What is an ice sheet? How do ice sheets work? How are ice sheets formed?
The ice sheets in Antarctica and Greenland are the largest volumes of fresh water in the world. Collectively, ice sheets contain more than 100 times more water than all the world’s lakes.
Ice sheets form over thousands of years because the snow that falls on them in winter never quite melts in the summer. In this way, each year a more snow accumulates. Eventually, the pressure of snow is enough to cause the snow to become ice.
Ice sheets begin to work when there is so much ice that the gravitational pressures cause the ice to begin to flow. It’s hard to imagine something as hard as ice flowing, but over very long periods of time it is definitely happening. Ice flow rates might be as high as 30 meters in a year. Because the landscape under the ice has valleys and peaks, and ice flows downhill, ice sheets rapidly develop areas where ice is deforming quickly (in the valleys), but surrounded by much slower moving ice (on the peaks). The quickly moving areas are called ice streams, because they end up looking like rivers in the ice sheet. In addition to flowing, ice that is deforming rapidly can melt. This causes liquid water to lubricate the interface between the ice and the rock or gravel that is under it. When this happens, ice streams can flow as fast as 10 km per year!
What is the difference between ice sheets, ice shelves, and glaciers?
Ice sheets are described above. Ice shelves occur when the ice streams reach the ocean. When this happens, two things could happen. Either the ice could break off into the ocean, a process called ‘calving’ that results in icebergs floating away from the ice stream. The other possibility is that the ice can smoothly transition from resting on the bed and floating in the ocean. When that happens, ice spreads rapidly over the ocean until it reaches some obstacle that pushes back on the floating ice. In most cases the thing that pushes back is an island, or the walls of a bay. Unlike situations that result in calving, this tends to be a stable configuration, and an ice shelf can remain in place over many thousands of years. Calving still takes place, but now it occurs further out, where the ocean and the ice shelf meet.
The distinction between glaciers and ice streams is a little fuzzy. Glaciers are typically strongly controlled by topography, for instance, being confined in a valley, and following the valley downhill. Ice streams are less controlled by topography and emerge due to the tremendous amount of ice being drained through them. Image you are watching water from waves run up onto a beach. Perhaps you’ve noticed that when the water runs back to the ocean, it forms channels, even though the whole beach is essentially flat in the direction perpendicular to the ocean. That would be like an ice stream. A glacier would be like water running down a gully, where all the water up on the slopes gets concentrated into flow down the bottom. It’s also confusing because the ice streams are also topographically controlled, but there is not clear threshold for calling one thing an ice stream and another a glacier.
What are the factors that influence a change in ice sheets?
An ice sheet can ‘feel’ the environment in several ways. First, the rate snow falls and melts on the ice sheet is called its mass balance. This means that if the snow were to stop, eventually all the ice would flow to the sea, and the ice sheet would be no more. This would take a very long time; hundreds of thousands of years.
Ice sheets feel air temperature in two ways. The first is by changing the amount of snow that melts on ice sheets in the summer. The second is by slowly warming the ice itself. Ice flows much better as it warms. However, ice is a good insulator, and it takes a long time for the interior of an ice sheet to feel a warmer temperature. If the temperature is very high, like it today in Greenland, ice on the surface of an ice sheet can melt. When this happens, very odd things can happen because the liquid water tunnels its way through the ice.
Finally, ice can feel the ocean at the point where the two meet. This is very important because a warmer ocean can melt the underside of ice shelves, potentially causing them to disintegrate.
How do the concepts of temperature, albedo, and surface elevation relate to ice sheets?
Temperature dealt with in the previous question. Albedo is high for an ice sheet, reflecting light back to space. Surface elevation matters as the ice sheet grows really large it produces its own climate because high altitudes are cold, and have less precipitation. In fact, very high regions of Antarctica have so little precipitation that the ice sublimes, or turns to water vapor, faster than snow falls, so paradoxically, the ice is getting thinner in the places where it is thickest. This is a very slow process, and is not significant in the overall mass balance.
How do viscosity, turbulent flow and laminar flow relate to ice sheets?
Ice flow is always laminar. The viscosity of ice is very temperature dependent, but is also dependent on the stresses in the ice itself.
How the concepts of conservation of mass, energy, and momentum relate to ice sheets?
Our understanding of ice flow is based entirely on how laws of conservation apply to ice dynamics. Let us consider each, and how it fits with our intuition
- Conservation of mass- In ice sheets matter is neither created nor destroyed. Mass is gained from accumulation of snowfall and the freezing of rain water. Mass is lost through the calving of icebergs into the sea, the summer melt of ice near the surface, and melting of the lower ice surface due to strain and frictional heating, or the transport of warm ocean water to the underside of floating ice shelves. The formal mathematical statement of the conservation of mass provides just enough additional information to solve the equations for the next conservation law, conservation of momentum.
- Conservation of momentum-Perhaps this is more easily understood as the sum of the forces being zero, or Newton's second law of motion with no change in momentum. We assume that ice is always in equilibrium and that the forces are zero. In ice, there is one large force arising from gravity and the slope of the ice surface. This force must be balanced by the forces in the ice arising from the traction at the bed, the drag along the sides of the glacier, and forces that may arise from ice downstream, which may push (or pull) on the ice. One interesting consequence of this conservation law is that all that is needed to compute the ice velocity is the ice geometry and temperature. One need not know how fast the ice was moving in the past.
- Conservation of energy- Energy is neither created nor destroyed, but flows through an ice sheet. Energy comes into the ice from the sun, and from the heat that flows up from the Earth's crust. As this heat flows through the ice sheet, the ice becomes warm enough to deform, and additional heat is produced by the deformation. A small additional source of heat arises from the ice sliding over the ground beneath it. When all of these heat sources, and subsequent flows are accounted for, the temperature everywhere in the ice sheet can be determined. This temperature can make very significant differences in ice flow, and is critical for modeling.