Arctic and Baltic Sea ice (CLIM 010) - Assessment published Nov 2012

Key policy question: What is the trend in the extent of Arctic and Baltic Sea ice?

Trend in Arctic Sea ice extent in March and September

Note: This figure shows the time series of Arctic Sea ice extent from 1979 to 2012. Trend lines and observation points for March (the month of sea ice extent maximum) and September (the month of sea ice extent minimum) have been indicated.

Data source:

• Sea Ice Extent provided by Ocean Monitoring and Forecasting (MyOcean)

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Maximum ice cover extent in the Baltic Sea

Note: The figure shows the maximum extent of ice cover in the Baltic Sea in the winters 1719/20–2010/11 (blue bars) and 15-year moving average (red line).

Data source:

• Ice Seasons 1996–2000 in Finnish Sea Areas provided by Baltic sea portal

• Maximum Annual Extent of Ice Cover in the Baltic Sea 1720–1995 provided by Baltic sea portal

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Key assessment

Past trends

In the period 1979-2012 the sea ice extent in the Arctic decreased by 45 000 km² per year in winter (measured in March) and by 98 000 km² per year in summer (measured in September) (see Figure 1). Winter sea ice loss has occurred in the peripheral seas, influenced by warmer oceans, while summer sea ice loss has developed in the Arctic Ocean driven by a warmer atmosphere (caused in part by warmer oceans south of Svalbard). This is evidenced by an earlier onset of summer surface melt [i]. In contrast Antarctic sea ice reached record high levels, with a monthly average Southern Hemisphere winter maximum extent in September 2012 of 19.39 million square kilometres. Scientists largely attribute the increase in Antarctic sea ice extent to stronger circumpolar winds, which blow the sea ice outward, increasing extent [ii].

Changes in Arctic Sea ice may trigger complex feedback processes. A longer melt season results in a lower sea ice extent in autumn and increased solar heat uptake by the ocean which delays the refreeze [iii]. However, a warmer atmosphere means more clouds and in summer these reflect sunlight, thus representing a negative feedback. Even so, some evidence suggests that winter regrowth of ice is inhibited by the warmer ocean surface [iv]. Thinner winter ice leads to more heat loss from the ocean and a warmer atmosphere, and hence a thicker cloud cover which inhibits the escape of heat to space [v], which is a positive feedback mechanism.  

The extent of the minimum sea ice cover at the end of the melt season in September 2007 broke all previously observed records. Comparison of recent sea ice coverage with older ship and aircraft observations suggests that sea ice coverage may have halved since the 1950s [vi]. Since the more reliable satellite observations started in 1979, summer ice has shrunk by 10.2 % per decade [vii]. The reduction in maximum winter extent is smaller, with a decrease of 2.9 % per decade [viii]. There is some evidence that the decline in summer ice has accelerated since 1999 [ix].

The Arctic Sea ice is also getting thinner and younger since less sea ice survives the summer to grow into thicker multi-year floes. Currently there is less multi-year ice than seasonal sea ice in the Arctic Ocean [x]. It is hard to calculate trends for the whole sea ice cover, but submarine data collected in the central Arctic Ocean considered to be the most representative suggest a decrease of 40 % in sea ice thickness from an average of 3.1 m in 1956–1978 to 1.8 m in the 1990s [xi]. British submarine data from 2007 also show continued thinning [xii].

Calculations of sea ice volume from satellite suggest that the Arctic autumn (winter) sea ice volume decreased by 1 237 km³ (862 km³) from 2004 to 2008 [xiii]. This estimate is consistent with the estimate by the Pan Arctic Ice-Ocean Modelling and Assimilation System (PIOMAS), which suggests that the mean monthly sea ice volume decreased by 2 800 km³/decade over the period 1979–2010. PIOMAS further suggests that sea ice volume has decreased by 70 % (September) and almost 40 % (March) relative to the period 1958–1978 [xiv].

Information on sea ice extent in the Baltic Sea goes back to 1720. The maximum sea ice extent has been decreasing most of the time since about 1800. The decrease in sea ice extent appears to have increased since the 1980s but the large interannual variability prohibits a clear assessment as to whether this increase is statistically significant. The frequency of mild ice winters, defined as having a maximum ice cover of less than 130 000 km², has increased substantially. The frequency of severe ice winters, defined as having a maximum ice cover of at least 270 000 km², has decreased (Figure 2).

Projections

Arctic summer sea ice is projected to continue to shrink. It may even disappear at the end of the summer melt season in the coming decades, although there will still be substantial ice in winter [xv]. So far summer-time melting of Arctic Sea ice has occurred much faster than projected by most climate models, which decreases confidence in projections of the future rate of Arctic Sea ice decline [xvi]. It has been suggested that the wide spread in future sea ice projections is due to the inability of many current climate models to properly represent the thickness distribution of Arctic Sea ice [xvii]. Projections of Baltic Sea ice extent under different emissions scenarios suggest that the maximal ice cover will continue to shrink significantly in the 21st century [xviii].

[i] Julienne Stroeve et al., „Recent changes in the Arctic melt season“, Annals of Glaciology 44, Nr. 1 (2006): 367–374, doi:10.3189/172756406781811583.

[ii] NSIDC, „Press Release: Arctic sea ice shatters previous low records; Antarctic sea ice edges to record high“, 2012, http://nsidc.org/news/press/20121002_MinimumPR.html.

[iii] Sharon Stammerjohn et al., „Regions of rapid sea ice change: An inter-hemispheric seasonal comparison“, Geophysical Research Letters 39 (März 16, 2012): L06501, doi:10.1029/2012GL050874.

[iv] Jennifer M. Jackson, William J. Williams, and Eddy C. Carmack, „Winter sea-ice melt in the Canada Basin, Arctic Ocean“, Geophysical Research Letters 39 (Februar 15, 2012): L03603, doi:10.1029/2011GL050219.

[v] Stephen P. Palm et al., „Influence of Arctic sea ice extent on polar cloud fraction and vertical structure and implications for regional climate“, Journal of Geophysical Research 115 (November 12, 2010): D21209, doi:10.1029/2010JD013900.

[vi] Walter N. Meier, Julienne Stroeve, and Florence Fetterer, „Whither Arctic sea ice? A clear signal of decline regionally, seasonally and extending beyond the satellite record“, Annals of Glaciology 46, Nr. 1 (Oktober 2007): 428–434, doi:10.3189/172756407782871170.

[vii] Josefino C. Comiso et al., „Accelerated decline in the Arctic sea ice cover“, Geophysical Research Letters 35 (Januar 3, 2008): L01703, doi:10.1029/2007GL031972; M.A. Killie und T. Lavergne, „Time series of the 1979 to 2010 monthly Arctic sea-ice extent in March (the month of sea-ice extent maximum) and September (the month of sea-ice extent minimum) in km2.“ (Source: data produced by the EUMETSAT OSI SAF (http://osisaf.met.no) and the CryoClim (http://www.cryoclim.net)  project, delivered through MyOcean), 2011).

[viii] Julienne Stroeve et al., „Arctic sea ice decline: Faster than forecast.“, Geophysical Research Letters 34 (2007): L09501, doi:10.1029/2007GL029703; Comiso et al., „Accelerated decline in the Arctic sea ice cover“; Killie and Lavergne, „Time series of the 1979 to 2010 monthly Arctic sea-ice extent in March (the month of sea-ice extent maximum) and September (the month of sea-ice extent minimum) in km2.“

[ix] Julienne C. Stroeve et al., „The Arctic’s rapidly shrinking sea ice cover: a research synthesis“, Climatic Change 110, Nr. 3–4 (Juni 8, 2011): 1005–1027, doi:10.1007/s10584-011-0101-1.

[x] R. Kwok et al., „Thinning and volume loss of the Arctic Ocean sea ice cover: 2003–2008“, Journal of Geophysical Research 114, Nr. C7 (Juli 2009), doi:10.1029/2009JC005312.

[xi] UNEP, Global Outlook for Ice and Snow (Arendal: United Nations Environment Programme, 2007), http://www.grida.no/publications/geo-ice-snow/.

[xii] P. Wadhams, N. Hughes, and J. Rodrigues, „Arctic sea ice characteristics in winter 2004 and 2007 from submarine sonar transects“, Journal of Geophysical Research 116 (2011): C00E02, doi:10.1029/2011JC006982.

[xiii] Kwok et al., „Thinning and volume loss of the Arctic Ocean sea ice cover“.

[xiv] Axel Schweiger et al., „Uncertainty in modeled Arctic sea ice volume“, Journal of Geophysical Research 116 (September 27, 2011): C00D06, doi:10.1029/2011JC007084.

[xv] G.A. Meehl and T. F. Stocker, „Global Climate Projections“, in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, ed. S. Solomon et al. (Cambridge: Cambridge University Press, 2007), Chapter 10, http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch10.html; I. Allison et al., „The Copenhagen Diagnosis“ (The University of New South Wales Climate Change Research Centre (CCRC), Sydney, Australia, 2009).

[xvi] Muyin Wang and James E. Overland, „A sea ice free summer Arctic within 30 years?“, Geophysical Research Letters 36 (April 3, 2009): L07502, doi:10.1029/2009GL037820.

[xvii] Kwok et al., „Thinning and volume loss of the Arctic Ocean sea ice cover“.

[xviii] Kirsti Jylhä et al., „Changes in frost, snow and Baltic sea ice by the end of the twenty-first century based on climate model projections for Europe“, Climatic Change 86, Nr. 3–4 (2008): 441–462, doi:10.1007/s10584-007-9310-z.