[flukus]

1. Thermal expansion. It's a tiny effect, you won't see it in a glass of water in science class but when you've got oceans a few kilometers deep and humans living a few meters above them it can have an impact.

2. It's a feedback effect. Less ice means less light reflected means more energy in our oceans/atmosphere. In turn this creates more ice loss that won't be sea ice.

[gaius]

Water expands as it freezes. That’s why pipes burst in winter.

[flukus]

Water crystallization is a different effect and not relevant here. It has the same mass so it doesn't effect sea levels when it's frozen and it's not a linear effect, it won't continue to shrink as it warms.

But like everything else (to various extents) it will expand when it's warmer.

[imtringued]

The maximum density of water is at 4 degrees. If you heat it above that it starts expanding quickly. If you freeze it below that it starts expanding slowly.