The sea ice covering the high-latitude oceans around both poles has changed in enigmatic ways during recent decades. In the Arctic, the ice cover has been rapidly retreating. In the Antarctic, by contrast, satellite observations indicate that the sea ice cover has been expanding, with an annual-mean rate 1/3 as fast as the Arctic sea ice retreat. These changes in both hemispheres disagree sharply with expectations based on comprehensive climate models, which consistently simulate too little sea ice loss in the Arctic and too much sea ice loss in the Antarctic (see graphic). Furthermore, there are surprisingly strong seasonal differences in the sea ice changes: the sea ice cover in the Arctic has largely recovered toward its long-term mean each winter, resulting in a year-to-year sea ice retreat that is far more pronounced in September than in any other month of the year. In this presentation, I will discuss work I have been doing toward addressing some of these enigmatic phenomena. This work involves the use of satellite-derived observations, simulations from comprehensive climate models, and idealized physical theories.