Sea levels on Earth are rising several times faster than they have in the past 2,800 years and are accelerating because of man-made global warming, according to new studies.
An international team of scientists dug into two dozen locations across the globe to chart gently rising and falling seas over centuries and millennia.
Until the 1880s and the world’s industrialization, the fastest seas rose was about 3cm to 4cm a century, plus or minus a bit. During that time global sea level really did not get much higher or lower than 7.5cm above or below the 2,000-year average. However, in the 20th century the world’s seas rose 14 cm. Since 1993 the rate has soared to 30 cm per century.
STUDIES IN JOURNAL
Two different studies published on Monday in the journal Proceedings of the National Academy of Sciences said that by 2100 the world’s oceans will rise between 28 cm and 131cm, depending on how much heat-trapping gas Earth’s industries and vehicles expel.
“There’s no question that the 20th century is the fastest,” said Rutgers University earth and planetary sciences professor Bob Kopp, lead author of the study that looked back at sea levels over the past three millennia. “It’s because of the temperature increase in the 20th century which has been driven by fossil fuel use.”
To figure out past sea levels and rates of rise and fall, scientists engaged in a “geological detective story,” said Ben Horton, a Rutgers marine scientist and co-author of the study.
DETECTIVE WORK
They went around the world looking at salt marshes and other coastal locations and used different clues to figure out what the sea level was at different times, and used single cell organisms that are sensitive to salinity, mangroves, coral, sediments and other clues in cores, Horton said.
On top of that they checked their figures by easy markers such as the rise of lead with the start of the industrial age and isotopes only seen in the atomic age, he said.
When Kopp and colleagues charted the sea level rise over the centuries — they went back 3,000 years, but are not confident in the most distant 200 years — they saw Earth’s sea level was on a downward trend until the industrial age.
HUMAN PROBLEM
Sea level rise in the 20th century is mostly of human origin, the authors said.
A separate, not-yet-published study by Kopp and others found that since 1950, about two-thirds of the US nuisance coastal floods in 27 locales have the fingerprints of man-made warming.
If seas continue to rise, as projected, another 46cm of sea level rise is going to cause lots of problems and expense, especially with surge during storms, said Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany, one of the study’s co-authors.
“There is such a tight relationship between sea level and temperature,” Horton said. “I wish there wasn’t, then we wouldn’t be as worried.”
The link to temperature is basic science, the study’s authors said. Warm water expands. Cold water contracts. The scientists pointed to specific past eras when temperatures and sea rose and fell together.
The Kopp study and a separate one published by another team projected future sea level rise based on various techniques.
SAME-SAME
They came to the same general estimates, despite using different methods, said Anders Levermann, a co-author of the second paper and a researcher at the Potsdam Institute.
If greenhouse gas pollution continues at the current pace, both studies project increases of about 57cm to 131cm. If countries fulfill the treaty agreed upon last year in Paris and limit further warming, sea level rise would be in the 28cm to 56cm range.
Jonathan Overpeck at the University of Arizona, who was not part of the studies, praised them, saying they show a clear cause and effect between warming and sea level rise.
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