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New research suggests that the world's oceans rose faster in the last 100 years than they did in nearly all of the preceding 3,000.
Between 1900 and 2000 seas swelled an average of 14 centimeters (5.5 inches) worldwide — a rate unprecedented in history. Using fossil records drawn from coastal sediment, a group of US and international scientists pegged the sea level rise to man-made climate change and concluded that rates of sea level rise are likely to accelerate.
"If you compare the 20th century to past centuries, there's a 95 percent probability that it's the fastest since 800 BC, the time of the founding of Rome," Robert Kopp, a Rutgers University climate scientist said.
Kopp, who led the ten researchers behind the study published in the Proceedings of the National Academy of Sciences, said that the data prior to Rome is too hazy to draw on with certainty, but that the forces behind the recent and rapid rise in oceans is clear.
Over the last century melting glaciers and the physical expansion of warming ocean waters have been the primary forces behind the spike in sea levels, which rose and fell much more gradually during the preceding millennia. Graphs of historic sea levels take the shape — notorious among climate watchers — of a hockey stick with a long roughly even period, "the handle," sharply curving up into a "blade" starting around 1850.
The graph of sea level rise hues closely to one published in 1999 by Penn State University climate researcher Michael Mann and two colleagues showing the rise in atmospheric temperatures. Kopp says the parallel "hockey sticks" of rising temperatures and oceans makes perfect sense and is well beyond scientific doubt.
"Basic physics tells you, if you have more greenhouse gasses in the air, you'll trap more heat on the planet, and if you trap more heat on the planet, the oceans will expand, glaciers will shrink, and you'll have sea level rise," said Kopp. "This is shown by a constellation of research that dates back to the 19th century."
Data from NASA shows sea levels are rising 3.4 millimeters each year — more than double what the study found to be the average between 1900 and 2000, suggesting a continued sharp acceleration. Modeling by Kopp and his colleagues suggests that this rate is likely to continue increasing even if global carbon emissions are cut. In their most optimistic low-emissions scenario, where global temperature rise is limited to no more than 2 degrees Celsius (3.6 Fahrenheit), the study predicts oceans will swell between 24 and 61 centimeters (0.7-2 feet) in the 21st century. The least optimistic scenario, where temperatures increase around 5 C, has oceans rising from 52 to 131 centimeters (1.7-4.3 feet).
These numbers are broadly mirrored by estimates put forward by several other peer reviewed studies and the Intergovernmental Panel on Climate Change. Kopp said they might prove too conservative as melt from the Antarctic and Greenland ice sheets flows into the ocean.
Higher sea levels mean more regular flooding for coastal communities. University of Miami climate researcher Shimon Wdowinski said that the eastern seaboard cities like New York and Boston are already getting a taste of what is to come, with seasonal high tides flooding greater amounts of coastal area. Miami is responding with construction of $400 million worth of new infrastructure to fortify coastal communities against flooding.
Climate change is expected to make extreme weather events like Hurricane Sandy, which caused havoc along the New Jersey shore and around New York City, more frequent and more severe. Both Wdowinski and Kopp said the risk of devastation like that wrought by the 2012 storm rises with the oceans.
"If you have higher sea levels, it takes less of anything else to cause flooding," said Kopp.
Follow Jake Bleiberg on Twitter: @jzbleiberg