Pack ice is a dense layer of ice that drifts on the ocean surface under the influence of winds and currents and independent of any landmass. Areas of pack ice grow in the winter and shrink in the summer. While warming air and water temperatures are generally melting Earth's ice, pack ice has increased in a few areas when ice sheets detach from land.

Pack ice is a crucial part of polar ecosystems. For instance, it provides protection for krill to breed without the danger of predators. However, large, thick masses of pack ice that persist over longer periods prevent other animals from accessing the krill, which can lead to a decline in the populations of birds and other species that need krill for food.

Arctic shipping routes are also affected by pack ice, with thick ice necessitating the use of icebreaking ships to clear the way. Because pack ice drifts around independent of land, it can cause unexpected hazards for ships navigating this area of the ocean.

NASA collects data to track the formation and growth of pack ice, providing a useful resource for studies of polar ecosystems, shipping transport, and global warming.