Good news for polar bears and seals: new study finds multiyear Arctic sea ice is getting thinner

The fact that multiyear sea ice got thinner between 2018 and 2021 as documented by a new study, is ultimately good news for polar bears: less multiyear ice compared to first year ice is better for all marine mammals in the Arctic. Polar bears and seals, for example, are dependent on the seasonal ice that forms every winter (Atwood et al. 2016; Durner et al. 2009). Multiyear ice is simply too thick for any purpose except as a summer refuge for polar bears (for which land will do just as well) and a platform for maternity dens over the winter, for which thick first year ice will often do just as well (Anderson et al. 2012; Rode et al. 2018).

According to the press release for the new paper (Kacimi and Kwok 2022):

– End-of-season Arctic multiyear sea ice is about 1.5 feet thinner in 2021 than in 2019

– Arctic Ocean sea ice lost one-third of its volume in the past 18 years

– New pan-Arctic snow depth suggests previous estimates of sea ice thickness may have been overestimated

The authors of the paper also emphasized that there had been ‘negligible’ changes in first-year ice over the same period, so it’s only multiyear ice that’s changing.

But don’t forget that old multiyear ice can be very thick indeed: 2-4 metres, sometimes quite a bit thicker, as is the ice north of Greenland which can be up to 20 metres thick.

Sea ice thickness and sea ice age are not the same thing, but sea ice age provides a proxy for thickness. A study published in 2007 found a dramatic change in the age of sea ice in the central Arctic Basin since the mid-1980s. In 1987, 57 percent of the ice pack was at least five years old, and a quarter of that ice was at least nine years old. By 2007, only 7 percent of the ice pack was at least five years old, and virtually none of the ice was at least nine years old (Maslanik et al, 2007). Multiyear ice coverage actually increased between March 2013 and March 2014, thanks to more ice surviving the summer melt season than had survived in the record-breaking summer of 2012. But overall, multiyear sea ice continues to decline in the Arctic (Perovich et al. 2014).

Areas in the central Canadian Arctic where multiyear ice has been largely replace by first year ice in recent years (such as M’Clintock Channel and Kane Basin) have seen polar bear population numbers grow.

References

Andersen, M., Derocher, A.E., Wiig, Ø. and Aars, J. 2012. Polar bear (Ursus maritimus) maternity den distribution in Svalbard, Norway. Polar Biology 35:499-508.

Atwood, T. C., Marcot, B.G., Douglas, D.C., et al. 2016. Forecasting the relative influence of environmental and anthropogenic stressors on polar bears. Ecosphere 7(6):e01370. Doi: 10.1002/ecs2.1370

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.

Kacimi, S. and Kwok, R. 2022. Arctic snow depth, ice thickness, and volume from ICESat-2 and CryoSat-2: 2018–2021. Geophysical Research Letters 49(5): e2021GL097448. https://doi.org/10.1029/2021GL097448

Rode, K.D., Olsen, J., Eggett, D., et al. 2018. Den phenology and reproductive success of polar bears in a
changing climate. Journal of Mammalogy 99(1):16-26. https://doi.org/10.1093/jmammal/gyx181