Ice
Sea ice and glacial ice hold about 2% of the water on Earth, but because the water is frozen ice plays a different role in the climate system than does ocean water.

Sea ice forms when sea water freezes, which happens at -1.9C. As the seawater freezes, all the salt is pushed out, so that the ice ends up less dense than the seawater, and floats atop it. Sea ice acts as an barrier that prevents the ocean from interacting with the atmosphere. When ice is present, heat from the ocean is not lost to the atmosphere (see below), and so the water can remain much warmer than the air.
Glacial ice occurs as mountain glaciers or continental ice sheets. Mountain glaciers can occur on anywhere in the world, but in the tropics they cannot form below 5 kilometers altitude, where it is too warm. There are currently two continental ice sheets in existence, which cover most of Greenland and Antarctica. If these ice sheets were to melt entirely, sea level could raise as much as 70 meters.

Ice plays an important role in reflecting incoming radiation from the sun back into the atmosphere, where it can become trapped by greenhouse gases.