The new Walsh dataset: Arctic sea ice extent since 1850

(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:

Walsh et al 2016 arctic sea ice extent 1850 2013 march september

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).

The following graph (source) shows how the ‘old’ Walsh dataset looked (green line, summer values):

Old Walsh dataset - seasonal extent arctic sea ice 1900 2010 Walsh Chapman Cryosphere TodayAnd 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):

Walsh et al. 2016 vs. Walsh & Chapman 2001 - Arctic sea ice extent 1850 1935 summer septemberThe 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:

New Walsh et al. 2016 vs Cea Piron MA y Cano Pasalodos JA 2016 nueva serie artico 1935 September Arctic sea ice extent - Arctic sea ice before satellitesThe same comparison in the following graph, but including also the old Walsh dataset (green line):

Walsh et al. 2016 vs Walsh & Chapman 2001 vs Cea Piron MA y Cano Pasalodos JA 2016 - Arctic sea ice extent 1935 - 20th century XXOur 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).


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9 respuestas a The new Walsh dataset: Arctic sea ice extent since 1850

  1. The graph below shows the comparison between the old and the new Walsh datasets extended back to 1900:

    Arctic sea ice extent 1850 1900 Walsh dataset new vs old summer september

    And the same but including our work:

    Arctic sea ice extent 1850 1900 Walsh dataset new vs old summer september

  2. The new Walsh dataset seems to have a higher correlation with Arctic temperature than the old one.
    The graph below shows a preliminary comparison between the old Walsh dataset (green line), the new one (blue line) and an estimation of sea ice extent based on the correlation between sea ice extent and Arctic temperature (CRUTEM4, Apr-Sept, 70-90N) during the satellite era (black line) from 1900 to present:

    Arctic sea ice extent 1850 1900 Walsh old new dataset summer temperature

    Although the time series we published earlier this year (red line) is even closer to the temperature regression:

    Arctic sea ice extent 1850 1900 Walsh old new dataset summer temperature cea cano 1935

  3. Pingback: Nueva serie de Walsh: la banquisa ártica desde 1850 hasta la actualidad | Banquisa en el Ártico: el blog del hielo marino

  4. Pingback: Walsh et al. 2016: the uncertainties | Banquisa en el Ártico: el blog del hielo marino

  5. Gridded Monthly Sea Ice Extent and Concentration, 1850 Onward, Version 2

    Minor changes. Uncertainties (and errors) remain.

    Error: during 1901-1956 Kelly grids are still misplaced by a month . That is, in August they are using July charts, in July they are using June charts, etc.

    Walsh et al. V2, August 1936. We can see the ice at Baffin Bay:

    DMI chart (the source where Kelly grids are derived from), August 1936. Baffin Bay is almost ice free:

    DMI chart, July 1936: sea ice at Baffin Bay:

    Walsh’s August map looks like DMI for July instead of August.

    If we look at Baffin Bay, we can clearly see the resemblance with July instead of August too.

    These charts are just an example, because this misplacement seems to happen from May to August, every year from 1901 to 1956.

    I hope they will correct it for a V3 version.

    “Note: A user let us know that the Kelly source fields appear to include other data sources that are temporally displaced by one month. For example, the August 1936 Kelly source appears to have included chart information from the July DMI charts or yearbooks. We suggest you delay downloading the V2 data until we are able to investigate this problem more throughly. When the issue is resolved we will remove this warning.”


    The error warning ( at has been removed. However, they have not corrected anything yet…

  6. Finally, NSIDC published a news announcement covering this issue:

    “NOAA@NSIDC would like to make users aware of a data update and an error notice for the NOAA/NSIDC Gridded Monthly Sea Ice Extent and Concentration, 1850 Onward data set.
    Error notice:
    A user has made us aware that the ice cover derived from the Danish Meteorological Institute Yearbooks may have been incorporated incorrectly into the Kelly grid source as it was used in G10010. The Kelly grids may have used the ice map from the subsequent month, rather than the intended month. The Kelly grids were used over April through August or September of 1901-1956. We are investigating the problem, and plan to correct the misalignment. We will inform registered users of the outcome. This apparent misapplication of the Kelly grid source is present in both V1 and V2.”

    A similar error warning was sent to registered users of this dataset too, and published in the User Guide:

    Let’s hope they are correcting this error in a next update.

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