Bias In Arctic Satellite Sea Ice Trend
Posted by Jeff Id on February 8, 2012
I wrote recently that I wanted to check local regions of inland lakes to look for bias in the sea ice satellite record. Satellites are not long lived creatures. They are built of lightweight materials very carefully constructed to survive in incredibly harsh environments. The result is design lifetimes far shorter than a decade. Why is that important? Because that means that the finest climate trend records, which are satellite based, are comprised of instruments continually re-calibrated to absolute detail but are knitted together wherever any switch of instrument occurs.
All kinds of things can affect measurements, time of day, altitude decay, instrument degradation, signal loss, on and on. Scientists work very hard to correct for these changes but sometimes they are unable to achieve a perfect result. Other times problems are missed.
One of the biggest critiques of satellite temperature data is the known offsets caused by orbital decay of the individual instruments and how they are knitted together during transitions to newer satellites. The details of the corrections are impressive and when things in science are not simple, that often means not certain. In the UAH and RSS series, that means that the scientists use additional data to re-knit the satellites. Less detail is available in knitting of multi-satellite sea ice data.
What is more important is that sea ice data changed sensor types at about 1987. I knew this before but had forgotten in which year this transition occurred.
For this experiment, I chose a section of Canada which included lakes that always will freeze over 100% in the winter and melt 100% in the summer. You would expect that sea ice area for these lakes would produce a sine wave with clipped peaks as the signal reached 100% and 0%.
I can really improve these graphics but this will get the point across:
By running the sea ice code presented previously here and incorporating this purple highlighted mask, we get the sea ice area plot below:
Several things stand out. First, there is little, yet a non-zero negative trend in the maximum ice from 3 lakes . Then we see that the minimum ice level from these 3 lakes never drops below 10000 sq km average. The daily unfiltered data approaches closer to zero but there is enough noise that zero isn’t ever reached in lakes which have zero ice. The very last cycle is “preliminary data” which has not been fully processed so it’s similar depth to the first cycles may be representative of the processing rather than the data itself. It makes one wonder what exactly was done to the different types of data.
Steve Fitzpatrick happened to be writing about the crazy thread on backradiation so I sent him this last plot with this comment:
Sea ice area from Great Bear, Great Slave, and Athabaska River.
Wouldn’t you expect that satellite data would show an even melt in the summer months?
Sea ice at these 3 lakes should have the same max/min. There is a satellite switch right at 87-88.
I don’t know if there is an instrument type change at the same point.
I had forgotten when the instrument type switch occurred but a bit of research turned up this:
This data set contains gridded brightness temperatures (TB) and sea ice concentrations, spanning October 1978 through August 1987, when the SMMR scanner was turned off.
In the end I realized that this is the point where the data turned from SMMR to SMM/I type sensors. I’m not familiar with the details of the sensors but it seems this offset likely biases the critical Northern Hemisphere sea ice trend to some extent.
Unfiltered area data per Carrick’s request:
As an update to the above post, I’ve created a latitude ring around the NH which is comprised of sea ice known to melt in the summer. I have verified the flat bottom of the melt cycle which lasts about 100 days for this entire range of data.
The anomaly has that same step in the middle we found for the sea ice south of the Arctic circle.
Magnifying the lower edge of the sea ice graph reveals similar patterns to what we have discovered. Remember, all of the sea ice in this band melts every year. Again, this isn’t a huge difference by any means, but it isn’t zero either.