Introduction

Approximately 10% of the Earth's surface is covered with ice today, most of which is in the East and West Antarctic ice sheets. The present area of the Antarctic ice sheets is approximately 11.5 million square kilometers, its volume is about 26 million cubic kilometers, and its maximum thickness is about 4 km. These Antarctic ice sheets contain more than 90% of the Earth's ice and about 75% of the Earth's freshwater. Despite the coldness of the Antarctic climate, large lakes (e.g., Lake Vostok) have recently been discovered underneath the Antarctic ice. The other main ice accumulation is on Greenland, with an area of about 1.7 million square kilometers, a volume of about 2.7 million cubic kilometers, and a maximum thickness of about 3.4 km. The Greenland ice sheet contains about 7% of the Earth's freshwater. The remainder of the ice on the Earth, only 3% by area, is in small ice caps and valley glaciers in mountainous regions. In addition to the areas of permanent ice, about 20% of the Earth's surface is affected by permafrost, i.e., permanently frozen.

Permanent ice requires abundant precipitation as snow and temperatures low enough to permit the snow to accumulate. These conditions occur at high latitudes and/or high altitudes. The lower limit of permanent snow and ice (the snowline) is at sea level in polar regions, but is at an elevation of 3 km in the temperate Alps, and at 6 km in the equatorial Andes.