What’s a good analogy for sea ice as essential polar bear habitat? Biologist Andrew Derocher claims that the soil in a forest is appropriate, because without the soil you can’t have the forest ecosystem. However, that’s a specious comparison because the amount of soil in a forest does not change markedly with the seasons the way that Arctic sea ice does.
A much better analogy is a big pond that dries up a bit every summer. The amount of habitat available to sustain aquatic plants, amphibians and insects is reduced in the dry season but many species have special adaptations for surviving reduced water availability. For the rest of the year, however, the pond provides an abundant and non-limiting habitat for all the creatures and plants that live there.
Derocher’s favourite meme is “no ice, no bears” — as if there is a real possibility that sea ice is on course to disappear completely, even in winter.
But that’s nonsense: even during past Interglacial periods that were much warmer than today, there was extensive sea ice in the winter and spring (Stein et al. 2017) and no Arctic species went extinct, even with much less summer sea ice than we have today.
Cronin and Cronin (2015) stated (my bold):
“Despite the scale, frequency and rapidity of Quaternary climate changes, Arctic marine ecosystems associated with sea-ice habitats were extremely resilient, adapting through geographic range expansion into the Arctic during warm periods, and south into extra-Arctic regions during glacial periods. The stratigraphic record of the last 1.5 Ma indicates that no marine species’ extinction events occurred despite major climate oscillations.”
Even the most pessimistic models that predict sea ice changes over the coming decades suggest that winter and spring sea ice will persist at the end of this century (ACIA 2005; AMAP 2017; Durner et al. 2009; Tremblay and Huard 2014; Wang and Overland 2012).
Most importantly, sea ice is necessary for polar bears only from late fall to late spring only — beyond that, sea ice is optional. Reduced sea ice extent from early summer through mid-fall has had little effect on the health or survival of bears that have had amply food available in the spring (Crockford 2017; Crockford and Geist 2017).
Bottom line: Any discussion of an “ice-free Arctic” refers to summer ice extent only. Arctic sea ice as an ecosystem is not in any danger of disappearing, despite Derocher’s hand-waving. Sea ice that fluctuates seasonally as a habitat for polar bears is nothing like the soil necessary to sustain a forest every day of every year.
ACIA 2005. Arctic Climate Impact Assessment: Scientific Report. Cambridge University Press. See their graphics package of sea ice projections here.
AMAP 2017. [ACIA 2005 update]. Snow, Water, Ice, and Permafrost in the Arctic Summary for Policy Makers (Second Impact Assessment). Arctic Monitoring and Assessment Programme, Oslo. pdf here.
Crockford, S.J. and Geist, V. 2018. Conservation Fiasco. Range Magazine, Winter 2017/2018, pg. 26-27. Pdf here.
Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 2 March 2017. Doi: 10.7287/peerj.preprints.2737v3 Open access. https://doi.org/10.7287/peerj.preprints.2737v3
Cronin, T. M. and Cronin, M.A. 2015. Biological response to climate change in the Arctic Ocean: the view from the past. Arktos 1:1-18 [Open access] http://link.springer.com/article/10.1007/s41063-015-0019-3
Durner, G.M., Douglas, D.C., Nielson, R.M., Amstrup, S.C., McDonald, T.L., et al. 2009. Predicting 21st-century polar bear habitat distribution from global climate models. Ecology Monographs 79: 25–58.
Stein, R., Fahl, K., Gierz, P., Niessen, F. and Lohmann, G. 2017. Arctic Ocean sea ice cover during the penultimate glacial and the last interglacial. Nature Communications 8 (373). https://www.nature.com/articles/s41467-017-00552-1
Tremblay, B. and Huard, D. 2014. Projected polar bear sea ice habitat in the Canadian Arctic Archipelago. PLoS One 9(11):e113746.
Wang, M. and Overland, J. E. 2012. A sea ice free summer Arctic within 30 years: An update from CMIP5 models. Geophysical Research Letters 39: L18501. doi:10.1029/2012GL052868