How Things Work: The Ice Cycle
by William Nicoll
Imagine a hot summer day in July. You?re sitting on the porch boiling in your own sweat when you decide nothing could be better than a cool, refreshing glass of ice water. You fill a glass with water and drop in a few ice cubes. You?re about to take that first glorious sip when ? freeze! ? you find yourself at the bottom of the world: Antarctica. Sure, it?s July, but July in the Southern Hemisphere lies in the depths of winter, where the average daily temperature is ?74?F and 24-hour darkness prevails. You quickly succumb to frostbite atop a windblown ice sheet. The prospects are chilling.
Now that you have an appreciation for ice, here?s some information on the ice cycle. The ice cycle can take place in any place where permanent snow or ice cover exists; these regions are known as cryospheres. Two obvious cryospheres exist in the extreme northern and southern latitudes. Permanent snow and ice cover exist near the poles for two main reasons: One, the intensity of solar energy, or solar flux, decreases near the poles. At the equator, energy from the sun strikes perpendicular to the surface of the Earth. This maximizes the sun?s energy over a given area. At the poles, solar rays travel almost parallel to the earth?s surface, and this minimizes the energy striking the surface. The existence of a cryosphere is also maintained by the fact that existing snow cover reflects much of the solar radiation back into space. Since little of the sun?s energy is being absorbed, the melting process is slowed.
Along with an average temperature below freezing, these conditions allow for a sustainable ice cycle to develop. Ice cycles can exist on two macroscopic scales: mountain glaciation and, on a larger scale, ice sheets. The dynamics of ice sheets are especially interesting, and will be the topic of further discussion.
Ice sheets didn?t just pop into existence overnight, rather they have been formed over millions of years. Today three ice sheets exist: one in Greenland and two in Antarctica. About three million years ago, our planet entered a period of cyclical cooling which continues today. What most people refer to as the last ?Ice Age? occurred 20,000 years ago when the last cycle completed. The accompanying climate changes of this long-term cooling allowed for the formation of the stable ice sheets we currently see.
For their gargantuan size, ice sheets have relatively benign beginnings; all it takes to create a sheet is snowfall. If winter snowfall survives from season to season, the snowflakes begin to compact. As successive snowfalls leave their remains atop the pack, deeper layers form coarse grains of ice. If enough snowfall accumulates, the pressure will cause these grains of ice to expand, filling tiny pockets of air and growing as large as footballs. True glacial ice contains less than two percent air in the form of tiny bubbles and takes about 1000 years to form. Scientists can analyze the chemical composition of such trapped air bubbles to measure climate changes over the existence of the ice sheet.
With three million years to accumulate, today?s ice sheets have grown to incredible thicknesses ? up to 4.5 km thick in some places. That is the equivalent of a building 28 times as high as the Cathedral of Learning! The weight of all this ice is so immense that it compresses the underlying landmass, so that the ice sheets exist in bowl-like depressions.
Depending on the surrounding geography, ice sheets can form ice cycles of two types: land-based or marine-based. Land-based ice cycles form relatively static ice sheets. Trapped in their bowl-like depressions, these ice sheets lose mass, or ablate, through narrow glacial channels. Ice moves within these ice sheets primarily through a process called internal deformation, in which gravity exerts a settling force. Imagine leaving a ball of uncooked dough on a table top. If left long enough, it would settle into a pancake shape. The same occurs in ice sheets, and over the course of a year an ice sheet might expand 10 feet outward.
Conversely, marine-based ice sheets spill out into the ocean over large areas. The West Antarctic Ice Sheet is an example of a marine-based ice cycle. These systems are considerably more dynamic than land-based cycles. When the ice sheet reaches the coastline, it spreads out over the water and forms what is called an ice shelf. Ice shelves anchor themselves on the rocky coastline and are continuously regenerated from the ice streams.
These streams are fast-moving conduits of ice that are carried by a thin layer of water and sediment at the bottom of the sheet. Because the pressure under the sheet is so great, ice can undergo a phase change and melt. This forms a thin layer of liquid water and sediment that facilitates the movement of the ice above it in a process called basal sliding. With the help of this process, ice streams move out to sea at up to two miles per year. Once the ice reaches the edge of the shelf, it can break off, or calve, to form icebergs. The icebergs float out to sea and eventually melt during the warm season.
To summarize the ice cycle, one can begin with snow accumulation. This accumulation eventually forms and continues to regenerate large continental ice sheets. Several internal mechanisms move ice outward from the ice sheet, eventually calving into the sea from smaller glacial channels or expansive ice shelves. Later these icebergs melt into the ocean in warmer seasons. Ocean water evaporates and becomes precipitation, repeating the cycle.
For the ice cycle to repeat, accumulation must be greater than or equal to ablation. Recent concerns over the effects of global warming on this cycle have raised fears of entire ice sheets? melting, raising the ocean levels by considerable amounts and flooding many of the population centers of the world. At present there is no conclusive confirmation for this theory, so don?t go running for the highlands quite yet. But the next time you toss a few ice cubes into your drink, be thankful you aren?t under 200 feet of water.