Global sea level rose faster in the 20th Century than in any of 27 previous

© Daniel Munoz
A group of scientist at Rutgers University pooled data from two dozen locations around the world and found sea levels rose several times faster in the 20th Century than in the past 2,800 years, speculating it is due to global warming, a new paper says.

The Rutgers University-led study found that global sea levels rose by about 5.5 inches (14 centimeters) in the 20th century, but the rate has soared to a foot per century (30 centimeters) since 1993. Without global warming, global sea levels would have risen by less than half the observed increase and might even have fallen, according to the study.

To figure out past sea levels and the rate of their rise, scientists conducted a “geological detective story,” according to Rutgers’ marine scientist Ben Horton, who co-authored the paper.

Horton said the international group of scientists combined studies on salt marshes, coral atolls, and archaeological sites that considered periods spanning three millennia to figure out the sea levels at different times. They also studied the reaction of single cell organisms that are sensitive to salinity to differences in sea levels, as well as mangroves, coral, and other clues in sediment cores. Using this data, they checked rates of rise against markers such as the rise of lead with the start of the industrial age and isotopes only seen in the atomic age.

The scientists said they have calculated with greater than 95 percent certainty that at least half, or more than 5 inches, of the sea level rise they detected during the 20th century had been directly caused by global warming.

“The 20th-century rise was extraordinary in the context of the last three millennia, and the rise over the last two decades has been even faster,” said Robert Kopp, an associated professor at Rutgers’ Department of Earth and Planetary Sciences and the study’s lead author, in a statement.

In a 2010 paper published by Southampton University, scientist estimated that the last time sea-levels had risen significantly was at the end of the last Ice Age, when water rose at a rate of 3.2 feet (1 meter) per century, interrupted by rapid “jumps” during which it rose by up to 8.2 feet (2.5 meters) per century. The findings were published in Global and Planetary Change.

The paper said that global sea levels rose by a total of more than 120 meters as the vast ice sheets of the last Ice Age melted. This melt-back lasted from about 19,000 to about 6,000 years ago, when the average rate of sea-level rise was roughly 3.2 feet (1 meter) per century.

In the new study, which limited its analysis to the three millennia up until the 1880s when the world was fully industrialized, the fastest rise in sea level was approximately 1 to 1.5 inches (3 to 4 centimeters) per century. Another interesting find was that global sea levels declined by about 3 inches (8 centimeters) from 1000 to 1400, a period when the planet cooled by about 0.4 degrees Fahrenheit (0.2 degrees Celsius).

“It is striking that we see this sea-level change associated with this slight global cooling,” Kopp said in a Rutgers’ statement. By comparison, the global average temperature today is about 1.8 degrees Fahrenheit [1 degree Celsius] higher than it was in the late 19th century.

Kopp’s study estimated that it’s very likely that global sea levels will rise by 1.7 to 4.3 feet in the 21st Century if the world continues to rely heavily upon fossil fuels. Phasing out fossil fuels could reduce levels to 0.8 to 2.0 feet, but that’s still an alarming estimate with more than one billion people living along shorelines around the world, and $11 trillion in assets sitting below the 100-year flood mark on the coasts.

The paper entitled Temperature-driven global sea-level variability in the Common Era was published on Monday in the Proceedings of the National Academy of Sciences.

A companion report published by Climate Central on Monday found that more than half of the 8,000 coastal floods observed at US tide gauge sites since 1950 would not have occurred if not for the global warming-induced component of sea-level rise.