Seas Rose Faster in 20th Century Than in Previous 27 Centuries

Man Wades Through Flooded Street
A man walks a flooded Atlantic City, New Jersey, street. Researchers found that as temperatures rose in the 20th century, so did sea levels. Ocean levels rose faster during the 1900s than in the previous 27 centuries. They predict more floods for the East Coast as temperatures continue to heat up. Tom Mihalek/Reuters

Had it not been for climate change, global sea levels would have risen by less than half the amount they did in the 20th century—and may even have fallen. Instead, the seas rose faster during those 100 years than in any of the previous 27 centuries, according to a Rutgers University–led study published in Proceedings of the National Academy of Sciences on Monday. And as oceans continue to rise, we can expect more flooding on the U.S. East Coast, researchers warned.

Global sea levels rose by about 5.5 inches from 1900 to 2000, the study found. Around the world, average temperatures have risen nearly 2 degrees Fahrenheit since the 1800s. Last year marked the hottest year on record—surpassing 2014, now the second hottest. Without global warming, the team led by Robert Kopp estimates ocean levels would have risen by only 2.75 inches during the 20th century, if at all. (From 1000 to 1400, when the planet cooled by about .4 degrees Fahrenheit, ocean levels took about a 3-inch dip.) Looking ahead, we can expect sea levels to rise another 1.7 to 4.3 feet in the 21st century as global temperatures continue to increase due to climate change, according to Kopp, an associate professor in Rutgers University's Department of Earth and Planetary Sciences.

Climate change can cause sea-level rise in two ways, reports the Union of Concerned Scientists, a Massachusetts-based nonprofit science advocacy group. First, as the global temperature rises, so do ocean temperatures. That in turn causes seawater to expand, just the opposite of how it contracts when frozen. When the warmer water expands, it finds itself trapped within a basin bounded by the continents, with nowhere to go but up. Another factor adding water to the oceans is melting land ice. When glaciers or polar ice sheets melt, water is released into the oceans.

It's difficult to arrive at global sea-level rise when data is recorded at particular locations. For this study, researchers devised a new statistical approach, allowing them to extrapolate global significance from regional records. With collaborators at Tufts University, the team worked with a data set of geological sea-level indicators from marshes, coral atolls and archaeological sites that spanned the last 3,000 years and represented 24 locations around the world. The analysis also tapped 66 tide-gauge records from the last 300 years. It was the largest and most detailed data set on sea levels yet to be analyzed in this way.

"As geologists, we can reconstruct how sea level changed at a particular site, and progress in the last 10 years has allowed us to do so with ever more detail and resolution," says Andrew Kemp, an assistant professor of earth and ocean sciences at Tufts University. "Gathering together and standardizing these reconstructions gave us a chance to look at what they had in common and where they differed, both of which can tell us about the causes of past, present and future sea-level change."

To calculate the likely sea-level change in a scenario with no global warming, Germany's Potsdam Institute for Climate Impact Research used the study's global sea-level reconstruction to estimate how temperatures relate to the rate of sea-level change. Based on this relationship, the study found that, without global warming, 20th century global sea-level change would have been somewhere between a decrease of 1.2 inches and a rise of 2.8 inches. The study also found that it's very likely global sea level 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 will reduce that to between 0.8 and 2.0 feet.

A companion report published by Climate Central—an independent organization of scientists and journalists reporting on climate science—found that more than half of the 8,000 coastal nuisance floods (those that lead to public inconveniences such as road closures) recorded since 1950 would not have occurred without sea-level rise due to global warming.

Seas Rose Faster in 20th Century Than in Previous 27 Centuries | Tech & Science