Land Ice flows over land and ocean
Land ice, such as glaciers and ice sheets, forms where more snow falls than melts, year on year. This mostly happens in the Arctic and near the South Pole, but there are some glaciers in high mountains in tropical regions too. Over hundreds or even thousands of years, the snow compresses down into ice and can start to flow very slowly. As long as there is a supply of ice flowing in from where the snow is accumulating, land ice also covers nearby areas that have more melt than snowfall.
There are almost 200,000 mountain glaciers on Earth at the moment, and two huge ice sheets covering Greenland and Antarctica. With such large bodies of ice, the flowing ice often extends some way over the water as an ice shelf or tongue, or can extend to the sea floor and form a sheer cliff. The ice then interacts directly with the ocean, and whole chunks can break off, forming icebergs.
Land Ice interacts with the climate and controls global sea level
About three-quarters of the Earth’s freshwater is stored in land ice. Seasonal glacier melt is an essential input to rivers and a source of freshwater for many parts of the world. Very importantly, water in land ice is water that is not part of the volume of the ocean. The amount of water locked up in glaciers and the enormous ice sheets of Antarctica and Greenland define sea-level and where coastlines are all around the world. Around 600 million people live in coastal areas less than 10 metres above mean sea level, in cities and communities that generate approximately US$1 trillion of global wealth.
Land ice interacts with the atmosphere
Ice, and the snow that is often found on top of it, make a surface that looks whiter than other parts of the Earth. This means that ice-covered areas reflect a lot of the sunlight that falls on them and since ice makes up 10% of the land surface area, this effect is a significant control on how much of the sun’s energy the planet absorbs.
As well as being an important part of the Earth’s energy budget, land ice has physical effects on the winds and storms in the atmosphere. Cooling down the air on steep slopes can create “katabatic” winds from glaciers. The mountainous ice sheets of Greenland and Antarctica also serve as huge obstacles to the flow of the atmosphere, with Greenland guiding the path of the storms that track across the North Atlantic from America to Europe.
Ice sheets interact with the ocean
Ice in contact with the ocean generally either melts directly, often creating currents of very cold, fresh water near the ice, or it can break off in solid chunks to form icebergs which can drift for hundreds of kilometres. Icebergs then cool and dilute the salt content of the ocean as they go. With meltwater coming from the rest of the ice too , land ice helps set the physical properties (such as, temperature, salinity and chemistry) of the ocean in polar areas. This is important for the chemistry and biology of life in these waters.
The enormous ice shelves that fringe Antarctica play a big role in the global ocean overturning circulation too. This is where waters near the surface become very dense, sink to the bottom of the ocean and are transported around the rest of the planet over hundreds of years. Katabatic winds blowing off the ice cool the surrounding ocean so much that it freezes into sea-ice, which is then blown away from the coast, opening up more water that can freeze. Sea-ice cannot hold most of the salt found in sea water, so as sea-ice forms salt is expelled into the water below, making the water very dense and very cold. This dense water can be cooled even more if it comes into contact with the underside of an ice shelf. It sinks to the bottom of the ocean and is transported all around the world as part of the global circulation.
Land Ice is melting and sea level is rising
As our climate warms, land ice around the world is melting more and covering less of our planet’s surface. In some areas, changes in the climate mean it snows on the ice more too, but nearly everywhere the increase in melt is bigger than the increase in snow.
- Warmer ice flows faster, draining ice away from the mountains and toward the sea.
- Glaciers almost everywhere are retreating, impacting water supplies for some parts of the world.
- The Greenland ice sheet is shrinking, mostly from increased melting at the surface.
- Antarctica is generally still too cold to experience much surface melting for now, but the floating ice shelves around the edge are melting more as the ocean warms. As the ice shelves thin we expect the flow of the ice sheet to change and deliver more water to the ocean.
Changes in the land ice on Earth do not just affect the climate we live in, they can physically determine where we can live. Increased melt from land ice contributes about half of the sea-level rise we currently see occurring, which will affect regions where many millions of people live and work.
There is a lot of uncertainty about how sensitive the Antarctic ice sheet is to changes in climate, which means it is very hard to predict exactly how fast the ice will melt, and when different countries will be affected.
Huge ice sheets, like those on Greenland and Antarctica, evolve slowly over hundreds or thousands of years so they won’t disappear entirely in the next few decades. However, they hold so much water that even small changes in these ice sheets can cause significant sea-level rise. Their slow pace of adjustment means that we haven’t yet seen all the impacts of the warmer climate we already have. Even if we were to stop human-induced climate warming tomorrow, we have already locked in changes in the ice sheets that we cannot reverse and that will continue for hundreds of years.