The molehill that was promoted to a mountain

molehill-mountain

I doesn’t happen often that record sea ice in the Antarctic is covered in the media. Yet that changes when at the same time this record could be minimized. A couple days ago a press release about Antarctic ice cover was announced. Its title: “Has Antarctic sea ice expansion been overestimated?“. The referenced paper “A spurious jump in the satellite record: has Antarctic sea ice expansion been overestimated?” was from Ian Eisenman et al and stated that “much” of the expansion could be due to a processing error:

New research suggests that Antarctic sea ice may not be expanding as fast as previously thought. A team of scientists say much of the increase measured for Southern Hemisphere sea ice could be due to a processing error in the satellite data.

The press release starts with defining the problem. It was the increase in the Antarctic sea ice cover in a warming world that puzzled the scientists. In the paper they also mention the inability of the models to capture the observed increase. The investigators in the paper now try to explain these contradictions by suggesting that much of the measured expansion just may be due to an error in the way the satellite data was processed. Culminating in the title that questions whether antarctic sea ice cover really is setting setting record highs.

When I first heard this, two questions crossed my mind. First, why didn’t they only find this now, after many decades of measurements? Secondly, how much of this increase is actually due to this error?

A couple days ago these questions were answered in one fell swoop by Pat Michaels and Paul Knappenberger. The first question was easily answered. According to the authors the error was found so late because the difference was not very visible in the noise. This triggered my curiousity to look into that press release (and later the paper). This is how they mentioned it in the press release:

“You’d think it would be easy to see which record has this spurious jump in December 1991, but there’s so much natural variability in the record – so much ‘noise’ from one month to the next – that it’s not readily apparent which record contains the jump. When we subtract one record from the other, though, we remove most of this noise, and the step-like change in December 1991 becomes very clear.”

If the difference was difficult to spot due to the noise, what does this want to say about the strength of that data in the first place?

Secondly, how big is that difference? It can’t be that big, otherwise that would emphasize even more the noisiness of the data. Some more explanation is needed here. What Eisenman et al found was a step change in the data after processing. The sensors are changed from time to time and needed to be calibrated. In one of those occasions it might had gone wrong and this was found after an update of the processing software (for example: Bootstrap). When the data was processed with both versions and then subtracted from each other, Eisenman found a step change in 1991, at the moment that the sensor was changed.

eisenman figure2

eisenman figure2

After 1991 the ice cover increased and stayed high since than. Therefor the conclusion that “much” of the expansion was due to this processing error, not because of an actual increasing ice cover. Now Michaels and Knappenberger estimated the difference to 200,000 km3, which is not really much comparing to the 1.3 million km3 cover. I think it will be even less. The step change is situated in the period from which the mean is calculated (from 1979 until 2008), so if the cover is lower after 1991 this will change the mean. Even if we subtract 200.000 of the current value, it still is quite an increase. Still not understood by the scientist and not projected by the models. Therefor Michaels and Knappenberger compared it with a molehill that became a mountain.

This was where Michaels and Knappenberger stopped. But after reading the press release and the paper, more questions arose than were solved. When looking at the Antarctic ice cover data, I would expect some step change in 1992 due to this processing error, but this was not the case. At the contrary, it seemed as if ice cover decreased a little in 1992. Plus, the graph was hovering around the zero line until around 2007 and then the ice cover started to increase rapidly. Look at the end year in figure 2, the so called “spurious” jump in the satellite record wasn’t even in data they presented in the press release.

Southern Hemisphere sea ice anomaly July 24, 2014

Southern Hemisphere sea ice anomaly July 24, 2014

According to Eisenman it is not clear which version is wrong. He sees two options. The first option is that Bootstrap version 1 is correct and version 2 introduced the problem after the update in 2007. This then means that the rate of Antarctic sea ice expansion has been overestimated in recent years, hence the title of the press release. The second option is that version 1 was wrong and the jump in 1991 was corrected with the update of version 2 in 2007.

The maintainer of the dataset doesn’t really agree. He claims there was indeed an error, but it was corrected in 2008 and the current version is correct:

The climate scientist who maintains the data set, Josefino Comiso of the NASA Goddard Space Flight Center in Greenbelt, Maryland, says he is confident that the current data set is correct. Comiso says that he inadvertently introduced a mistake into the record at some point after 1991, but corrected it unknowingly when he updated the file in 2008.

Comiso and other climate scientists reject the suggestion that his data set may overestimate the recent trend in Antarctic sea-ice growth – by as much as two-thirds, according to Eisenman’s analysis. Another NASA sea-ice data set, processed using the other standard algorithm, shows a growth trend similar to that in Comiso’s current data.

That put a fresh light on the case to say the least.

What does this means? We saw a press release that basically says that Antarctic sea ice expansion has been overestimated and “much” of the increase in Antarctic sea ice cover is due to processing error. Yet when we look at what was found, “much” seems not really that “much” after all. At least not the “significant error” they talked about. There is even the possibility that the error was found already and was corrected in 2008. The molehill seems to be sold to the public as a mountain. Those who don’t look at the numbers will have the impression that Antarctic ice cover is not worth much talking about, while in fact when the value of the step change is subtracted from the current values, it is still quite an increase.

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