Antarctic sea ice
From Climate Change Reconsidered, a work of the Nongovernmental International Panel on Climate Change
A number of claims have been made that CO2-induced global warming is melting sea ice in the Arctic and Antarctic and that such melting will accelerate as time passes.
Antarctic Sea Ice
Utilizing Special Sensor Microwave Imager (SSM/I) data obtained from the Defense Meteorological Satellite Program (DMSP) for the period December 1987-December 1996, Watkins and Simmonds (2000) analyzed temporal trends in different measures of the sea ice that surrounds Antarctica, noting that “it has been suggested that the Antarctic sea ice may show high sensitivity to any anthropogenic increase in temperature,” and that most climate models predict that “any rise in surface temperature would result in a decrease in sea ice coverage.”
Contrary to what one would expect on the basis of these predictions, the two scientists observed statistically significant increases in both sea ice area and sea ice extent over the period studied; and when they combined their results with results for the preceding period of 1978-1987, both parameters continued to show increases over the sum of the two periods (1978-1996). In addition, they determined that the 1990s also experienced increases in the length of the sea ice season.
Watkins and Simmonds’ findings, i.e., that Southern Ocean sea ice has increased in area, extent, and season length since at least 1978, are supported by other studies. Hanna (2001) published an updated analysis of Antarctic sea ice cover based on SSM/I data for the period October 1987-September 1999, finding the serial sea ice data depict “an ongoing slight but significant hemispheric increase of 3.7(±0.3)% in extent and 6.6(±1.5)% in area.” Parkinson (2002) utilized satellite passive-microwave data to calculate and map the length of the sea-ice season throughout the Southern Ocean for each year of the period 1979-1999, finding that although there are opposing regional trends, a “much larger area of the Southern Ocean experienced an overall lengthening of the sea-ice season … than experienced a shortening.” Updating the analysis two years later for the period November 1978 through December 2002, Parkinson (2004) reported a linear increase in 12-month running means of Southern Ocean sea ice extent of 12,380 ± 1,730 km2 per year.
Zwally et al. (2002) also utilized passive-microwave satellite data to study Antarctic sea ice trends. Over the 20-year period 1979-1998, they report that the sea ice extent of the entire Southern Ocean increased by 11,181 ± 4,190 square km per year, or by 0.98 ± 0.37 percent per decade, while sea ice area increased by nearly the same amount: 10,860 ± 3,720 square km per year, or by 1.26 ± 0.43 percent per decade. They observed that the variability of monthly sea ice extent declined from 4.0 percent over the first 10 years of the record, to 2.7 percent over the last 10 years.
Yuan and Martinson (2000) analyzed Special SSM/I data together with data derived from brightness temperatures measured by the Nimbus-7 Scanning Multichannel Microwave Radiometer. Among other things, they determined that the mean trend in the latitudinal location of the Antarctic sea ice edge over the prior 18 years was an equatorward expansion of 0.011 degree of latitude per year.
Vyas et al. (2003) analyzed data from the multi-channel scanning microwave radiometer carried aboard India’s OCEANSAT-1 satellite for the period June 1999-May 2001, which they combined with data for the period 1978-1987 that were derived from space-based passive microwave radiometers carried aboard earlier Nimbus-5, Nimbus-7, and DMSP satellites to study secular trends in sea ice extent about Antarctica over the period 1978-2001. Their work revealed that the mean rate of change of sea ice extent for the entire Antarctic region over this period was an increase of 0.043 M km² per year. In fact, they concluded that “the increasing trend in the sea ice extent over the Antarctic region may be slowly accelerating in time, particularly over the last decade,” noting that the “continually increasing sea ice extent over the Antarctic Southern Polar Ocean, along with the observed decreasing trends in Antarctic ice surface temperature (Comiso, 2000) over the last two decades, is paradoxical in the global warming scenario resulting from increasing greenhouse gases in the atmosphere.”
In a somewhat similar study, Cavalieri et al. (2003) extended prior satellite-derived Antarctic sea ice records several years by bridging the gap between Nimbus 7 and earlier Nimbus 5 satellite datasets with National Ice Center digital sea ice data, finding that sea ice extent about the continent increased at a mean rate of 0.10 ± 0.05 x 106 km² per decade between 1977 and 2002. Likewise, Liu et al. (2004) used sea ice concentration data retrieved from the scanning multichannel microwave radiometer on the Nimbus 7 satellite and the spatial sensor microwave/imager on several defense meteorological satellites to develop a quality-controlled history of Antarctic sea ice variability covering the period 1979-2002, which includes different states of the Antarctic Oscillation and several ENSO events, after which they evaluated total sea ice extent and area trends by means of linear least-squares regression. They found that “overall, the total Antarctic sea ice extent (the cumulative area of grid boxes covering at least 15% ice concentrations) has shown an increasing trend (~4,801 km²/yr).” In addition, they determined that “the total Antarctic sea ice area (the cumulative area of the ocean actually covered by at least 15% ice concentrations) has increased significantly by ~13,295 km²/yr, exceeding the 95% confidence level,” noting that “the upward trends in the total ice extent and area are robust for different cutoffs of 15, 20, and 30% ice concentrations (used to define the ice extent and area).”
Elderfield and Rickaby (2000) concluded that the sea ice cover of the Southern Ocean during glacial periods may have been as much as double the coverage of modern winter ice, suggesting that “by restricting communication between the ocean and atmosphere, sea ice expansion also provides a mechanism for reduced CO2 release by the Southern Ocean and lower glacial atmospheric CO2.”
Three papers on Antarctic sea ice were published in 2008. Laine (2008) determined 1981-2000 trends of Antarctic sea-ice concentration and extent, based on the Scanning Multichannel Microwave Radiometer (SSMR) and SSM/I for the spring-summer period of November/December/ January. These analyses were carried out for the continent as a whole, as well as five longitudinal sectors emanating from the south pole: 20°E-90°E, 90°E-160°E, 160°E-130°W, 130°W-60°W, and 60°W-20°E. Results indicated that “the sea ice concentration shows slight increasing trends in most sectors, where the sea ice extent trends seem to be near zero.” Laine also reports that “the Antarctic region as a whole and all the sectors separately show slightly positive spring-summer albedo trends.”
Comiso and Nishio (2008) set out to provide updated and improved estimates of trends in Arctic and Antarctic sea ice cover for the period extending from November 1978 to December 2006, based on data obtained from the Advanced Microwave Scanning Radiometer (AMSR-E), the SSM/I, and the SMMR, where the data from the last two instruments were adjusted to be consistent with the AMSR-E data. Their findings indicate that sea ice extent and area in the Antarctic grew by +0.9 ± 0.2 and +1.7 ± 0.3 percent per decade, respectively.
A study that “extends the analyses of the sea ice time series reported by Zwally et al. (2002) from 20 years (1979-1998) to 28 years (1979-2006)” by Cavalieri and Parkinson (2008) derived new linear trends of Antarctic sea ice extent and area based on satellite-borne passive microwave radiometer data. Results indicate “the total Antarctic sea ice extent trend increased slightly, from 0.96 ± 0.61 percent per decade to 1.0 ± 0.4 percent per decade, from the 20- to 28-year period,” noting the latter trend is significant at the 95 percent confidence level. Corresponding numbers for the Antarctic sea ice area trend were 1.2 ± 0.7 percent per decade and 1.2 ± 0.5 percent per decade. Both sets of results indicate a “tightening up” of the two relationships: Over the last eight years of the study period, both the extent and area of Antarctic sea ice have continued to increase, with the former parameter increasing at a more rapid rate than it did over the 1979-1998 period.
Cavalieri, D.J. and Parkinson, C.L. 2008. Antarctic sea ice variability and trends, 1979-2006. Journal of Geophysical Research 113: 10.1029/2007JC004564.
Cavalieri, D.J., Parkinson, C.L. and Vinnikov, K.Y. 2003. 30-Year satellite record reveals contrasting Arctic and Antarctic decadal sea ice variability. Geophysical Research Letters 30: 10.1029/2003GL018031.
Climate Change Reconsidered: Website of the Nongovernmental International Panel on Climate Change. http://www.nipccreport.org/archive/archive.html
Comiso, J.C. 2000. Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements. Journal of Climate 13: 1674-1696.
Comiso, J.C. and Nishio, F. 2008. Trends in the sea ice cover using enhanced and compatible AMSR-E, SSM/I, and SMMR data. Journal of Geophysical Research 113: 10.1029/2007JC004257.
Elderfield, H. and Rickaby, R.E.M. 2000. Oceanic Cd/P ratio and nutrient utilization in the glacial Southern Ocean. Nature 405: 305-310.
Hanna, E. 2001. Anomalous peak in Antarctic sea-ice area, winter 1998, coincident with ENSO. Geophysical Research Letters 28: 1595-1598.
Laine, V. 2008. Antarctic ice sheet and sea ice regional albedo and temperature change, 1981-2000, from AVHRR Polar Pathfinder data. Remote Sensing of Environment 112: 646-667.
Liu, J., Curry, J.A. and Martinson, D.G. 2004. Interpretation of recent Antarctic sea ice variability. Geophysical Research Letters 31: 10.1029/2003GL018732.
Parkinson, C.L. 2002. Trends in the length of the Southern Ocean sea-ice season, 1979-99. Annals of Glaciology 34: 435-440.
Parkinson, C.L. 2004. Southern Ocean sea ice and its wider linkages: insights revealed from models and observations. Antarctic Science 16: 387-400.
Vyas, N.K., Dash, M.K., Bhandari, S.M., Khare, N., Mitra, A. and Pandey, P.C. 2003. On the secular trends in sea ice extent over the antarctic region based on OCEANSAT-1 MSMR observations. International Journal of Remote Sensing 24: 2277-2287.
Watkins, A.B. and Simmonds, I. 2000. Current trends in Antarctic sea ice: The 1990s impact on a short climatology. Journal of Climate 13: 4441-4451.
Yuan, X. and Martinson, D.G. 2000. Antarctic sea ice extent variability and its global connectivity. Journal of Climate 13: 1697-1717.
Zwally, H.J., Comiso, J.C., Parkinson, C.L. Cavalieri, D.J. and Gloersen, P. 2002. Variability of Antarctic sea ice 1979-1998. Journal of Geophysical Research 107: 10.1029/2000JC000733.