How has the sea ice that surrounds Antarctica varied over the period for which there exist comprehensive satellite data? In what follows, we review what has been learned about the subject -- in the order in which it was learned -- starting with the very first year of the current millennium.
Noting that "Antarctic sea ice may show high sensitivity to any anthropogenic increase in temperature" -- as per the canary-in-the-coal-mine concept of high-latitude amplification of CO2-induced global warming -- while further noting that most climate models suggest that an increase in surface temperature "would result in a decrease in sea ice coverage," Watkins and Simmonds (2000) analyzed temporal trends in different measures of the sea ice that surrounds Antarctica, using Special Sensor Microwave Imager data obtained from the Defense Meteorological Satellite Program for the nine-year period December 1987-December 1996, in search of the suspected signal. But contrary to what one would expect on the basis of the model simulations, and especially in light of what climate alarmists call the unprecedented warming of the past quarter-century, the two scientists observed statistically significant increases in both sea ice area and extent; and when they combined their results with those of the preceding nine-year period (1978-1987), both parameters continued to show increases over that expanded time period. In addition, they found that the 1990s also experienced increases in the length of the sea-ice season.
In a contemporary assessment of Antarctic sea ice behavior, Yuan and Martinson (2000) also utilized Special Sensor Microwave Imager data, but they additionally analyzed brightness temperatures obtained by the Nimbus-7 Scanning Multichannel Microwave Radiometer; determining that the mean trend in the latitudinal location of the Antarctic sea ice edge over the prior 18 years was an equatorward extension of 0.011 degree latitude per year, in harmony with the findings of Comiso (2000), who analyzed Antarctic temperature data obtained from 21 surface stations, as well as from infrared satellites operating from 1979 to 1998, and discovered a 20-year cooling trend of 0.042°C per year in the satellite data and 0.008°C per year in the station data.
That Antarctic sea ice had indeed increased in area, extent and season length since at least 1978 was also supported by several subsequent studies. The very next year, for example, Hanna (2001) published an updated analysis of Antarctic sea ice cover -- also based on Special Sensor Microwave Imager data, but for the extended period of October 1987-September 1999 -- finding "an ongoing slight but significant hemispheric increase of 3.7(±0.3)% in extent and 6.6(±1.5)% in area." And one year later, 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 were opposing regional trends, a "much larger area of the Southern Ocean experienced an overall lengthening of the sea-ice season ... than experienced a shortening."
Concurrently, 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. And in contradiction of the ancillary climate-alarmist claim that various aspects of earth's climate should exhibit greater variability when it is warmer than when it is colder, they observed that the variability of monthly sea ice extent declined from 4.0% over the first ten years of the record to 2.7% over the last ten years (which were supposedly the warmest of the prior millennium, according to the world's climate alarmists).
One year later, 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 had been derived from space-based passive microwave radiometers carried aboard earlier Nimbus-5, Nimbus-7 and DMSP satellites, in order to study secular trends in sea ice extent about Antarctica over the period 1978-2001. This 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 addition, the six researchers 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," which finding they described as "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 data sets with National Ice Center digital sea ice data, finding that sea ice extent about Antarctica rose at a mean rate of 0.10 ± 0.05 x 106 km² per decade between 1977 and 2002. Likewise, Liu et al. (2004) employed sea ice concentration data retrieved from the Scanning Multichannel Microwave Radiometer on the Nimbus 7 satellite, plus the Special Sensor Microwave Imager on several defense meteorological satellites, to develop a quality-controlled history of Antarctic sea ice variability over the period 1979-2002 (which included 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. This work revealed, in their words, that "overall, the total Antarctic sea ice extent has shown an increasing trend (~4,801 km²/yr)," and that "the total Antarctic sea ice area has increased significantly by ~13,295 km²/yr, exceeding the 95% confidence level."
Shortly thereafter, Parkinson (2004) reviewed the history of satellite observations of sea ice extent in the Southern Ocean about Antarctica, concentrating on data obtained from the Scanning Multichannel Microwave Radiometer aboard the Nimbus 7 satellite and subsequent satellite-based Special Sensor Microwave Imagers, because these platforms provided, in her words, "the best long-term record of changes in the full Southern Ocean ice cover."
The resulting plot of 12-month running-means of Southern Ocean sea ice extent, which extended from November 1978 through December 2002, revealed significant multi-year variability in the data, which began at the top of a peak and ended at the bottom of a trough. But in spite of the high beginning point and low end point of the data, which would mitigate against a long-term upward trend, the data exhibited just such a feature, the least-squares-fit slope of which revealed a 12,380 ± 1,730 km2 upward trend in sea ice extent per year.
In considering this result, it is interesting to note that over the period of time that climate alarmists claim has experienced the most extreme global warming of the past millennium or more, and in spite of the fact they have historically claimed such warming should be most evident in earth's polar regions, and that it should lead to a decrease in polar sea ice extent, just the opposite had occurred to this point in time in the Southern Ocean that surrounds Antarctica. But what is doubly damning to their dogma is the fact that the Southern Ocean's sea ice extent is extremely sensitive to warming, decreasing from a 24-year-average maximum monthly value of 18.23 x 106 km2 in September to a similarly-calculated minimum monthly value of 2.98 x 106 km2 in February. This decrease represents the disappearance of nearly 84% of each year's maximum sea ice cover; and, therefore, it can be appreciated that given just a little extra seasonal warmth, it would disappear altogether each February. But it hasn't. In fact, it continues to slowly, but ever so surely, grow in the mean.
Focusing on the spring-summer period of November/December/January (1981-2000) some four years later, Laine (2008) determined trends in Antarctic ice-sheet and sea-ice surface albedo and temperature, as well as sea-ice concentration and extent, based on Advanced Very High Resolution Polar Pathfinder data in the case of ice-sheet surface albedo and temperature, and the Scanning Multichannel Microwave Radiometer and Special Sensor Microwave Imagers in the case of sea-ice concentration and extent. These analyses were carried out for the continent as a whole, as well as for 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. This work revealed, in Laine's words, that "all the regions show negative spring-summer surface temperature trends for the study period." In addition, the Finnish researcher found that "sea ice concentration shows slight increasing trends in most sectors, where the sea ice extent trends seem to be near zero." Laine also found that "the Antarctic region as a whole and all the sectors separately show slightly positive spring-summer albedo trends." Consequently, over the last two decades of the 20th century, Antarctica successfully bucked the world's supposedly unprecedented global warming trend by (1) cooling a bit, (2) acquiring slightly more sea ice, and (3) becoming a tad more reflective of incoming solar radiation.
Several other studies of the subject were also conducted in 2008. Noting that earth's polar regions "are expected to provide early signals of a climate change primarily because of the 'ice-albedo feedback' which is associated with changes in absorption of solar energy due to changes in the area covered by the highly reflective sea ice," for example, Comiso and Nishio (2008) set about to provide updated and improved estimates of trends in Antarctic sea ice cover for the period extending from November 1978 to December 2006, based on data obtained from the Advanced Microwave Scanning Radiometer, the Special Scanning Microwave Imager and the Scanning Multichannel Microwave Radiometer. And in doing so, they found that the 28-year trends in Antarctic sea ice extent and area were +0.9 ± 0.2 and +1.7 ± 0.3% per decade, which is definitely not a "signal" of global warming.
In another study employing satellite-borne passive microwave radiometer data that extended the analyses of the sea ice time series reported by Zwally et al. (2002) from 20 years (1979-1998) to 28 years (1979-2006), Cavalieri and Parkinson (2008) found that "the total Antarctic sea ice extent trend increased slightly, from 0.96 ± 0.61% per decade to 1.0 ± 0.4% per decade, from the 20- to 28-year period." The Antarctic sea ice area trend, however, remained constant at 1.2 ± 0.7% per decade. Its variability, however, like that of sea ice extent, declined (from ± 0.7% to ± 0.5% per decade), so that both sets of results indicated a "tightening up" of the two relationships. And why were these things so? The two researchers state that "what is driving the observed changes remains unanswered, and the physical mechanisms explaining these changes remain to be determined."
Most recently, Turner et al. (2009) reviewed the history of Antarctic sea ice extent derived from satellite observations, after which they attempted to derive an explanation for the empirical data being what they are, based on climate model simulations. Citing the work of Zwalley et al. (2002), they first noted that over the period 1979-1998, sea ice extent surrounding Antarctica increased at a mean rate of 0.98% per decade, and that Comiso and Nishio (2008) derived a value of 0.9% per decade for the period 1978-2006. This sea ice extent increase, according to their modeling work, was largely driven by an autumn increase in the Ross Sea sector that they suggest "is primarily a result of stronger cyclonic atmospheric flow over the Amundsen Sea." And they say that "the trend towards stronger cyclonic circulation is mainly a result of stratospheric ozone depletion, which has strengthened autumn wind speeds around the continent, deepening the Amundsen Sea Low through flow separation around the high coastal orography." On the other hand, and much more simply, the nine researchers report that "statistics derived from a climate model control run suggest that the observed sea ice increase might still be within the range of natural climate variability."
In light of these contrasting possibilities, it is clear that the true cause of the near-three-decade-long increase in Antarctic sea ice extent cannot be stated with any confidence. The only thing we can conclude at this point in time, therefore, is that for some still-unproven reason, and in spite of the supposedly unprecedented increases in mean global air temperature and atmospheric CO2 concentration that the planet has experienced since the late 1970s, Antarctica sea ice extent has stubbornly refused to do what climate models say it should be doing, as it just keeps on growing.
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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.
Comiso, J.C. 2000. Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements. Journal of Climate 13: 1674-1696.
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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.
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Turner, J., Comiso, J.C., Marshall, G.J., Lachlan-Cope, T.A., Bracegirdle, T., Maksym, T., Meredith, M.P., Wang, Z. and Orr, A. 2009. Non-annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent. Geophysical Research Letters 36: 10.1029/2009GL037524.
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.
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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.Last updated 30 December 2009