As sea level rises, satellite helps map the changes along coastline
The next generation of sea- level monitoring satellites is now in space, building upon nearly 30 years of data that has helped researchers measure the accelerating rate at which the ocean is rising and threatening coastal communities.
“These satellites measure us, literally, changing the shape of two- thirds of our planet,” said Josh Willis, lead NASA scientist for the mission that launched Saturday.
The Sentinel- 6 Michael Freilich satellite, a joint U. S.- European mission, launched atop a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California at 11:17 a. m. Saturday. It is the fifth satellite since 1992 to follow a carefully selected orbit ideal for measuring sea level.
More than 2 billion people live near the coast. As the ocean gets warmer — it absorbs more than 90 percent of the heat trapped by greenhouse gases — its molecules take up more space, and water begins encroaching on homes and businesses.
But this accounts for only onethird of the reason why water is rising. The other two- thirds come from melting glaciers and ice sheets ( glacial ice that covers large landmasses, found in Antarctica and Greenland, is called
an ice sheet).
Globally the ocean is rising between 4 and 5 millimeters a year — double from about 2 millimeters a year in the 1990s, Willis said. And while millimeters might not sound significant, it’s enough to erode beaches and drown wetlands along the Texas coast.
“Our coast is changing very rapidly. This is not a prediction,” said John Anderson, the retired Maurice Ewing professor of oceanography at Rice University. “We’re seeing increased erosion of barrier islands. We’re seeing changes in the bays and estuaries that are unprecedented.”
The Sentinel- 6 Michael Freilich is the first of two satellites that will be launched this decade to continue tracking the height of the ocean. Its twin satellite, which was built at the same time as the Sentinel- 6 Michael Freilich to cut down costs, will be launched in 2025.
“It’s important to overlap these,” Willis said, “because if there’s a break in the record, we lose information and it’s hard to repair.”
The Sentinel- 6 Michael Freilich will orbit 830 miles above the Earth. It uses an altimeter to send radio waves down to the ocean, where they bounce off the water and return to the satellite. The height of the ocean is then determined based on the satellite’s precise location, which is provided by GPS and other positioning systems, and the distance to the water as measured by the radar.
The Sentinel- 6 Michael Freilich also has a radiometer that measures the amount of water vapor between the satellite and the ocean. Water vapor can affect the radar pulses, making the ocean look higher or lower than it actually is. The radiometer corrects this effect.
The satellite will measure 90 percent of the ocean every 10 days. And advances with this satellite allow the Sentinel- 6 Michael Freilich to collect sea level measurements closer to the coastline than previous satellites.
“If your family lives by the sea, the sea has been coming closer to you,” Thomas Zurbuchen, NASA’s associate administrator for the Science Mission Directorate, said in an interview with the Chronicle. “The same is true for our NASA infrastructure.”
NASA has launch pads, laboratories, airfields and testing facilities near the coast. The agency understands the concerns of the rising ocean, and it wants to provide data that can help families, businesses, municipalities and cities protect current infrastructure and make decisions for the future.
The satellite’s data can also help with other forecasts, such as hurricanes. Warm water is 5 to 10 centimeters higher. If there’s a big warm patch hundreds of feet beneath the ocean’s surface, the satellite will notice that, Willis said.
But it’s not just hurricanes pushing water onto Galveston. The island is becoming increasingly prone to “nuisance flooding.” This term describes flooding that causes minor inconveniences, such as road closures, and minimal property damage but isn’t considered a widespread catastrophe.
With the ocean at a higher level than when the island was developed, all it takes is persistent wind ( not hurricane strength) blowing water into Galveston Bay to cause nuisance flooding, said Anderson, the retired Rice professor. And sometimes, this flooding happens for reasons that aren’t completely understood. It’s possible that currents are moving closer to the shore and, coupled with the higher sea, are causing nuisance flooding.
Anderson said data from the Sentinel- 6 Michael Freilich and other satellites can help answer these questions and more. For instance, it can also help researchers understand why the ocean isn’t rising uniformly. Some parts of the world are seeing water rise higher than the global average, but other areas are seeing the ocean retreat. In Galveston, he said, water is rising at about 3 millimeters a year.
The Sentinel- 6 Michael Freilich and its twin satellite make up the Sentinel- 6/Jason- CS ( Continuity of Service) mission, which is a partnership between NASA, the European Space Agency, the European Organisation for the Exploitation of Meteorological Satellites and the National Oceanic and Atmospheric Administration. The European Commission is providing some funding, and the French space agency CNES is providing technical and scientific support.
And the satellite launched Saturday is named for the former director of NASA’s Earth Science Division. Freilich was a leading figure in advancing ocean observations from space. He died Aug. 5.
His children were in California on Saturday to watch the launch.
“It’s my first time seeing a launch from out here,” Freilich’s son, Daniel, said in an interview with NASA. “And to just see that was incredible. It means so much to see — and feel it. I heard him talk about feeling it. And feeling that as it goes is incredible.”