Historical Perspective

During the last ice age, huge ice sheets covered parts of North America, northern Europe, and parts of Asia. Because so much of Earth's water supply was locked up in ice, global sea level was 394 feet (120 m) lower than it presently is. Once the ice sheets began to melt, sea level began to rise. During that process, there were intermittent intervals when sea levels rose in rapid spurts called "meltwater pulses." One such pulse occurred between 14,600 and 13,500 years ago, increasing sea levels by 52-79 feet (16-24 m). Water originated from both North America and Antarctica. Rates of melting and sea level rise then declined after that during the Younger Dryas cold period, followed by another surge 11,500 to 11,000 years ago, when sea level may have risen as much as 92 feet (28 m). Another meltwater pulse occurred 8,200 to 7,600 years ago. By the mid-Holocene Period (6,000-5,000 years ago), glacial ice age melting had run its course: In the following years, there was very little sea-level rise—until recently.

According to NASA, 20th-century sea-level trends are significantly higher than the trends of the past few thousand years. Based on the analysis of coastal sediments retrieved from several locations, a new phase of sea-level rise began in the mid to late 1800s and has continued. Based on readings taken from tidal gauges in coastal harbors, global sea level has been increasing about 0.07 inch (0.17-0.18 cm) per year, which scientists attribute to global warming, specifically the melting of the world's glaciers and the thermal expansion of water. Recent analysis of the TOPEX/Poseidon data indicates a trend of a 0.1-inch (0.3-cm) increase per year in global sea-level rise.

According to NASA, an area of concern based on recent satellite observations of the Greenland and West Antarctic Ice Sheets is a thinning at the lower elevations, which is causing glaciers to empty at a faster rate into the ocean. Researchers have determined that this alone has added 0.08-0.02 inch (0.02-0.06 cm) per year to the oceans within the last decade. If either ice sheet melted completely, it could raise sea level by 16-23 feet (5-7 m). If a global temperature increase of 3.3-8.3°F (2-5°C) occurred, it could destabilize Greenland enough that it would cause an irreversible melting and trigger that magnitude of a sea-level rise. This magnitude of temperature rise falls within those predicted in climate projections for the 21st century. NASA clarifies, however, that any significant meltdown would take many centuries and views it as highly unlikely that annual rates of sea-level rise would exceed those of previous postglacial meltwater pulses.

0 0

Post a comment