The definition of an ice sheet is, as you might imagine, a "mass of ice." To meet the criteria of a "sheet," however, the mass must extend more than 20,000 square miles.
That's a lot of ice. Today, glacial ice covers 10 percent of Earth’s land mass. Sixty-eight percent of the world's freshwater is bound up in glaciers. And that's a lot of water. Since we depend on water for our very existence, those ice sheets act as a kind of bank of our future survival.
Ice sheets are formed over thousands of years. As snow falls, melts and freezes, the sheet gains mass and compresses like a cake of many layers.
Glaciers also move under the force of their own weight. They creep and flow downhill like slow rivers. Not much can stand in the way of an ice sheet. Not forests. Not mountains.
Ice sheets can be so heavy that they force the land to buckle beneath them and become concave, like a bowl. The physical geography of the planet has been shaped, in large part, by ice.
Through a process called "compression melting," they ooze, grind and pirouette over the terrain on their own meltwater. Ice sheets behave more like a liquid than a solid, so much so that geologists call this attribute "plasticity.”
During the last Ice Age about 18,000 years ago, ice sheets covered most of North America, Asia and Europe. Today, we are in what climatologists call an “interglacial,” the relatively warm lull between times of freezing temperatures.
Currently, we have only two ice sheets: the Antarctic ice sheet and the Greenland ice sheet. Scientists study ice cores from these sheets to learn about global climate history, and global climate change.
The Greenland ice sheet was in the news in 2015 for a very bad reason: It's melting. This is to be expected in a warming world, but the Greenland ice sheet—which is on average 1 mile thick and three times the size of Texas—has been melting dramatically. Greenland currently contributes twice as much as the Antarctic to rising sea levels.
This is due to multiple factors, including lower than expected snowfall, higher land and ocean temperatures and even rain, which bores pits into ice sheets. These pits grow into lakes of blue water on the ice sheet’s surface. Because they’re darker than the surrounding reflective white ice, glacial lakes speed melting by absorbing the sun’s rays and its heat.
If the Greenland ice sheet were to completely melt, it would raise world sea levels by more than 20 feet. That's enough to put coastal cities around the world underwater. It would shift global ocean currents and change weather patterns worldwide.
Global sea level rise is one of the major environmental challenges of the 21st century. To gather data—and to better understand what's happening in Greenland and how it will affect us—NASA launched a project called Oceans Melting Greenland. Its acronym is OMG.
However, this science is critical and serious. We Marylanders will be changed by a rising ocean.
Over the next four years, OMG will deploy climatologists, radar, sonar, aircraft and over 200 robot sensors to gather field data on the Greenland ice sheet and report back. And depending on how we choose to interpret it, our future will be shaped by that data. OMG!
To learn more about ice and glaciers and how they have formed our landscape, visit aqua.org/ablueview.