Sea level isn't, well, level. Nor is the rate by which sea level has been rising over the past few decades, but the trend is clearly up.
Global sea level has risen an average of three millimeters (0.1 inch) per year since 1993. Rising seas have the potential to affect billions of people around the globe, not just those living near coastlines.
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Researchers say that
about half of the rise in global sea level since 1993 is due to thermal expansion of the ocean and
about half to melting ice. As Earth warms, these proportions are likely to change with dramatic results.
"More heat is coming into Earth's atmosphere than is going out," says Dr. Lee-Lueng Fu, project scientist for the Jason mission. "Over the past 40 years, the ocean has absorbed 84 percent of this excess heat--enough heat to warm the entire atmosphere by 27 degrees Celsius (49 degrees Fahrenheit)." The ocean has been able to absorb this heat by mixing warm surface water with much colder water from its depths, he explains. "The question is how long can it continue to do this."
Add more heat to the oceans, already Earth's largest storehouse of solar radiation, not only does global sea level rise due to thermal expansion but circulation patterns could change and affect the ocean's ability to store more heat in the future.
Excess heat that doesn't go into the ocean has to go somewhere. If it's melting ice, the effect on sea level will be immense.
Melting, not warming, has the biggest potential to raise sea level. "If you warm up the ocean, it will rise perhaps half a meter (1.6 feet)," says JPL researcher Dr. Eric Rignot. "If you melt land ice, you could raise sea level by 70 meters (230 feet). The real concern over the long term is the ice sheets in Greenland and Antarctica. With thermal expansion, the coastlines erode; with the ice sheets melting completely, you are talking about cities and states under water."