Melting Ice Raised Sea Levels More Than Previously Thought, Study Says

Relying on tidal gauges in the Northern Hemisphere skewed previous estimates over the 20th century, leaving the world's coasts more vulnerable than expected.

Readings from coastal tide gauges around the world—the most reliable historical water-level records—have underestimated 20th century sea level rise caused by various melting ice caps and glaciers in the Northern Hemisphere by between 5 and 28 percent, said a new study published in the journal Geographical Research Letters.

Using historical tide gauge observations as well as climate models, the researchers found that the least amount of global sea level rise that could have occurred last century is about 5.5 inches.

"The most likely amount," the study concluded, "is closer to 6.7 inches," with implications for the hundreds of millions of people who live along the world's coasts.

The readings come mainly from 15 gauges in North America and Europe, where sea level rise has likely been slower than the global average, skewing earlier estimates.  The study shows that melting ice raises sea level faster than the global average in areas farthest from the melt sources, like the southern Pacific Ocean and equatorial regions.

Sea level rise due to the Greenland ice melt, for instance, has been underestimated by 28 percent, the study said, while the sea level drop from the melting Alps was underestimated by 5 percent.

"If you want to understand the future, you have to understand how much sea level rise was caused by past warming," said lead author Philip Thompson, associate director of the University of Hawaii Sea Level Center.

The study is also important because it helps identify regional patterns of sea level rise, information that can help communities adapt to rising water in coming decades.

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