Sealevel Rise

Sea levels are currently rising and are expected to continue rising, possibly even at an accelerated rate, over the next century and beyond. It is not caused by one simple mechanism; it is the result of several processes, such as:

• the melting of glaciers and ice caps from continents (the melting of ice already in the water does not affect the sea level)

• thermal expansion of the oceans' upper layers

• the melting of the Antarctic and Greenland Ice Sheets

• redistribution of terrestrial water storage

• oceanographic factors, such as changes in ocean circulation or atmospheric pressure

• vertical land movement

The Intergovernmental Panel on Climate Change (IPCC), in its fourth report, issued in 2007, predicts that global mean sea level will continue to rise. The exact amount will not only depend on the above factors but will also be affected by the anthropogenic factor, specifically the emission of fossil fuels. If anthropogenic factors are not kept in check, melting and thermal expansion will increase, causing sea levels to rise further. The table on page 94 reflects the IPCC's projections of future sea-level rise.

The two main processes that contribute to sea-level rise are thermal expansion and the melting of glaciers, ice caps, and ice sheets. The more

global warmíng cycles

Sea-Level Change (inches/centimeters)

YEAR

LOW

MEDIUM

HIGH

2GSG

4/10

6/16

9/22

21GG

8/20

14/3S

2G/SG

Source: IPCC, 2007

heat water absorbs, the more volume it occupies; the density of seawater is determined by temperature. Thermal water expansion is one of the few aspects associated with global warming that can be calculated directly from basic physics. When ice melts, it is added to the ocean through surface melting and runoff, as well as ice directly flowing into the ocean.

According to the IPCC, one of the biggest uncertainties of future sea-level rise is how the large ice sheets of Antarctica and Greenland will behave as the atmosphere continues to warm. There is enough ice on Greenland and the western portion of Antarctica to raise sea levels by 40 feet (12 m) should it melt. Even if this amount of ice melted over a long period of time, it would impact millions of people who live along the world's coastlines. According to James E. Hansen of GISS, these ice sheets' potential collapse could be the biggest long-term risk that humans face from global warming. More than 100 million people live within 3.3 feet (1 m) of mean sea level. In fact, some island states exist entirely at that low elevation, and their existence is threatened by sea-level rise. Coastal wetlands are also endangered. Property destruction is a serious issue, and many countries that would be negatively impacted could not afford the economic impacts of the destruction caused by rising water levels.

If there were to be substantial melting of the Greenland or Antarctic ice cap, sea levels would increase for centuries. Examining proxy data in rock formations, geologists at USGS have identified ancient beaches that existed far above the present sea level, confirming these areas were formed during warmer climates of Earth's past. During the last major interglacial period 125,000 years ago, Earth's temperature was comparable to Earth's predicted temperature in the next few centuries due to global warming. Sea level 125,000 years ago was 20 feet (6 m) higher than it is today (during this time, Antarctica was still covered in ice). It has been estimated that if the ice on Greenland were to melt, present sea level would rise roughly 20 feet (6 m), destroying the world's coastal communities.

Another issue the IPCC brought up in its 2007 report concerned the concept of ice surges. Analysis of satellite radar data in 2006 determined that the velocities of large ice streams in southern Greenland had doubled over the past five years and then slowed again. These sporadic surges surprised many of the scientists studying them, leading them to believe that the surges were more sensitive to global warming than initially thought. In addition, gravitational force measurements collected by the satellite determined there were significant changes in the mass of the ice sheets on a yearly basis, indicating that both ice sheets were losing significant amounts of ice to the ocean. In its 2007 report, the IPCC for the first time acknowledged the serious possibility of ice surges and their role in potentially adding significantly to sea-level rise before the year 2100, as well as beyond.

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