Sea ice is riddled with salty brine channels that host whole food webs
Frozen seawater covers about 7 percent of Earth's surface, yet it is far from solid. It is a mix of ice, trapped brine, air bubbles and salt, and those brine channels shelter microbes at the base of unique food webs. Its white surface also bounces sunlight back to space, helping cool the planet.
Because ice is less dense than liquid water, frozen seawater floats. It covers roughly 12 percent of the oceans, mostly in the Arctic and Antarctic ice packs, which swell each winter and shrink each summer. It is not the same as an iceberg, which is a chunk broken off a glacier or ice shelf, though icebergs can become embedded in it.
The mix of ice, brine and air governs how it conducts heat, how strong it is and how dense. While growing it usually holds under 5 percent brine and 1 to 2 percent air, but warming can raise air content to 15 percent in summer. First-year ice ranges from about 910 to 920 kilograms per cubic metre in winter down to 860 to 880 in summer, and older ice is lighter still. Those shifts introduce errors of 0.3 to 0.4 metres when satellites use radar or lasers to estimate thickness.
Winds, currents and temperature swings keep it constantly moving and deforming. Ice frozen to the shore, shoals or grounded icebergs is called fast ice; farther out, drift ice moves freely, divided into a shear zone, a central pack and a marginal ice zone bordering open water. Drift ice is made of floes at least 20 metres across, graded from small up to giant, which exceeds 10 kilometres. Submariners call the whole cover the ice canopy.
Its influence reaches well beyond the poles. It steers shipping routes, shapes regional weather and affects global ocean circulation, and it insulates the sea, limiting exchanges of heat, water vapour and carbon dioxide with the air. Satellites have recorded a marked fall in Arctic ice extent and thickness in recent decades, tied to climate change, while Antarctic ice varies more regionally but has recently declined as well.
Source: Sea ice
