Hobbies And Interests
Formation of Ice Sheets
Ice sheets form when the mass of snow gain per annum in an area consistently outpaces the volume of snow lost during the summer thaw through melt and evaporation. This cycle is called the mass balance of a glacier. The snow which survives the summer melt becomes denser and sintered, and is referred to as firn. Glacier accumulation takes place over tens of thousands of years. As this procedure continues on a yearly basis, the snow becomes buried deeper and deeper, and thus more tightly packed and dense. The older snow is pushed deeper and outward because of the overlying weight of the snow and firn. As such, ice sheets are thicker with higher elevation towards the center and lower elevations toward the edges.
Forcing Functions
The cycle between glacial and interglacial periods, and the accumulation of ice sheets in the northern hemisphere, is thought to be tied to the Milankovich cycles of summer insulation. This asserts that the length and intensity of the summer months, and thus the ability of snow to survive the summer months, is tied to a number of characteristics of the Earth's orbit in relation to the Sun: the obliquity, also known as the axial tilt, which is the angle between an object's rotational axis and a line perpendicular to its orbital plane; the precession, which refers to a change in the Earth's rotational axis; and the eccentricity, which is the amount the Earth's orbit deviates from a perfect circle. These factors, relating to the tilt of the earth towards the Sun and the maximum and minimum orbital distances from the Sun, effect the trend of summer temperatures and thus effect the growth of ice sheets.
Examples of Ice Sheets
Today, we live in an interglacial period and, as such, many of our glaciers are much reduced compared to, say, 8,000 years ago when the Last Glacial Maxima occurred. However, there are some examples of extensive ice sheets. For instance, the Antarctic and Greenland ice sheets, the two largest, together cover about 10 percent of the Earth's surface area and contain 77 percent of the world's fresh water. Both have an average thickness of approximately 6,900 feet, and the Antarctic ice sheet has an area of about 5 square miles by 41 square miles, around 10 times the ice volume of the Greenland ice sheet.
Ice Sheets in the Past
During the past 2.6 million years or so, the Earth has experienced numerous periods of glaciation, when the volume of fresh water locked up in ice was much higher than it is today. While the exact volume of ice locked up during the last ice age is hotly debated by geologists, geophysicists and glaciologists, it is thought that the global sea level was at least 410 feet lower than it is today than was about 19,000 years ago. Evidence indicates that the northern ice sheets spread south to cover the regions where New York, Berlin and London now stand. In the past, the southern hemisphere was also home to the Patagonian Ice sheet, but it has shrunk to below ice sheet status during the current interglacial period.