🧊 Sea ice

The sea round the poles freezes every winter. What happens when it melts?

The sea can freeze
Round the North Pole it gets so cold that the top of the sea freezes. This sea ice is a floating crust of frozen seawater, a few metres thick. White shows how far it reaches at the end of winter (drawn simpler than it really is).
It grows and shrinks
Sea ice comes and goes with the seasons. In the long dark of winter it spreads far out. In the summer sun most of the new ice melts away again. Slide through the year and watch the night and the ice move together.
The south does the opposite
Antarctica has winter when the Arctic has summer. So its sea ice is biggest in September, just when the Arctic’s is smallest. Slide through the year again, from the bottom of the world.
Melt it: does the sea rise?
Sea ice is already floating in the sea. If it all melted, sea level would not rise at all, like an ice cube melting in a full glass. Try it.
White bounces, dark soaks
Sea ice is white, so it bounces most of the sunlight back into space. Open sea is dark, and soaks it up. How much a surface bounces back is called its albedo.
Less and less
Satellites have measured the sea ice every day since 1979. Each September, the Arctic keeps less ice than it used to. Slide through the years.
Quiz time
The white is the sea ice at the end of the northern winter. Which ocean is most of it on?

⭐ Remember

Sea ice is frozen seawater floating at the poles. It grows in winter and melts in summer.
Melting sea ice doesn’t raise the sea, but it uncovers dark water that soaks up more sunlight.
Since 1979, the Arctic has lost about 12% of its September ice every ten years.

The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).