Floods may become more frequent and more damaging. click to enlarge image   Extreme tides caused this 1997 Maryland flood. Source: NOAA Drains and sewer systems can become stressed with overflow of seawater. If these systems fail, disease outbreaks become more likely. People who don't have safe drinking water or sanitation systems are at even higher risk of illness. Food shortages could occur as the breeding cycles of fish are disrupted. Farms may become flooded with saltwater, which eventually destroys crops. Many coastal towns were early settlements, so their historic buildings, museums and archives, monuments, and infrastructure could be threatened.

Sea level is not fixed in one place. Oceans have risen and fallen since the formation of Earth. Twenty thousand years ago, the planet was shivering through the last ice age. Glaciers covered huge chunks of land, and enormous amounts of seawater froze. Sea level then was about 100 meters (300 feet) lower than it is today. When things finally warmed up around 12,000 years ago, the ice melted and sea levels rose.

In fact, they're still rising. Researchers estimate that the ocean is now 15 centimeters (6 inches) higher than it was in 1900. The crust of the Earth is always changing, and continents are able to move up and down. So measuring sea level is tricky. It's hard to tell whether the ocean is higher or the land is lower.

This 1921 “tide house” measured water levels on the coast of southeastern Alaska. Source: NOAA

Fortunately, with satellite-based global positioning systems, scientists can gather much more precise data. Unfortunately, this data suggests that sea level is climbing four times faster than in 1900. If this continues, by 2030 the world's oceans will be about 20 centimeters (8 inches) higher.

• Higher temperatures have caused glaciers and polar ice caps to melt more than usual, and the melting water feeds into the ocean. There has been an estimated 10 percent decrease in snow cover since the 1960s. Arctic sea ice thickness (in summer) has decreased by 40 percent in recent decades.

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Source: United Nations Environment Programme

• Water, like other liquids, expands as it gets hotter. This is called thermal
  expansion. So warmer oceans increase in volume.

Even if melting ice were not adding more water to the oceans, thermal expansion alone would still be enough to make sea levels rise. The two changes together yield higher and higher sea levels. Scientists studying global warming predict that sea level could rise 88 centimeters (35 inches) by 2100.

Technology—and money—often help wealthy nations protect their harbors and seaports. But that's not a realistic option in many developing countries. Some people don't have any choice but to uproot themselves and move to higher ground, where they also could face competition for housing, food, jobs, and resources.

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Sea level increases could devastate Bangladesh, one of the most crowded nations on Earth. Source: United Nations Environment Programme

Those who are forced to move into crowded, unsanitary refugee camps or heavily populated areas with inadequate sanitation run the risk of contracting cholera and other infectious diseases.

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