Global sea levels rise and fall depending on the volume of the ocean basins versus that of the water in them. Changes in the volume of ocean basins occur over millions of years and are not directly responsive to climate, whereas changes in water volume may occur relatively rapidly and depend on global temperatures. Temperature directly changes the volume of water in the oceans. When ice melts to a liquid at 0°C, water decreases in volume by 9% (which is why icebergs float with about 9% of their volume above the surface), reaching a minimum at 4°C and then expands with temperature.
Continental ice sheets sequester water that would otherwise be found in the oceans. In particular, the ice sheets of Antarctica and Greenland contain, respectively, about 2% and 0.2% of the water on Earth, and their accretion or attrition alters sea levels. Warmer global temperatures and the melting of glaciers over the past century have increased the global volume of water in the oceans. These factors potentially could raise sea levels roughly 0.7 millimeters per year (mm y–1). Two independent research approaches, however, indicate that sea levels are rising at more than twice this rate.
One approach involves tide gauges, instruments usually located on piers that monitor the height of the sea relative to a nearby geological marker. Periodic surveys determine whether the pier has shifted or whether the elevation of land in the area has changed from natural forces, such as tectonic movements, or anthropogenic ones, such as groundwater pumping and mining. Extensive and reliable data from tide gauges are available worldwide since 1870 and show that sea levels have risen an average of 1.7 mm y–1 over the past century and 3.0 mm y–1 over the past decade.
The other approach involves the satellites TOPEX/Poseidon (in operation from 1993 through 2005) and Jason-1 (in operation from 2001 through the present) that were specially designed for monitoring sea levels. These satellites follow a path that transects the globe uniformly from 66°S to 66°N every ten days. They bounce microwaves off the oceans and monitor the time it takes between sending and receiving the microwaves (i.e., their time of flight) to determine their distance from the surface. Global Positioning System (GPS) receivers on the satellites, along with laser range finders, establish their orbital height. Sea level is taken as the difference between the orbital height of a satellite and its distance from the sea surface, with an error of less than 1 centimeter. The satellite data show that average sea level has risen 2.9 mm y–1 over the past decade.
Why do theory and practice differ? In this case, the theory predicts that changes in the volume of ocean water should produce a sea level rise of only 0.7 mm y–1, whereas both tide gauges and satellites indicate an average rate of greater than 1.7 mm y–1. Most likely, the Antarctic and Greenland ice sheets are melting faster than previously estimated.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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