Thongchai Thailand

Antarctic Sea Ice: 1979-2018

Posted on: August 6, 2018







  1. Sea ice consists of free floating floes in constant motion driven by wind and ocean currents. Sea ice extent is defined as a contiguous surface area of the sea, measured in millions of square kilometers (MSK), where at least 15% of the sea surface consists of floating ice. On average, the dispersion of sea ice within a sea ice extent varies from 50% to 80% (Munshi, Trends in polar sea ice extent, 2015). Both sea ice extent and the degree of dispersion within the extent can be estimated in the brightness data of passive microwave images taken from satellite mounted instruments (Comiso, 1997). Dispersed sea ice extent corrected for dispersion is reported as concentrated sea ice extent and referred to as sea ice area (NSIDC, 2016). An intense interest in polar sea ice extent in the climate change era derives from the proposition that a multi-year decline in sea ice extent serves as an index of the impact of anthropogenic global warming (AGW) (Lacis, 2010) (Hansen, 1981) (IPCC, 2014) (Comiso, 2002) and also because the reduction in albedo due to lost polar sea ice could accelerate AGW and further complicate its effects on the climate system (Comiso, 2008) (Winton, 2006) (Perovich, 2007) (Serreze, 2007).
  2. Sea ice area undergoes a deep seasonal cycle in both poles. The seasonal cycle in the Antarctic is the reverse of that in the Arctic. Here the summer minimum is reached in February (2 MSK) with a winter maximum (14.5 MSK) in September. The amplitude of these seasonal changes is much greater than the differences implied by long term declining trends. (Parkinson, 2002) (Cavalieri, 2012) (Munshi, Trends in polar sea ice area, 2015). Because of these large seasonal changes, trend analyses of sea ice area is usually restricted to the summer minimum and winter maximum months or carried out for each calendar month separately.
  3. The charts above show that air temperature and sea ice area are inversely related in the average seasonal cycle. This visual intuition is confirmed by the correlation between temperature and sea ice area in their average seasonal cycles . The computed values are R = -0.8607 in the Arctic where changes in sea ice area are mostly a seasonal phenomenon, and R = -0.9123 in the Antarctic where changes in sea ice area are almost entirely a seasonal phenomenon. The observed month to month seasonal relationship between air temperature and sea ice area suggests that long term inter-annual trends in sea ice area for each month of the calendar year should be related to air temperature and that this relationship should be measurable at an annual time scale.
  4. Long term full span trends, 1979-2018 for January to July and 1979-2017 for August to December, for both sea ice area and temperature are tabulated above for each calendar month. The relevant measure of global warming for the Antarctic is taken as the zonal mean for lower troposphere temperature anomalies for oceans in the south polar region as reported by the University of Alabama Huntsville (Christy/Spencer, 2018). The table above shows mostly rising trends in sea ice and mostly cooling trends in temperature but no statistically significant long term trend is found in the Antarctic for either temperature or sea ice area.
  5. Quite unlike the Arctic, the Antarctic shows neither global warming nor year to year sea ice decline. There is no sign of global warming in this portion of the globe. The observed trends and their statistical insignificance are depicted graphically in the two charts above the table of trend and correlation values. They show that for the two seasonal extremes, the February Minimum and the September Maximum sea ice area, no trends are seen either for warming or for sea ice area. These data do not support the usual assumption that global warming is causing a decline in Antarctic sea ice.



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