BARCELONA - Global warming caused by humans was almost certainly responsible for more than half the increase in sea levels in the 20th century, when oceans rose at a faster rate than in the preceding 27 centuries, scientists have found.
Worldwide, seas rose by about 14 centimetres (5.5 inches) from 1900 to 2000, according to a study led by Rutgers University in the United States. But without global warming, it would have risen by less than half of that, and may even have fallen.
"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, the study's lead author and an associate professor at Rutgers' Department of Earth and Planetary Sciences.
In contrast, the study, published in the Proceedings of the National Academy of Sciences, found that global sea level declined by about 8 centimetres from 1000 to 1400, a period when the planet cooled by about 0.2 degree Celsius (0.4 degrees Farenheit).
Today, global average temperatures are around 1 degree Celsius higher than in pre-industrial times, in the late 19th century.
The researchers compiled a new database of geological sea-level indicators from marshes, coral atolls and archaeological sites in 24 locations around the world, spanning close to 3,000 years. They also used 66 tide-gauge records from the last 300 years.
A separate report, drawing on the Rutgers-led study, found that, of some 8,700 days of "nuisance" flooding identified from 1950 through 2014 at 27 sites in the United States, only one third would have happened without the sea-level rise induced by global warming.
Nuisance flooding is flooding that closes coastal area roads, overwhelms storm drains and compromises infrastructure.
"There are human fingerprints on thousands of U.S. coastal floods — and countless more the world over," Benjamin Straus, vice president for sea level with independent science group Climate Central, wrote in a commentary on his team's work.
AVOIDING COSTLY DAMAGE
In other related research, led by the Potsdam Institute for Climate Impact Research (PIK), scientists predicted that sea-levels worldwide would likely rise 50 to 130 cm (20 to 51 inches) by the end of this century if planet-warming emissions are not reduced rapidly.
“With all the greenhouse gases we (have) already emitted, we cannot stop the seas from rising altogether, but we can substantially limit the rate of the rise by ending the use of fossil fuels,” said PIK researcher Anders Levermann, who co-authored the study.
If governments put in place ambitious policies following December's agreement on a new global deal to curb climate change, sea levels would rise by less - an increase of 20 to 60 cm (8 to 24 inches) by 2100, the study projected.
“This is quite a challenge, but less expensive than adaptation to unabated sea-level rise which in some regions is impossible”, Levermann said.
“If the world wants to avoid the greatest losses and damages, it now has to rapidly follow the path laid out by the U.N. climate summit in Paris a few weeks ago.”
The PIK scientists said they would make their computer simulation code available online so other experts could use the information and the estimation tool in their risk assessments.
“We try to give coastal planners what they need for adaptation planning, be it building dikes, designing insurance schemes for flooding, or mapping long-term settlement retreat,” Levermann added.
(Reporting by Megan Rowling; editing by Laurie Goering. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, climate change, women's rights and trafficking. Visit http://news.trust.org)