Ice and sea
In some parts of the world the ocean becomes so cold that it begins to freeze. The ice that grows (called sea ice) isn’t as dense as seawater so it floats on top. Starting off as a slushy mixture of ice and water, the ice crystals gradually stick together to make a solid layer. As the ice gets thicker it forms a barrier which stops much heat, light or gas getting through, and reduces the effect of the wind stirring the upper ocean. Some parts of the Arctic Ocean are covered in ice all year round (called multiyear ice), but at the edges of the Arctic Ocean and around Antarctica the ice comes and goes with the seasons (called seasonal ice). Although sea ice can sometimes get stuck to the land and remain in the same place for months at a time (called landfast ice), it is generally always moving, being pushed around by the wind and the ocean currents. This motion can make sheets of ice crash into each other causing ice to pile up into thick ridges.
When the sea freezes, it changes how salty the seawater is. As sea ice starts to form, most of the salt from the seawater drains through the ice crystals, making the seawater underneath even saltier. This super salty water (called brine) is heavy because of the extra salt and sinks deep into the ocean. So when the ice warms up and starts to melt back into water, there isn’t much salt left in it. The cold melt water stays floating on the top of the ocean, similar to how oil floats on water. Changing where the salt is in the ocean can affect how the sea moves (ocean currents) and where creatures live in the sea. Just like plants that live on land, the tiny plants that live in seawater (called phytoplankton) need light and food to grow. If they are living in the surface layer of melt water they will have plenty of light but will quickly use up all the food nearby. It is difficult for more food to get into the fresh surface water from below, just as it is difficult for the tiny plants living below the melt water to get up to the surface to find the light.
Sea ice also plays a key role in the climate system by: 1) insulating the relatively warm ocean from the cold winter atmosphere and 2) reflecting the sun’s radiation back into the atmosphere during the summer (due to the high albedo of ice and snow). Even when the sea ice cover is quite extensive there will be some regions that remain ice free. Long cracks (called leads) can form in the ice or the wind can blow all the ice away from some coastal areas (called polynyas). These gaps in the ice expose the underlying seawater to intense winter heat loss 10-100 times greater than over sea ice. Without the insulating sea ice blanket, the seawater cools rapidly and more ice forms. During the summer, as the surface of the ice begins to melt, pools of water begin to accumulate in depressions in the ice (called melt ponds). These melt ponds are not as reflective as the surrounding ice and absorb more heat (they have a low albedo) which causes more ice to melt (this is an example of a positive feedback).
Icebergs are not the same as sea ice. They are formed when chunks of glaciers break off into the sea and as such are made up of snow that fell at the head of the glacier. However, once they are in contact with the warm ocean they will gradually melt and release fresh water into the sea with similar oceanographic impacts as those described above for sea ice.
