evidence: Sea ice is frozen seawater that floats on the ocean surface. Bookending our planet, sea ice waxes and wanes with the polar seasons. In the Arctic winter, sea ice stretches its tentacles into seas and coastlines far from the poles. Since land confines most of the Arctic, sea ice can only spread so far southward in many places. By contrast, Antarctic sea ice fringes a massive land mass, growing as far as winter freezing temperatures let it. Come spring, however, sea ice extent shrinks in the Arctic, with a lot of ice persisting, whereas almost all Southern Ocean or Antarctic sea ice is "seasonal ice," meaning it melts away and reforms annually. [...] Both Arctic and Antarctic sea ice extent are characterized by variations from year to year. The monthly average extent can vary by as more than 1 million or even 2 million square kilometers (386,000 square miles) from the long-term monthly average. [...] Historically, Antarctic sea ice has exhibited a minimal long-term trend with significant year-to-year variability. In the austral winter of 2014, Antarctic sea ice reached its highest extent in the modern satellite record dating back to 1979. In 2016, however, Antarctic sea ice began to experience a series of below-average extents, including a series of record-low extents, prompting questions about a possible regime change.