A 40-y record reveals gradual Antarctic sea ice increases followed by decreases at rates far exceeding the rates seen in the Arctic
Claire L. Parkinson
PNAS first published July 1, 2019 https://doi.org/10.1073/pnas.1906556116
Contributed by Claire L. Parkinson, May 24, 2019 (sent for review April 16, 2019; reviewed by Will Hobbs and Douglas G. Martinson)
A newly completed 40-y record of satellite observations is used to quantify changes in Antarctic sea ice coverage since the late 1970s. Sea ice spreads over vast areas and has major impacts on the rest of the climate system, reflecting solar radiation and restricting ocean/atmosphere exchanges. The satellite record reveals that a gradual, decades-long overall increase in Antarctic sea ice extents reversed in 2014, with subsequent rates of decrease in 2014–2017 far exceeding the more widely publicized decay rates experienced in the Arctic. The rapid decreases reduced the Antarctic sea ice extents to their lowest values in the 40-y record, both on a yearly average basis (record low in 2017) and on a monthly basis (record low in February 2017).
Following over 3 decades of gradual but uneven increases in sea ice coverage, the yearly average Antarctic sea ice extents reached a record high of 12.8 × 106 km2 in 2014, followed by a decline so precipitous that they reached their lowest value in the 40-y 1979–2018 satellite multichannel passive-microwave record, 10.7 × 106 km2, in 2017. In contrast, it took the Arctic sea ice cover a full 3 decades to register a loss that great in yearly average ice extents. Still, when considering the 40-y record as a whole, the Antarctic sea ice continues to have a positive overall trend in yearly average ice extents, although at 11,300 ± 5,300 km2⋅y−1, this trend is only 50% of the trend for 1979–2014, before the precipitous decline. Four of the 5 sectors into which the Antarctic sea ice cover is divided all also have 40-y positive trends that are well reduced from their 2014–2017 values. The one anomalous sector in this regard, the Bellingshausen/Amundsen Seas, has a 40-y negative trend, with the yearly average ice extents decreasing overall in the first 3 decades, reaching a minimum in 2007, and exhibiting an overall upward trend since 2007 (i.e., reflecting a reversal in the opposite direction from the other 4 sectors and the Antarctic sea ice cover as a whole).
Since the late 1990s, it has been clear that the Arctic sea ice cover has been decreasing in extent over the course of the multichannel passive-microwave satellite record begun in late 1978 (1⇓–3). The decreases have accelerated since the 1990s and have been part of a consistent suite of changes in the Arctic, including rising atmospheric temperatures, melting land ice, thawing permafrost, longer growing seasons, increased coastal erosion, and warming oceans (4, 5). Overall, it has been a consistent picture solidly in line with the expectations of the warming climate predicted from increases in greenhouse gases. In particular, modeled sea ice predictions showed marked Arctic sea ice decreases, and the actual decreases even exceeded what the models predicted (6).
The Antarctic situation has been quite different, with sea ice extent increasing overall for much of the period since 1978 (7⇓⇓⇓–11). These increases have been far more puzzling than the Arctic sea ice decreases and have led to a variety of suggested explanations, from ties to the ozone hole (12, 13; rejected in refs. 14, 15); to ties to the El Niño–Southern Oscillation (ENSO) (16), the Interdecadal Pacific Oscillation (17), and/or the Amundsen Sea Low (10, 13, 17); to ties to basal meltwater from the ice shelves (18; rejected in ref. 19). None of these has yet yielded a consensus view of why the long-term Antarctic sea ice increases occurred.
In the meantime, while the unexpected, decades-long overall increases in Antarctic sea ice extent are still being puzzled out, the sea ice extent has taken a dramatic turn from relatively gradual increases to rapid decreases. On a yearly average basis, the peak sea ice extent since 1978 came in 2014. Since then, the decreases have been so great that the yearly averages for 2017 and 2018 are the lowest in the entire 1979–2018 record, essentially wiping out the 35 y of overall ice extent increases in just a few years. This dramatic reversal in the changes occurring in the Antarctic sea ice will provide valuable further information to test earlier suggested explanations of the long-term Antarctic sea ice increases. We now have a 40-y multichannel passive-microwave satellite record of the Antarctic sea ice cover, all of which resides in the Southern Ocean. The purpose of this paper is to present that record both for the Southern Ocean as a whole (labeled “Southern Hemisphere” in the figures, to emphasize the inclusion of the entire hemispheric sea ice cover) and for the breakdown of the Southern Ocean into the 5 sectors identified in Fig. 1.