The Antarctic paradox: sea ice extent and thickness trends during the last three decades

In polar regions, ocean–atmosphere interactions are strongly influenced by sea ice and its thickness. Since satellite passive microwave observations became available in the 1970s, significant progress has been made in the study of snow depth and sea ice concentration/extent in these regions. Satellite observations collected in the last three decades show that Arctic sea ice has been reduced drastically while Antarctic sea ice has been stable or even increasing (slowly but to record levels). This expansion is at odds with general climate model results. These different behaviors in Arctic and Antarctic sea ice constitute a challenging paradox to be explained by the cryosphere community. While climate deniers claimed record Antarctic sea ice was a refutation of climate change, in reality it showed they could not fathom that the cryosphere and the Earth climate are complex processes which we are still trying to understand. In this framework the polar paradox is a key issue in climate science to be convincingly resolved, through appropriate accounting of controlling factors, in order to advance Earth system models to achieve projections with a significantly reduced uncertainty. A number of hypotheses have been proposed to explain the polar paradox: i) stratospheric ozone depletion may affect atmospheric circulation and wind patterns sustaining the Antarctic sea ice cover, ii) warm deep ocean currents are contributing to ice shelves melt thus shielding the surface ocean from the warmer deeper waters, iii) decrease in sea ice growth may reduce salt rejection and upper-ocean density to enhance thermohaline stratification, iv) Antarctica lacks of additional heat sources such as warm river discharge as is the case in the Arctic.Estimating what is happening under the sea ice surface, instead, turned out to be considerably more difficult. Recent studies using several satellite products (IceSat, Cryosat-2, SMOS) show that also sea ice thickness is drastically decreasing in the Arctic, while few studies have been provided about Antarctica. In this study we used the estimations provided by the SIT algorithm (Aulicino et al., 2014) including passive microwave satellite observations to demonstrate that no evidences of negative sea ice thickness trends can be detected in two of the most important regions of the Southern Ocean (i.e., the Ross and Weddell Seas), over the last two decades (1992-2015). We also show that these regions behave differently in the study period: a dipolar behavior is evident in the first decade of analysis (1992-2002) while not present in the following decade. Connections to the main climate indexes suggest that the study of regional scales is fundamental for a detailed comprehension of the Antarctic sea ice dynamics and evolution.