Guest essay by Dr. Susan Crockford of Polar Bear Science
Svalbard in the western Barents Sea has recently had less sea ice extent than it had in the 1980s, especially in the west and north, but this is not unprecedented.
New evidence from clams and mussels with temperature-sensitive habitat requirements confirm that warmer temperatures and less sea ice than today existed during the early Holocene period about 10.2–9.2 thousand years ago and between 8.2 and 6.0 thousand years ago (based on radio carbon dates) around Svalbard. Barents Sea polar bears almost certainly survived those previous low-ice periods, as they are doing today, by staying close to the Franz Josef Land Archipelago in the eastern half of the region where sea ice is more persistent.
As this sea ice chart for 18 April 2018 shows, ice this month has been virtually absent from the west and north coasts of the Svalbard Archipelago, while Franz Josef Land to the east is surrounded by highly concentrated pack and land-fast ice.
From a new paper by Jan Mangerud and John Svendsen (2018) [my bold]:
Svalbard, located between 74° and 81°N, is the warmest place on Earth at this latitude (Drange et al., 2013). This is because of the North Atlantic Current and large-scale atmospheric circulation which transport warm water and air masses from lower latitudes northwards across the Atlantic and along the coast of Norway to Svalbard (Figure 1). Yet, during the Holocene Thermal Maximum, the climate of Svalbard was considerably warmer than at present.
The transition from Younger Dryas cold to Holocene Thermal Maximumwarm conditions took place very rapidly, according to records from nearby Greenland (Taylor et al. 1997), warming in “steps” of about five years each over a period of about 40 years. This was at last as fast, if not faster than, recent Arctic warming between the 1980s and 2015. And since polar bears of the Barents Sea and adjacent Arctic areas appear to have survived this change to Holocence Thermal Maximumconditions, it challenges the notion that recent warming has been (or will be) too fast to allow polar bears to survive without huge changes in their present distribution (Amstrup et al. 2007).
The summer water temperature map from Mangerud and Svendsen (below) not only illustrates why western Svalbard is subject to periods of no or low sea ice in winter but why Franz Josef Land to the east (surrounded by near-zero temps (in blue), even in summer) is the perfect refugium for polar bears during low-ice years (Aars 2015; Aars et al. 2017; Andersen and Aars 2016; Barr 1985; Chernova et al. 2014; Descamps et al. 2017; Fauchald et al. 2014), see previous post here.
Franz Josef Land provides the most stable sea ice habitat for Barents Sea polar bears because it is largely beyond the influence of warm water influxes from the North Atlantic.
The schematic below from Mangerud and Svendsen shows the warm water incursions from the Atlantic flowing past the west coast of Svalbard at about 11 thousand years ago, when the Laurentide Ice Sheet still covered the eastern half of Canada and the northern US, excluding fish, seals and polar bears from most of Canadian Arctic and Hudson Bay.
Here is the abstract from Mangerud, J. and Svendsen (2018) [my bold, link added]:
“Shallow marine molluscs that are today extinct close to Svalbard, because of the cold climate, are found in deposits there dating to the early Holocene. The most warmth-demanding species found, Zirfaea crispata, currently has a northern limit 1000 km farther south,indicating that August temperatures on Svalbard were 6°C warmer at around 10.2–9.2 cal. ka BP, when this species lived there. The blue mussel, Mytilus edulis, returned to Svalbard in 2004 following recent warming, and after almost 4000 years of absence, excluding a short re-appearance during the Medieval Warm Period 900 years ago. Mytilus first arrived in Svalbard at 11 cal. ka BP, indicating that the climate was then as least as warm as present. This first warm period lasted from 11 to 9 cal. ka BP and was followed by a period of lower temperatures 9–8.2 cal. ka BP. After 8.2 cal. ka, the climate around Svalbard warmed again, and although it did not reach the same peak in temperatures as prior to 9 ka, it was nevertheless some 4°C warmer than present between 8.2 and 6 cal. ka BP. Thereafter, a gradual cooling brought temperatures to the present level at about 4.5 cal. ka BP. The warm early-Holocene climate around Svalbard was driven primarily by higher insolation and greater influx of warm Atlantic Water, but feedback processes further influenced the regional climate.”
Survival of Barents Sea polar bears during low-ice years does not require emigration to another sea ice ecoregion or even another subpopulation area. The eastern Barents Sea (located in Russian territory), as defined by the Polar Bear Specialist Group (see map below), provides ample habitat for polar bears to thrive despite extended fluctuations in seasonal sea ice cover in the western portion. Although it must be frustrating for Norwegian researchers and their colleagues to see “their” bears abandoning Svalbard for Franz Josef Land because of recent low ice levels, they are not witnessing a biological catastrophe.
Bottom line: Barents Sea polar bears are loyal to this region because the eastern portion has the habitat they require to thrive even when sea ice cover in the western portion essentially disappears for thousands of years at a time.
Aars, J. 2015. Research on polar bears at Norwegian Polar Institute. Online seminar (‘webinar”), January 14. pdf here.
Aars, J., Marques,T.A, Lone, K., Anderson, M., Wiig, Ø., Fløystad, I.M.B., Hagen, S.B. and Buckland, S.T. 2017. The number and distribution of polar bears in the western Barents Sea. Polar Research36:1. 1374125. doi:10.1080/17518369.2017.1374125
Aars, J., Marques, T.A., Buckland, S.T., Andersen, M., Belikov, S., Boltunov, A., et al. 2009. Estimating the Barents Sea polar bear subpopulation. Marine Mammal Science 25: 35-52. https://onlinelibrary.wiley.com/doi/full/10.1111/j.1748-7692.2008.00228.x
Amstrup, S.C., Marcot, B.G. & Douglas, D.C. 2007. Forecasting the rangewide status of polar bears at selected times in the 21st century.US Geological Survey. Reston, VA. Pdf here
Amstrup, S.C., Marcot, B.G., Douglas, D.C. 2008. A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Pgs. 213-268 in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications, E.T. DeWeaver, C.M. Bitz, and L.B. Tremblay (eds.). Geophysical Monograph 180. American Geophysical Union, Washington, D.C. http://onlinelibrary.wiley.com/doi/10.1029/180GM14/summary and http://alaska.usgs.gov/science/biology/polar_bears/pubs.html
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