Saturday, October 29, 2016

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

By Bob Berwyn
Oct 27, 2016

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.


Total global ice melt since 1900 is about equal to the amount of water in Lake Superior, about 2 quadrillion gallons, which is enough to cover North and South America in a foot of water. It's pouring into the oceans at a steady, gradually increasing rate, but it's not as simple as filling a bathtub. Propelled by the spin of the Earth, shifting gravitational fields, seasonal changes and wind, all that water sloshes around in a pattern that satellites can see.


The researchers used data from NASA's Gravity Recovery and Climate Experiment, satellites that show how melting ice caps change the Earth's gravitational field. NASA scientists Surendra Adhikari developed a new climate model to blend ice, ocean, atmosphere and solid earth data to create a global picture of how ocean mass is redistributed due to ice melting.


While the regional variations may be small and subtle, they are important from an impact point of view, he said. "It's the changing of extremes we're worried about. Much smaller storm surges give same flooding with higher sea level."

Even without storm surges, places like Miami are currently experiencing so-called sunny day, or nuisance flooding, during high tides.

"A hundred years ago, there was no sunny day flooding in Miami. Twenty millimeters of sea level rise makes all the difference," he said.

In the past 15 years, sea level has been rising by about 0.11 inches per year, already double the average rate of the 20th century, and in some areas, it's much more than that. In the South China Sea and around Indonesia, the rate has been about 0.4 inches annually for the past two decades, projected to increase to three times that much by 2100, Haigh said.

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