Antarctica is a continent about one and a half times the size of the United States, which lies mostly south of the Antarctica Circle. About 98 percent of it is covered by a thick continental ice sheet thousands of feet thick, and immense glaciers form ice shelves along half of its coastline. Antarctica contains 90 percent of the ice on earth. The Arctic (the North Pole) is much smaller, but it, too, is crucial to the welfare of the planet. Most worrisome for us human inhabitants, both have shown signs that they are getting smaller because global warming is making them melt.
Some of the ice shelves in the northern part of the continent, known as the Antarctic Peninsula, have been collapsing over the last few years and warming of 4.5 degrees F has been recorded since 1945. The 770-square-mile Larsen A shelf collapsed suddenly in 1995, and in 1998 and 1999 two more ice shelves, with a combined area of 1,150 square miles, fell into the sea. Scientists are worried that if the large West Antarctica ice sheet disintegrates, it could raise the sea level worldwide by as much as 20 feet.
In the Arctic, a series of NASA studies in 2002 found that "perennial" sea ice (ice that remains all year round) is melting at a disturbing rate of 9 percent per decade. Ice also reflects the sun's light. So at both poles, the less ice there is, the less of the sun's rays are reflected, and global warming accelerates.
But scientists cannot agree on one important consequence of these global changes: whether the climate will get hotter or colder. At the moment our planet is in an interglacial period-the ice has retreated to the poles, so how could global warming lead to the next ice age?
The answer lies with the Gulf Stream, the current that takes warm water from the West Indies to the North Atlantic, without which Europe would be at least five degrees colder with bitter winters. The Gulf Stream is part of a larger system of currents called the North Atlantic Meridional Overturning Circulation (MOC), but the basic mechanism runs something like this: salty cold water in the Atlantic sinks because it is dense. As it sinks more water flows north to replace it. This thermohaline circulation (from the Greek words for heat and salt) is a crucial factor in the climate of the earth.
The problem with melting ice is that it is fresh water. When it flows into the Atlantic, it makes the ocean less salty and therefore less dense, so the flow would sink less rapidly, and the Gulf Stream would slow down. Samples of ice (ice cores) show evidence of dramatic climate changes in the past and that thermohaline circulation have been responsible.
One thing is certain: the climate is changing, and in ways that may not have been experienced in several million years.
