Sea ice concentration

The fraction of a patch of ocean covered by ice rather than open water, usually given as a percentage. Satellites map it across the whole Arctic and Antarctic.

Why it matters

Sea ice reflects sunlight, insulates the ocean, and shapes polar ecosystems and shipping, so its concentration is a headline climate indicator — its long-term shrinkage is one of the clearest signs of warming.

Where you’ll meet it

• AMSR2 passive microwave maps daily sea ice concentration through clouds and darkness.
• Earlier SSM/I sensors built the multi-decade sea ice record.
• MODIS and VIIRS add fine detail to ice mapping under clear skies.

In plain terms

It’s like looking at a glass of icy water from above and estimating how much of the surface is ice cubes versus liquid — across an entire polar ocean.