The Arctic is almost entirely covered by water, much of it frozen. Some frozen features, such as glaciers and icebergs, are frozen freshwater. In fact, the glaciers and icebergs in the Arctic make up about 10% of Earth’s supply of freshwater. Most of the Arctic, however, is the liquid saltwater of the Arctic ocean basin. Some parts of the ocean’s surface remain frozen all or most of the year. This frozen seawater is called sea ice. Often, sea ice is covered with a thick blanket of snow. Sea ice helps determine Earth’s climate.
Sea ice has a very bright surface, which means about 80% of sunlight that strikes sea ice is reflected back to space. The dark surface of the liquid ocean, however, absorbs about 90% of solar radiation. Due to thermohaline circulation, the Arctic’s thick, reflective sea ice moderates ocean temperatures around the world. As sea ice melts in the summer, it exposes the dark ocean surface. Instead of reflecting 80% of the sunlight, the ocean absorbs 90% of the sunlight. The oceans heat up, and Arctic temperatures rise further.