In January each year, about 41 percent of all land on Earth is covered by snow. In March, sea ice covers an area the size of Antarctica in the Northern Hemisphere, and in September, the same is true in the Southern Hemisphere.
Snow is white, ice is almost white, and both reflect more sunlight back into space than the ground or ocean beneath would do. This cools the Earth. But that is just one of the effects climate scientists need to consider when dealing with ice. Learn more about how ice affects climate on Earth in the sections below.
Sea ice and glaciers may be the first things to come to our mind when thinking about ice on Earth. But if we are concerned about the area covered, nothing beats the seasonal snow that falls during the Northern Hemisphere winter. At the maximum in January, 47 million square kilometers are covered with snow.
The difference through the year is also large. The Southern Hemisphere has less land masses than the Northern Hemisphere, and with the exception of Antarctica, there is little land in the far south. As a result, the maximum snow cover in the Southern Hemisphere is less than one million square kilometers.
Snow is white and reflects sunlight back into space. It is also a good insulator, shielding the ground below from the icy cold winter atmosphere. As snow reflects so much more of the radiation from the sun than rock or forests do, snow makes the ground colder than it would otherwise be. This can affect weather system such as blocking high pressure areas.
The age of the snow plays a large role in determining the albedo of snow, of how much sunlight it reflects. Fresh snow has a very high albedo. As the snow ages, the grains in the snow grow and the snow reflects less light. You can clearly see the difference between the whiteness of fresh snow and the gray, older snow later in the season. Older snow will also have a higher density, providing less insulation between the air and the ground below the snow.
During the winter season, snow accumulates layer by layer, each coming from a single snowfall event. Snow models also represent the snow as layers with different properties.
Ice on Land
Ice covers only about 10 percent of the Earth’s land area, but stores 75 percent of our freshwater. Most of the ice is found in the huge ice sheets of Antarctica and Greenland, the rest in smaller ice caps and glaciers.
Sea level rise is the greatest concern when ice melts as the global temperature increases. If all the ice melted, the sea level would rise by 70 meters. To fully account for this, we need to understand, not just the thermodynamics of melting ice, but how the ice moves. Large ice streams carry ice from the top of Greenland and Antarctica into the ocean, where ice bergs float away from the coast. The speed of ice streams affects how fast the ice disappears.
Glacier ice flows, not just in the sense that ice floats on water, but as motion within the ice itself. Like a bread dough, some would say, or like ketchup.
Sea ice moves. When it forms, ice floes will often cover parts of the region, with narrow leads of open water between them. The area covered by sea ice in winter is the same in both hemispheres, but the seasonal variation is larger in the south. There, little of the ice survives the summer, so the ice will be thin, first-year ice. The average ice thickness in the Southern Ocean is less than one meter, compared to about three meters in the Central Arctic.
If you want to know how much sea ice there is, you need to know three things: How much ice is formed, how much ice melts, and how the ice moves. These factors must also be included in an ice model.
– Forming ice –
Snow on top of the ice can be converted into ice, as can rain. But mainly, sea ice grows at the base. In the cold winter, heat is lost from the ocean, through the ice and into the atmosphere. As the water below the ice gets sufficiently cold, it freezes to the underside of the ice. As the ice gets thicker, this process slows down. The ice insulates the ocean from the cold atmosphere. As a result, thin ice grows faster than thick ice.
New, fresh sea ice is very salty. Gradually, brine is ejected through channels in the ice. After a couple of years there is little salt left. As salt is released to the ocean, the water gets heavier. This may influence the circulation in the ocean.
In an Earth system model, the ice module kicks in when the ocean model tells it that there is surface water at the freezing point somewhere. The coupler is the messenger between the two modules. In the ice module, ice is then formed in part of a grid cell and allowed to grow thicker or melt. The salt in the model ice is allowed to gradually drain out, as in the real world.
This ice grows quickly. As the ice gets thicker, it insulates the ocean from the cold winter air, making ice growth less likely. If the ice is still, with no movements creating ridges, it will seldom get thicker than two meters.
– Melting ice –
Melting of the ice can occur from the top, from below and from the sides. Once the ice starts to melt, several feedback processes enhance the melting. Meltwater collects in pools on top of the ice, increasing the absorption of sunlight. As the ice gets thinner, the ocean also receives more radiation and becomes warmer. This increases melting from below and from the sides.
There is a major difference between melting sea ice in the Arctic and in Antarctica. The waters around Antarctica are relatively warm, causing the ice to melt from below. In the Arctic, freshwater from the large rivers that enter the Polar Ocean acts as a cold layer on top, shielding the ice from the warmer ocean water below. As a result, melting from above is much more important in the Arctic.
– Drifting ice –
Ice floes move. Large packs of ice move. The movement of the ice, with leads and polynyas opening between the floes, exposes the ocean to the atmosphere. When this happens, there is a huge exchange of heat and moisture between the atmosphere and the ocean. Areas of open, exposed water increases the formation of sea ice.
Wind and ocean currents transports ice into other regions than where it formed. Ice with leads in-between the floes drifts freely, while there will be more tension in densely packed ice.
Wind and storms make ice floes crash into each other, forming ridges and ice that is thicker than the two meters upper limit otherwise seen. The thickest ice can be 3–5 meters thick.
– Snow on the ice –
As on land, snow falls on the sea ice, and this influences both the growth and the melting of ice in various ways.
The snow on the ice can be seen largely as composed of two layers. The lower layer consists of large, hollow crystals called depth hoar. This layer has less density, and the conductivity is low, meaning that it insulates the ice well. The upper layer is more compact, packed by the wind, especially in the Arctic. In Antarctica, snows fall more frequently, giving a softer upper layer.