Vast expanses of sea ice make the Arctic and Antarctic oceans perilous and inhospitable, but they’re also vital guardians of Earth’s climate.
Sea ice forms when ocean water is cooled to the freezing point by cold polar air. Freezing begins in fall, when the amount of light from the sun decreases, and ice continues to expand and thicken during the dark winter months. When summer returns, the sun’s energy warms the surface of the ice, causing portions of it to melt.