Nineteen subpopulations of polar bears (
The International Union for Conservation of Nature (IUCN) Polar Bear
Specialist Group (PBSG) recognizes 19 subpopulations of polar bears
(
Polar bear subpopulation region names, abbreviations, and areas. See Fig. 1 for a map of the regions. The area of each region includes the marine portion only, not land. The number of cells is the number of SSM/I grid cells. The percent of total area is with respect to all regions (last row). The percent of area shallower than 300 m and deeper than 300 m are given in the last two columns. The pole hole (second to last row) is the circular area around the North Pole excluded from analysis due to the satellite orbits. The Arctic Basin region (AB) surrounds the pole hole but does not include it. All regions includes all 19 subpopulation regions plus the pole hole.
Map of the 19 PBSG polar bear subpopulation regions, with shallow
depths (
Multiple approaches have been taken to construct sea-ice metrics for studies of survival and body condition in specific polar bear subpopulations (Table 2). These have generally focused on subpopulation-specific metrics such as the number of ice-free or ice-covered days per year (Obbard et al., 2007; Regehr et al., 2010, 2015; Hamilton et al., 2014), the dates of spring sea-ice breakup and/or fall sea-ice freeze-up (Stirling and Parkinson, 2006; Regehr et al., 2007; Lunn et al., 2014; Laidre et al., 2015a; Obbard et al., 2016), or the sea-ice concentration (Rode et al., 2012; Peacock et al., 2012, 2013). Sea-ice metrics have mainly been selected based on the specific region under study or developed for single studies or data sets. There is a need to develop standardized circumpolar metrics of polar bear habitat based on the satellite record of sea ice that allow for regional comparisons of habitat change and for tracking changes into the future, e.g., as in Vongraven et al. (2012). Thus the objective of this study is to propose and produce metrics of polar bear sea-ice habitat that are also relevant to other Arctic marine mammal (AMM) species.
In this study we used daily sea-ice concentration data to calculate several
sea-ice metrics for each of the 19 polar bear subpopulation regions for the
period 1979–2014. The metrics are date of spring sea-ice retreat, date of
fall sea-ice advance, average sea-ice concentration from 1 June to 31
October,
and the number of ice-covered days per year. We calculated each metric
for the total marine area of each region and for the shallow depths only
(
Several previous studies have divided the Arctic into distinct regions and calculated the sea-ice area trend in each region (e.g., Stroeve et al., 2012; Perovich and Richter-Menge, 2009; Parkinson and Cavalieri, 2008). While this is a straightforward and useful way to document changes in sea ice, other metrics of sea-ice habitat are more relevant to marine mammals whose life history events, such as hunting and breeding, depend on the annual retreat of sea ice in the spring and advance in the fall. Many ecologically important regions of the Arctic are ice covered in winter and ice free in summer and will probably remain so for a long time into the future. Therefore the dates of sea-ice retreat in spring and advance in fall, and the interval of time between them, are key indicators of climate change for ice-dependent marine mammals (Stirling et al., 1999; Stirling and Parkinson, 2006).
Recent literature where sea-ice metrics were used for analysis of polar bear habitat. Note that these studies examined habitat for a single polar bear subpopulation (or geographically close set of subpopulations). Bold text gives names of sea-ice metrics. Abbreviations: PM (passive microwave), SIC (sea-ice concentration), CIS (Canadian Ice Service).
Continued.
As in Laidre et al. (2015a) we used the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (Cavalieri et
al., 1996) data set available from the National Snow and Ice Data
Center (NSIDC) in Boulder, CO. This product is designed to provide a
consistent time series of sea-ice concentrations (the fraction, or
percentage, of ocean area covered by sea ice) spanning the coverage of
several passive microwave instruments. The sea-ice concentrations are
produced using the NASA Team algorithm, and are provided in a polar
stereographic projection (true at 70
Concerning the accuracy of the sea-ice concentration data, the product
documentation states that it is within
The spatial coverage of the sea-ice concentration data excludes a small circle around the North Pole, due to the satellite orbits. This “pole hole” is entirely surrounded by the Arctic Basin region (AB in Fig. 1 and Table 1). Although the size of the pole hole became smaller in 1987 with the advent of a new satellite and instrument, we use the larger pre-1987 pole hole for consistency of calculations throughout the period 1979–2014. Our Arctic Basin region does not include the pole hole; it surrounds the pole hole.
To identify shallow depths (
Sea-ice area is defined as
We next looked for outliers in each time series: excessively large or small values that may be the result of erroneous sea-ice retrievals due to extreme weather events or other errors. Outliers were identified by comparing each value in the time series with a five-point median-filtered version of the time series. If the difference between the actual value and the median-filtered value exceeded a certain threshold (15 % of the mean March sea-ice area), then the actual value was replaced by the median value. The outlier rate was less than three values per 10 000. This procedure also led to the identification of an anomaly on 14 September 1984 that turned out to be an error in the passive microwave source data, which was subsequently re-processed by NSIDC.
We next used linear interpolation to fill in the every-other-day gaps up to 9 July 1987. We also used linear interpolation to span a data gap from 3 December 1987 to 13 January 1988. The end result was a complete time series of daily sea-ice area for each region, 1979–2014.
The date of spring sea-ice retreat is defined here as the date when the sea-ice area drops below a certain threshold on its way to the summer minimum. The date of fall sea-ice advance is defined as the date when the sea-ice area rises above the threshold on its way to the winter maximum. These dates may or may not occur in what is normally considered to be spring or fall; they are meant to mark the transitions between winter and summer sea-ice conditions.
Arctic sea ice typically reaches its maximum area in March and its minimum area in September. Accordingly, we chose the transition threshold for each region as follows. We calculated the mean March sea-ice area over the period 1979–2014, and the mean September sea-ice area over the same period, and then chose the transition threshold to be halfway between these means. This is illustrated for the Baffin Bay region in Fig. 2 and for the other regions in Supplement Fig. S1. (Figs. 2–9 use Baffin Bay as a sample region for purposes of illustration).
Daily sea-ice area in Baffin Bay (all depths), January–December,
1979–2014 (gray curves). The colored curves are decadal averages, as
indicated in the legend. The upper horizontal dotted line (at 613
Figure 3 illustrates the method for finding the dates of spring retreat and
fall advance in Baffin Bay in one particular year. The daily sea-ice area
(gray curve) exhibits small daily fluctuations that can be attributed to the
uncertainty in the underlying sea-ice concentration data. We smooth the
daily values with a low-pass Gaussian-shaped filter in which 87 % of the
weight is within
Occasionally the smoothed sea-ice area time series may cross the threshold
more than once in spring and/or fall. Our method always chooses the crossing
date that is closest in time to the summer minimum. In practice, out of 2736
crossing dates (36 years
Determination of the spring and fall transition dates for the year
2005 in Baffin Bay. The gray curve is the daily sea-ice area; the black
curve is a smoothed version. The horizontal dotted line (at 311
For each region we calculated the mean sea-ice concentration for 1 June–31 October for each year, from 1979 to 2014. While it has already been established that the sea-ice concentration in every region of the Arctic except the Bering Sea is declining in every month of the year (e.g., Perovich and Richter-Menge, 2009), the winter sea-ice cover will likely continue to provide suitable polar bear habitat for at least several more decades (especially in the Canadian high Arctic; Amstrup et al., 2008; Hamilton et al., 2014), whereas the summer sea-ice cover may not. A summer sea-ice metric, therefore, measures the change in polar bear habitat during the season when that habitat is most vulnerable to change.
We calculated the number of days per year that the sea-ice area in each subpopulation region exceeded the threshold defined in Sect. 3.2 (i.e., 50 % of the way from mean September to mean March sea-ice area). For example, in Fig. 3, the sea-ice area in Baffin Bay was greater than the 50 % threshold for 220 days in the year 2005. This sea-ice metric was used as a measure of polar bear habitat in the IUCN Red List assessment of polar bears (Wiig et al., 2015).
In all 19 regions, the date of spring sea-ice retreat is trending earlier
and the date of fall sea-ice advance is trending later. Along with this, the
length of the summer season is increasing, the summer sea-ice concentration
is decreasing, and the number of ice-covered days per year is decreasing,
for the period 1979–2014 (Table 3). Nearly all the trends (88 of 95) are
statistically significant. Trends in the date of spring sea-ice retreat are
on the order of
Figure 4 illustrates results for the Baffin Bay region (see Fig. S2
for similar plots for other regions). Sea-ice retreat in spring is changing
by
Trend in date of spring sea-ice retreat (days decade
Dates of sea-ice retreat (red) and sea-ice advance (blue) in Baffin Bay (all depths) for 1979–2014. The red and blue lines are least-squares fits. The vertical green lines indicate the time interval between retreat and advance (i.e., length of summer season). See Table 3 for trends. See Fig. S2 for similar plots for other regions.
We also calculated the number of ice-covered days based on a 15 % threshold of sea-ice area, as illustrated in Figs. 7 and 8 (see Fig. S5 for similar plots for other regions). The 15 and 50 % thresholds intersect the annual cycle of sea-ice area at different levels and therefore contain information about the shape of the annual cycle. In Baffin Bay, the rate of decline in the number of ice-covered days is about the same for both thresholds (Fig. 8). However, in the Chukchi Sea region (Fig. S5) the rate of decline is faster for the 15 % threshold, meaning that the rise and fall of the annual cycle of sea-ice area is steepening, leading to faster transitions between summer and winter sea-ice coverage. In the Barents Sea (Fig. S5) the opposite is occurring. Further analysis of changes in the shape of the annual cycle of sea-ice area is possible but is beyond the scope of the present study.
Length of the summer season (from spring sea-ice retreat to fall
sea-ice advance) vs. year for Baffin Bay (all depths), with least-squares
line in red (slope:
Summer (June through October) sea-ice concentration vs. year for
Baffin Bay (all depths), with least-squares line in red (slope:
Sea-ice area in Baffin Bay (all depths), 1979–2014. Top green line is mean March sea-ice area; bottom green line is mean September sea-ice area. Two thresholds are shown: 15 and 50 % of the way from the mean September area to the mean March area.
Number of ice-covered days in Baffin Bay (all depths), 1979–2014, based on two thresholds: 15 % (blue) and 50 % (red) (see also Fig. 7). Least-squares lines are also shown. See Fig. S5 for similar plots for other regions.
Figure 4 shows that there is year-to-year variability about the trend lines in the dates of spring sea-ice retreat and fall sea-ice advance. Subtracting out the trend lines leaves residuals. We calculated the correlation of the spring residuals with the fall residuals (Table 3, last column). The correlation is negative in most regions, often significantly so. This means that an early spring sea-ice retreat (relative to the trend line) tends to be followed by a late fall sea-ice advance (relative to the trend line), and vice versa. The de-trended spring and fall dates for Baffin Bay are shown in Fig. 9. The negative correlations are likely the result of the ice–albedo feedback, discussed in Sect. 5.4.
In regions with a strong negative correlation, this suggests a method for
predicting the date of fall sea-ice advance, once the date of spring sea-ice
retreat has been observed. (1) Find the slope (
The spatial pattern of trends in the date of spring sea-ice retreat (Fig. 10) shows that all trends over shallow depths are statistically significant
except in the Northern Beaufort, Viscount Melville, and Norwegian Bay
regions. Otherwise, the continental shelves around the Arctic show
significantly earlier spring retreat, generally
Note that in this analysis, the Chukchi Sea region extends south of Bering Strait into the northern Bering Sea. We know from other analyses (e.g., Laidre et al., 2015a; Parkinson, 2014) that there has been a slight increase in sea ice in the Bering Sea. Therefore the negative trends for the Chukchi Sea reported here, while still statistically significant, are relatively small because of the inclusion of the northern Bering Sea within the Chukchi Sea region. Similarly, the trends for the Arctic Basin region are relatively large because that region includes the northern Chukchi Sea, where summer sea ice has been rapidly disappearing (e.g., Frey et al., 2015; Parkinson, 2014).
The calculation of the spring and fall transition dates is based on a sea-ice area threshold that is halfway between the mean September sea-ice area and the mean March sea-ice area for each region. Different thresholds would lead to different transition dates. How sensitive are the transition dates to the actual choice of threshold? The answer can be seen in Fig. 2 (and Fig. S1). The rate of change of sea-ice area (i.e., its slope) is relatively steep at the times of threshold crossing, indicating that sea ice diminishes quickly in spring and grows back quickly in fall compared to the rate of change in winter and summer. Therefore the transition dates are relatively insensitive to the threshold, in the sense that a small change in the threshold would lead to a small change in the transition dates.
Many studies in the last 10 years have considered changes in the timing of
sea-ice advance and retreat in the context of polar bear ecology. Stirling
and Parkinson (2006) used daily sea-ice concentration from satellite passive
microwave data to calculate the date of sea-ice breakup (50 %
concentration) in spring in Baffin Bay for each year from 1979 through 2004,
finding a statistically significant trend toward earlier breakup (
Other researchers have considered changes in the timing of sea-ice advance
and retreat without specific emphasis on polar bears. Stammerjohn et al. (2012) used daily sea-ice concentration from satellite passive microwave
data (1979–2007) to calculate trends in the dates of sea-ice retreat and
advance at every 25
Date of fall sea-ice advance (de-trended) vs. date of spring
sea-ice retreat (de-trended) for Baffin Bay (all depths). The de-trended
dates have correlation
Trend map of the date of spring sea-ice retreat for the shallow parts of each PBSG region. Trends are also given in Table 4.
Parkinson (2014) used daily passive microwave data (1979–2013) to calculate
and map the number of days per year with sea-ice concentration
Frey et al. (2015) used daily passive microwave data (1979–2012) to study the timing of sea-ice breakup, freeze-up, and persistence in the Beaufort, Chukchi, and Bering seas, finding trends toward earlier breakup and later freeze-up in the Beaufort and Chukchi seas, with steeper trends since 2000. They also used wind and air temperature data to determine that for the localized areas that are experiencing the most rapid shifts in sea ice, those in the Beaufort Sea are primarily wind driven, while those offshore in the Canada Basin are primarily thermally driven.
Steele et al. (2015) looked at the timing of sea-ice retreat in the
southeastern and southwestern Beaufort Sea using daily sea-ice
concentration data (1979–2012). They found no trend in the date of retreat
in the southeastern Beaufort Sea but did find a trend toward earlier retreat in the southwestern
Beaufort Sea. Furthermore, an increase in monthly mean easterly winds of
Our methods in the present study are based on our previous work. Laidre et al. (2015a) calculated the timing of sea-ice advance and retreat in 12 Arctic regions (1979–2013) for the Conservation of Arctic Flora and Fauna (CAFF) Arctic Biodiversity Assessment (ABA). Laidre et al. (2015b) focused on polar bear habitat in East Greenland, including changes in the timing of sea-ice advance and retreat. Laidre et al. (2012) examined narwhal sea-ice entrapments and the timing of fall sea-ice advance in six narwhal summering areas of Baffin Bay. Heide-Jørgensen et al. (2013) considered changes in the timing of spring sea-ice retreat in the North Water Polynya. All these studies found trends toward earlier spring sea-ice retreat and later fall sea-ice advance from the 1980s to present.
Our sea-ice metrics are currently being used in the IUCN PBSG Status Table (
Trend map of the date of fall sea-ice advance for the shallow parts of each PBSG region. Trends are also given in Table 4.
While the metrics reported here were tailored specifically to polar bears
and polar bear ecology, they can be considered relevant for a range of other
AMM species. Besides the polar bear, AMMs are
typically considered to be three cetacean species (the narwhal,
The dates of sea-ice advance and retreat, as shown in Figs. 4 and S2, vary about the trend lines. Some regions such as East Greenland have high year-to-year variability, while other regions such as Foxe Basin have low year-to-year variability (as measured, for example, by the standard deviation of the residuals about the trend line). The high variability is likely due to advection of sea ice through the region due to wind and currents, while the low variability indicates a lack of such advection, as noted by Laidre et al. (2012), who found that three sheltered sites on the western side of Baffin Bay had low variability in fall freeze-up dates, while sites near the North Water Polynya in northern Baffin Bay, and in the East Greenland Current, had high variability. In regions where sea-ice advance and retreat are primarily driven by thermodynamics, the year-to-year variability will be lower than in regions where wind and currents are strong.
Trend map of the length of the summer season for the shallow parts of each PBSG region. Trends are also given in Table 4.
Same as Table 3 but for the shallow (
The negative correlations between the de-trended dates of sea-ice retreat
and advance (Tables 3 and 4) are likely the result of the ice–albedo
feedback, noted also by Stammerjohn et al. (2012). When sea ice retreats
earlier than average in spring, the ocean has more time to absorb heat from
the sun. The extra heat is stored in the upper ocean through the summer, and
must be released to the atmosphere in the fall before sea ice can begin to
form, thus delaying fall freeze-up. Conversely, a late spring sea-ice
retreat prevents the ocean from absorbing as much heat, allowing sea ice to
form earlier in the fall (e.g., Perovich et al., 2007). The negative
correlations are not perfect because other factors contribute to the timing
of sea-ice retreat and advance, such as short-term weather events and
long-term climate patterns. This is also discussed in more detail by
Blanchard et al. (2011), who attributed the “re-emergence of memory” in
the fall to the several-month persistence of sea surface temperatures (SSTs)
over the summer, enhanced by the ice–albedo feedback. We calculated the
correlation of the date of fall sea-ice advance in year
Some sea-ice studies use sea-ice extent, rather than sea-ice area, to characterize sea-ice coverage. Sea-ice extent is the total area of all grid cells with sea-ice concentration greater than 15 %, i.e., not weighted by the sea-ice concentration. If the sea-ice concentration in a grid cell exceeds 15 %, the entire area of the grid cell counts toward the sea-ice extent. This is useful in some contexts, but we believe that sea-ice area is a better measure of how much usable sea ice is actually present for polar bears. Also, sea-ice extent is a highly nonlinear function of sea-ice concentration, which leads to more abrupt jumps in its time series than sea-ice area.
Some investigators have approached the idea of seasonal transitions in the Arctic by examining the dates of melt onset in the spring and freeze-up in the fall, based on the presence of liquid water in the surface layer of the ice or snow (Winebrenner et al., 1994, 1996; Smith, 1998; Belchansky et al., 2004; Markus et al., 2009; Stroeve et al., 2014). In these studies, melt onset and freeze-up are closely tied to the surface air temperature, but they are not indicators of sea-ice coverage or condition. For example, at the SHEBA station in the Beaufort Sea in 1997–1998 (Perovich et al., 1999), melt onset occurred on 29 May when rain fell, but the sea ice did not actually break up until the end of July when a storm passed through. Similarly in fall, melt ponds on the surface of the ice began to freeze in mid-August but the sea ice did not actually consolidate into winter-like pack ice until early October (Stern and Moritz, 2002). Melt onset and freeze-up dates are useful as climate metrics, but for ice-dependent marine mammals, transition dates between seasons are best measured by the sea-ice coverage itself, rather than proxies tied to air temperature.
The NCA summarizes the impacts of climate change across the United States, now and into the future, with the goal of better informing public and private decision-making at all levels. The third NCA report was released in May 2014 (Melillo et al., 2014). It documents the decline of Arctic sea-ice extent, thickness, and volume, but not changes in the timing of sea-ice advance and retreat. One of the motivations of the present study was to develop a sea-ice climate metric (or indicator) with relevance to marine mammals that could be used in future NCA reports. The timing of sea-ice advance and retreat satisfies all the qualifications for climate indicators put forward by the NCA (NCA, 2011).
It is well established that the area of Arctic sea ice is declining in all
months of the year, based on satellite passive microwave data from 1979 to
the present (Fetterer et al., 2016; IPCC, 2013). In this study we looked
instead at the timing of sea-ice retreat in spring and advance in fall,
because the duration of the sea-ice season (or equivalently the ice-free
season) is important for polar bears. We found that there has been a
consistent and large loss of habitat for polar bears across the Arctic. In
17 of the 19 subpopulation regions there are significant trends toward
earlier spring sea-ice retreat, mostly ranging from
General circulation models (GCMs) predict ice-free Arctic summers by mid-century or sooner (IPCC, 2013; Overland and Wang, 2013). Spring sea-ice retreat will continue to arrive earlier and fall sea-ice advance will continue to arrive later, with no reversal in sight. Barnhart et al. (2015) used daily sea-ice output from a 30-member GCM ensemble, driven by the business-as-usual emissions scenario (RCP 8.5), to map the annual duration of open water in the Arctic through 2100. They found that by 2050 the entire Arctic coastline and most of the Arctic Ocean will experience an additional 1 to 2 months of open water per year, relative to present conditions, which is consistent with extrapolation of the trends in Table 3.
What are the implications of these physical changes for the global population of polar bears? Their dependence on sea-ice means that climate warming poses the single most important threat to their persistence (Stirling and Derocher, 2012; USFWS, 2013). Changes in sea ice have been shown to impact polar bear abundance, productivity, body condition, and distribution (Stirling et al., 1999; Durner et al., 2009; Regehr et al., 2010; Rode et al., 2012, 2014; Bromaghin et al., 2015; Obbard et al., 2016). Furthermore, population and habitat models predict substantial declines in the distribution and abundance of polar bears in the future (Durner et al., 2009; Amstrup et al., 2008; Castro de la Guardia et al., 2013; Hamilton et al., 2014). This study offers standardized metrics with which to compare polar bear habitat change across the 19 subpopulations and provides a starting point for including sea-ice habitat change in circumpolar polar bear management and conservation plans.
Harry L. Stern carried out the sea-ice calculations in consultation with Kristin L. Laidre; Harry L. Stern and Kristin L. Laidre prepared the manuscript.
This work was supported by NASA under the programs Development and Testing of Potential Indicators for the National Climate Assessment, grant NNX13AN28G (PI: Harry L. Stern), and Climate and Biological Response, grant NNX11A063G (PI: Kristin L. Laidre). We also acknowledge support from the Greenland Institute of Natural Resources. We thank the National Snow and Ice Data Center in Boulder for sea-ice concentration data, and NOAA for bathymetry data (ETOPO1). We thank Eric Regehr, Steve Amstrup, and Cecilia Bitz for conversations about sea-ice metrics. We thank the PBSG for input during the development of the metrics. We thank Andy Derocher and one anonymous reviewer for comments that helped to improve the paper. Edited by: C. Haas Reviewed by: two anonymous referees