(en español aquí)
Recently Walsh et al. have published a paper presenting their new Arctic sea ice extent dataset. The new record covers the period spanning from 1850 to 2013. The data and the documentation describing the new sea ice dataset are available at NSIDC (G10010).
The graph below (source) shows their results. We can see Arctic sea ice extent in March, when the ice is near its annual maximum, and in September, when ice extent reaches its yearly minimum:
The winter maximum barely changes, except for the slight downward trend in recent decades. Regarding the summer minimum, the sea ice cover remains steady until the 30s, 40s and 50s, when it becomes somewhat lower. Higher values in the 60s, and a downward trend since then. This downward trend has accelerated in the last two decades, leading to the lowest values of the whole record.
The main conclusions of the paper are that recent sea ice extent is the smallest on record, and that the current downward trend has no precedent in duration or scales since 1850.
Without calling into question these conclusions, I would like to point out that the reliability of the record is much lower before 1935, especially in September. Prior to 1935, there are not September direct observations at all, and the extent numbers before that year rely almost completely on interpolation. (Likewise the extent numbers during 1935-1952 have larger uncertainties than the values from 1953 onwards).
And the chart below presents a comparison between the old (green line) and the new (blue line) Walsh datasets from 1935 to present day (I have scaled the summer values of the old dataset to match the September ones of the new Walsh record in the satellite era):
The most striking difference is that the new dataset shows much lower values during 1935-1952 than the old one. In fact, according to the old dataset the 40s were the period with the highest extent of the whole record, whereas the new dataset shows the 60s as the decade with the largest sea ice cover. In addition, according to the old dataset every year during 1935-1952 had a higher extent than any year of the satellite era. That is not true according to the new record.
Earlier this year we published a paper that presented a new time series of September Arctic sea ice extent from 1935 to 2014. Our time series included data for the Siberian sector (AARI operational charts) that hadn’t been used for the previously existing time series (The old Walsh dataset, HadISST). In addition, we also adjusted the data before 1979 to be consistent with the satellite record.
And how does the new Walsh dataset compare with the time series that we presented earlier this year? The graph below shows a comparison between our work (red line) and the new Walsh dataset (blue line) since 1935:
Our time series and the new Walsh dataset are reasonably consistent. We can see that the new Walsh time series is much closer to our work than to the old Walsh dataset (or to HadISST, that is very similar to the old Walsh record).
Nevertheless, our numbers before 1979 run always somewhat lower than those of Walsh et al. This is probably due to the fact that Walsh et al. have not adjusted the pre-satellite values, whereas we adjusted them. Walsh et al. are aware of the likely lack of consistency between the passive microwave satellite record and the earlier data. In order to address that, they are planning to present an alternative time series with different values for the satellite era (I guess that they could use NIC charts, that usually present higher values than passive microwave ones).
(Curiously, our numbers and those from the new Walsh record present some discrepancies during the satellite era as well. This is due to the fact that we have used two different datasets from 1979 onwards. While we have used the well known NSIDC monthly means (G02172), Walsh et al. have used mid month daily values from a different dataset: NSIDC’s G02202. It’s remarkable that the difference between the two time series looks larger in the 80s than in the 90s and 2000s. I guess that it could be related to how the records from SMMR and SSM/I sensors have been merged in both datasets).