The Ice Sheets—A Latent Threat
During most of the 20th century, global sea level has been steadily creeping upward at a rate of ~0.07 in per year (1.7 to 1.8 mm/yr), increasing to nearly 0.12 in per year (3 mm/yr) within just the last decade. Most of this rise in sea level comes from warming of the world’s oceans and melting of mountain glaciers, which have receded dramatically in many places especially within the last few decades of the 20th century. According to a 2001 report by the Intergovernmental Panel on Climate Change, global warming of 2.5° to 10.4° F (1.4°--5.8° C) could lead to a sea level rise of 4 inches to 2.9 feet (0.09-0.88 meters) by 2100.However, recent trends from Greenland and the West Antarctic ice sheet are potentially more worrisome. Satellites detect a thinning of parts of the Greenland Ice Sheet at lower elevations, and glaciers are disgorging ice into the ocean more rapidly, adding 0.01-0.02 in/yr (0.23 to 0.57 mm/yr) to the sea within the last decade. The West Antarctic Ice Sheet is also showing some signs of thinning. Global warming could cause further thinning of these ice sheets. Either ice sheet, if melted completely, contains enough ice to raise sea level by around 23 ft (7 m). Some scientists suggest that a global temperature rise of only 5.4° F (3° C) may be enough to destabilize Greenland irreversibly. Although such a temperature rise lies within the range of several future climate predictions for the 21st century, the meltdown would probably occur over multiple centuries.