Sea ice, which forms from the freezing of ocean water, is a major component of the cryosphere, covering millions of square kilometers of the polar oceans each year. Pack ice represents the vast, mobile expanse of this sea ice, acting as a dynamic boundary layer between the ocean and the atmosphere. Its scale and constant change influence polar environments, affecting local weather and global climate patterns.
Defining Pack Ice and Its Stages of Formation
Pack ice is a general term for any large area of floating sea ice that is not attached to the coastline, consisting of numerous individual pieces of ice called floes. Formation begins when seawater cools to its freezing point of about -1.8 degrees Celsius. The first sign is the appearance of frazil ice, which are fine, needle-like crystals suspended near the surface. These crystals then coagulate to form a soupy, viscous layer known as grease ice, giving the sea a matte appearance.
If the water is calm, a thin, elastic crust called nilas forms, typically less than 10 centimeters thick. In turbulent waters, the grease ice or broken nilas churns, creating pancake ice. Pancake ice consists of circular pieces ranging from 30 centimeters to three meters in diameter with distinctive raised rims caused by collisions. The final transition occurs as these pieces raft and freeze together, forming larger, more rigid ice floes. Once the ice has thickened beyond 30 centimeters, it is classified as first-year ice.
The Dynamics of Mobile and Fixed Sea Ice
Sea ice cover is categorized into two types based on movement and location. Fast ice, or land-fast ice, is anchored to the coastline, seabed, or other fixed objects, remaining stationary throughout the winter. This fixed ice provides a stable platform along the edges of polar landmasses.
In contrast, the vast majority of pack ice is classified as drift ice, carried along by wind stress and ocean currents. This mobile ice field is constantly in motion; individual floes collide to form pressure ridges or separate to create open-water features called leads and polynyas.
The thickness of the ice is defined by its age. Seasonal or first-year ice melts completely each summer, typically reaching one to two meters in thickness. Multi-year ice, often called perennial ice, has survived at least one summer melt season, making it thicker and more resilient, often averaging three to five meters in the Arctic.
Essential Role in Polar Ecosystems and Climate
Pack ice is a specialized habitat that supports a complex food web. Microscopic ice algae grow within the brine channels and pores of the ice structure, forming the base of the polar marine food chain. This primary production supports grazers like Antarctic krill, which are a major food source for numerous species, including fish, seals, and whales.
Larger marine mammals, such as seals and polar bears, depend on the ice surface for breeding, hunting, and resting platforms. The presence of pack ice insulates the warmer ocean water from the frigid polar atmosphere, regulating the transfer of heat and moisture between the two.
The ice also regulates global temperatures through its high reflectivity, known as the albedo effect. The bright white surface of the pack ice reflects a high percentage of incoming solar radiation back into space. When the ice melts, it exposes the darker ocean water, which absorbs more solar energy, leading to further warming and melting in a self-reinforcing cycle. This ice-albedo feedback makes the pack ice a globally significant component of the planet’s energy budget.