that variation reflects real changes, but much of it is due to limitations in the accuracy of the data or short-term weather effects, such as storm front blowing the ice one direction or another for a short period of time.
Thus, to reduce the noise and better reflect the seasonal trends we decided to use a 5-day average (5 days is a reasonable, though arbitrary, time period to reduce synoptic effects). We chose a centered average because that seemed the most logical. This means the average value is always 2 days behind the latest extent value. However, people wanted to see “today’s” value. So, we decided to provide preliminary values for those last two days by using a simple linear extrapolation. When we got enough data for a full centered 5-day average, we replaced that with the final values. However, this means that the values for the last two days change and one can get a “wiggle” in the data, particularly where there is a day or two of steep change because that day or two gets extrapolated out to 5 days. This can be misleading because, at least for a day or two, the slope may look more extreme than it really is.
I think you’re both familiar with this because it’s been commented on in the past, but I provide the background again for the full context. We refrained from changing it because of three reasons. First, after initial confusion, people understood it, so changing it could cause more confusion. Second, changing the averaging method would slightly change things in comparison with our previous analyses, namely, the date when minimum and maximum extents occur (a shift of two days). This is a minor change, but could cause some confusion. And finally, third, we wanted to make a few other changes and needed to plan resources to do them, so we put this on the list of things to do.
Last week we started to work on some changes. This was simply planning – looking at our processing, assessing what needed to be change. In the process, it was noted that changing the 5-day average would be simpler than we expected and could be done quickly. So I gave the go ahead to do this and was informed a couple days later that it had been done. However, there was some miscommunication. I was expecting that we wouldn’t put it into production immediately, but our developers assumed that it was good to go, so it went into production. Though the change had been discussed amongst all of us, the decision to do it right away happened fairly quickly and I don’t think Julienne was aware that it was in the process of being done.
In any event, what we have now implemented is a 5-day trailing average – in other words, the value plotted for a day is the average of that day and the four previous days. What this means is that there should no longer be a little. The data that we plot on a day should not change and we won’t be doing extrapolation. We think this is a better way to display the data and I think most would agree.
Another issue that wasn’t immediately noticed was that the climatology shifted more than the daily. This is because the climatology used a 9-day average. I don’t remember exactly why this was chosen, but I believe it was to make it look just a bit cleaner, though since it is an average, it already is pretty smooth. And since we were using a centered average, 5-day vs. 9-day, makes little difference. For example, the 5-day average for April 17 is 14.797 million sq km and the 9-day average is 14.801, a difference of 0.004 (4,000 sq km). Effectively, there is no difference because we estimate the precision to be on the order of 0.05 (50,000 sq km). So as long as both the daily and the climatology used a centered average, there was a consistent comparison.
However, when the centered average is moved to a trailing average there is a relative change between the 5-day daily, which slides 2 days, and the 9-day climatology, which slides 4 days. Thanks to Steve for noticing this and pointing it out. We should have it changed to a 5-day by tomorrow so that the comparison plot will again be consistent.
As for the timing of this, as mentioned above, it was mostly simply due to opportunity – we had a chance to make the change, so we decided to do it. Also, knowing that we’re heading toward the summer melt season, it was advantageous to make the change sooner rather than later. As the extent line steepens going through spring and into summer, the “wiggle” is often more noticeable. So making the change now would remove the issue for this summer’s melt season.
The fact that we made this change as the daily extent was nearing the average was entirely coincidental. It never actually entered my mind because I didn’t think it would make any difference (and it shouldn’t once we implement a 5-day average for the climatology). In fact, the change should help because we won’t be using extrapolation that can misleadingly make lines on the plot look closer than what the data really indicate.
Even using a 5-day average, short-term changes in the extent should be taken with some caution. It would be interesting if we did match or exceed the climatology, simply because it’s been several years since it happened. However, the ice near the edge now is all seasonal ice and quite thin and will melt fairly quickly. Any anomaly now will have little to no effect on the summer extent or the amount and thickness of multiyear ice.
As a final, personal note let me make a more general comment. I am saddened that some people have become so cynical about climate scientists and climate data. I can appreciate that scientists have brought some this on themselves. And of course, a healthy dose of skepticism is essential to science. But it is disappointing to see people immediately jump to conclusions and assume the worst. I hope people will take from this explanation that NSIDC, and scientists in general, are working hard to the best we can, both in understanding the science and communicating it. We’re not perfect, we make mistakes. When we find them or hear of them, we try to fix them as quickly as we can and to explain what happened as best we can. I’m proud of our team for working very hard today to address the issues, fix them, and answer questions. I think they did a great job today. And in my experience with other climate scientist, I’ve seen nothing other than that same level of dedication.