Tracking polar bears in the Southern Beaufort, with a sea ice surprise – April 2015 map

USGS biologists were clearly busy this spring putting more satellite radio collars and glue-on tags on Southern Beaufort Sea polar bears but there’s some surprising information in their April 2015 tracking map about current sea ice conditions.

From the 2013-2014 issue of  “Polar Bear News” (USFWS).

From the 2013-2014 issue of “Polar Bear News” (USFWS).

What’s interesting is that the sea ice maps they use show less dark spots that might be open water this year than were present last year in late April. Oddly, this phenomenon has one prominent biologist worried about “challenging” polar bear habitat developing this year – without mentioning last year at all.

The USGS track map for April 2015 is copied below.1

A new crop of bears was added in April – it appears that 14 new females joined the eight remaining from last month (collared spring of 2014), making a total of 22 females to track this year. An additional 8 males and/or juveniles got glue-on ear or shoulder tags. That gives the USGS folks a total of 30 bears to track for a while, hopefully a few of those are males that they can track through the mating season (April/May).

However, last year those glue-on tags didn’t last long: by the end of June 2014, all 7 bears with tags deployed that April either moved out of the area (unlikely) or their tags came off and stopped transmitting.

Figure 1. Original caption: “Movements of 30 satellite-tagged polar bears for the month of April, 2015. Polar bears were tagged in 2014 and 2015 on the spring-time sea ice of the southern Beaufort Sea. Twenty-two of these bears have satellite collar transmitters and 8 of these bears have glue-on satellite transmitters. Polar bear satellite telemetry data are shown with MODIS imagery from 27 April, 2015.”   Click to enlarge.

Figure 1. Original caption: “Movements of 30 satellite-tagged polar bears for the month of April, 2015. Polar bears were tagged in 2014 and 2015 on the spring-time sea ice of the southern Beaufort Sea. Twenty-two of these bears have satellite collar transmitters and 8 of these bears have glue-on satellite transmitters. Polar bear satellite telemetry data are shown with MODIS imagery from 27 April, 2015.” Original here. Click to enlarge.

Note that the circles with the polar bear icons are the end points (final positions for the month), while the tail ends of the strings are the positions at the first of the month. Compare to last month here.

The map used to show the 2014 crop of tagged and collared bears used a sea ice map for 26 April (Fig. 2, below), just a day before the ice map used this year (Fig. 1, above): compare the dark spots that might be areas of open water, low ice concentration, or melt ponds on ice. There was clearly more of this ice type last year:

From original caption: “Movements of 24 satellite-tagged polar bears for the month of April, 2014. Polar bears were tagged in 2013 and 2014 on the spring-time sea ice of the southern Beaufort Sea. Seventeen of these bears have satellite collar transmitters and 7 of these bears have glue-on satellite transmitters. Polar bear satellite telemetry data are shown with MODIS imagery from 26 April, 2014.”  Click to enlarge. Original image here.

Figure 2. From original caption: “Movements of 24 satellite-tagged polar bears for the month of April, 2014. Polar bears were tagged in 2013 and 2014 on the spring-time sea ice of the southern Beaufort Sea. Seventeen of these bears have satellite collar transmitters and 7 of these bears have glue-on satellite transmitters. Polar bear satellite telemetry data are shown with MODIS imagery from 26 April, 2014.” Click to enlarge. Original image here.

The difference isn’t huge but does show that a bit of open water and/or broken ice is probably not unusual for this time of year in the Southern Beaufort – it’s the beginning of the melt season, after all. April 2013 had little open water as did April 2012 but April 2010 had only a bit (although the archive of earlier maps deletes the captions, so we can’t tell what dates were for the ice maps; previous months show that end of the month images were not consistently used).

Sea ice map for 3 May 2015 (Fig. 3, below) shows sea ice breakup in progress on the east coast of Canada and beginning in Hudson Bay and Hudson Strait, as well as the low concentration in the Southern Beaufort (click to enlarge):

Figure 3.  Canadian Ice Service sea ice concentration map for 3 May 2015.

Figure 3. Canadian Ice Service sea ice concentration map for 3 May 2015. Click to enlarge.

The animated version of the ice map above shows that the relatively large patch of “open water” in the Southern Beaufort appeared suddenly (as it appears today) on 30 April (not 27 April) and remained unchanged until 3 May (same is true for the slightly lower concentration ice areas (coloured grey and blue-grey) in eastern Hudson Bay and Hudson Strait). In contrast, the sea ice off Labrador and Newfoundland changes daily in the animation.

That suggests a potential algorithm issue to me (well-known problem with the computer software that interprets the satellite images and turns them into maps we can understand, which have difficulties correctly interpreting melt ponds on ice (interpreted as open water) and areas of broken ice).2

[From my July post on the minutes of the last IUCN Polar Bear Specialist Group meeting (June 2014) of the about the problem of bears with collars showing up on sea ice that does not exist (original document here):

There was further discussion on sea ice thresholds and that telemetry and sea ice do not always line up. For example, recent analysis shows that up to 20% of locations of collared females are in areas where the ice data indicates there is no ice.” Pg. 23, “recent analysis” not identified, but may be S. Beaufort tagging study.]

Note the map for 2 May 2015 posted by NSIDC (Fig. 4, below) shows no region of apparent “open water” or low concentration in the Southern Beaufort, nor does the most recent MASIE map (Fig. 5).

Figure 4. NSIDC map for 2 May 2015.

Figure 4. NSIDC map for 2 May 2015. Click to enlarge.

Figure 5. MASIE sea ice map for 2 May 2015, day 122.

Figure 5. MASIE sea ice map for 2 May 2015, day 122. Click to enlarge.

That suggests that area of what appears to be “open water” in the Southern Beaufort may not necessarily be “challenging habitat” and “tough for bears,” as one chronic pessimist has suggested (Fig. 6, below). [repeated again, for emphasis, with another map later the same day].

Even if this is truly open water and low concentration sea ice, there was more of it last year and yet a polar bear catastrophe did not follow. Some folks just see a crisis brewing everywhere they look.

Figure 6. Tweet from polar bear biologist Andrew Derocher, 30 April 2015

Figure 6. Tweet from polar bear biologist Andrew Derocher, 30 April 2015. Click to enlarge. Original here.

Footnote 1. This is the April 2015 follow-up to my post on the July 2013 track map for female polar bears being followed by satellite in the Beaufort Sea by the US Geological Survey (USGS) – “Ten out of ten polar bears being tracked this summer in the Beaufort Sea are on the ice.” See that post for methods and other background on this topic, and some track maps from 2012 (also available at the USGS website here).

Footnote 2. Here is what Scott and Marshall (2010:156) had to say about the systematic underestimation of microwave data for sea ice maps in the melt season:

The accuracy of the concentration data is stated to be at its worst, at ±15% of the actual sea-ice concentration (Cavalieri et al., 1992), during summer in the Arctic, which unfortunately corresponds to the time of breakup we are interested in. The low accuracy is largely due to the effects of surface melt ponds on the sea ice, which can lead to underestimation of the sea-ice concentration. In addition, at the ice margins and areas of ice breakup, the many different concentrations that can exist within one pixel will be smoothed to an average figure. Several studies have shown that because of the surface melt, in particular, the passive microwave-derived data tend to systematically underestimate ice concentrations (e.g. Agnew and Howell, 2003; Shokr and Markus, 2006). When compared to sea-ice charts, the passive microwave sea-ice concentrations derived from the NASA Team algorithm were found to underestimate concentration during summer melt by 20.4% to 33.5%. The improved NASA Team 2 algorithm has been shown to underestimate concentration by 18.35% on average, with a standard deviation of 16.8% (Shokr and Markus, 2006). These studies suggest that although a random error of around 15% is still reasonable, there will be a systematic underestimation of ice concentration in the passive microwave concentration data set.” [my bold]

Scott and Marshall (2010:157) also suggested that Canadian Ice Service (CIS) daily and weekly charts may be “more accurate than passive microwave data for estimates of ice concentration, particularly in the presence of surface melt,” although more satellite data are now more likely to be incorporated by CIS into their analyses than previously (i.e., before 1987).

Finally, Scott and Marshall (2010: 162), concluded that:

“The use of passive microwave–derived concentration is a poor technique for examining summer breakup because it is well known that the greatest errors in the data are related to this season (Cavalieri et al., 2008).”

Or, as NSIDC put it [accessed 3 May 2015]:

“Particular care is needed to interpret the sea ice concentrations during summer when melt is present…”

References
Scott, J.B.T. and Marshall, G.J. 2010. A step-change in the date of sea-ice breakup in western Hudson Bay. Arctic 63:155-164. Open access http://arctic.synergiesprairies.ca/arctic/index.php/arctic/issue/view/55

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