Sea ice

This is sea ice in the Bellingshausen Sea in West Antarctica, seen from 2,000 feet up, on October 21.

"Sea ice formation and melt have a really strong impact on ocean circulation, which acts like a huge heat pump keeping our climate stable," according to Thorsten Markus in the Operation Ice Bridge FAQ. "This 'thermohaline circulation' is driven by temperature and salinity. The interesting part of this circulation is that the deep, bottom water masses of the ocean only make contact with the atmosphere only at polar latitudes, in the Arctic or the Antarctic. Change ocean salinity--by growing or melting sea ice, which is inherently salt-free--and you can affect global circulation. The process is complex, but that's basically why it's so critically important."