A satellite scheduled to launch from California later this month will measure sea level rise and provide other crucial data to scientists who study how global warming is affecting the Earth's oceans.
Melting ice has already caused sea levels to rise by about 8 inches since 1880, and the trend is accelerating. The Earth's oceans have soaked up the vast majority of the extra heat, and about one quarter of the extra carbon dioxide, that humans have generated by burning fossil fuels.
The new satellite, named Sentinel-6 Michael Freilich after the former director of NASA's Earth Science division, will measure sea level around the globe for the next five years. At that point a second satellite of the same type will take its place, providing scientists with a full decade of reliable data about the Earth's oceans. The mission is a collaboration between NASA and the European Space Agency.
"Sea level is continuing to rise and we can't stop measuring it," says Josh Willis, the mission's project scientist at NASA's Jet Propulsion Laboratory. "Every year, every decade, we're remaking the climate and raising sea levels higher and higher."
The satellite is the latest in a parade of sea level measurement missions going back to the early 1990s. The Sentinel-6 mission, with two state-of-the-art satellites over 10 years, is an indication of how much demand there is for reliable, high-resolution data about climate change.
"We know that the oceans are rising because of human-caused interference with the climate. And, to see that, you really have to see sea level rise over the entire planet," Willis says. "That is what this satellite does best."
Sentinel-6 will orbit about 800 miles up and use radar to measure the surface of the ocean. An instrument on the satellite sends a radar wave down to Earth. The radar bounces off the surface of the ocean and returns to the satellite. By measuring how long it took for the radar to go down and back — and accounting for moisture in the atmosphere which slows the radar down — scientists can measure how far away the surface of the ocean is from the satellite.
In other words, the satellite can tell scientists on Earth how high the oceans are, and how that height is changing over time.
"It's really kind of an incredible feat of technology," Willis says. "We can accurately measure the water level with an accuracy of 1 inch from 800 miles up."
Rising seas affect coastal areas of the U.S. in tangible, often destructive, ways. Ocean water has infiltrated drainage systems, flooded streets and removed entire island communities from existence in the Chesapeake Bay, Gulf of Mexico and Pacific coasts.
But the higher, hotter water also affects people far from the coasts. What happens in the oceans does not stay in the oceans. For example, ocean currents and temperatures affect weather and fish populations.
And because water expands as it gets hotter, sea level data can help scientists understand other things about the ocean.
"We can also use the sea level measurements to understand how currents are changing, how the ocean is storing heat," says LuAnne Thompson, an oceanographer at the University of Washington. Thompson and her colleagues use sea level rise data from satellites every day, and she says it's indispensable for her research about how much heat the oceans are storing.
"By knowing the sea level, we have an indication of how much the ocean has expanded because of warming," Thompson explains.
There is still a lot that scientists don't know about how the oceans will change in the coming decades. Climate models are clear that sea levels will continue to rise rapidly, and that sea level rise will accelerate as the Earth gets hotter. But exactly what that will look like at the local level requires detailed, continuous data from both satellites and tide gauges down on Earth.
RACHEL MARTIN, HOST:
The oceans are rising. Globally, the average sea level is more than eight inches higher now than it was in 1880, and the trend is accelerating. NPR's Rebecca Hersher has the story about a new satellite that could help scientists understand how climate change is changing our seas. Here's her story.
REBECCA HERSHER, BYLINE: If you live near the coast, you've probably seen buoys and other contraptions along the water's edge that measure what's going on in the ocean, including how high the water is. But when it comes to understanding global climate change, there is no substitute for satellite data.
JOSH WILLIS: From space, you can see the whole thing.
HERSHER: Josh Willis is a scientist at NASA. He's leading the U.S. team that's launching a new satellite called Sentinel-6 in collaboration with the European Space Agency. Sentinel-6 will zip around the globe 800 miles up and look at the surface of all the oceans.
WILLIS: It's really kind of an incredible feat of technology. We can actually measure the water level with an accuracy of about one inch from 800 miles up.
HERSHER: Sentinel-6 uses radar to make continuous measurements.
WILLIS: A radar beam comes down out of the satellite. It bounces off that surface, and then it measures the signal coming back. And by figuring out how long it takes to go down and come back, you can tell how far away the water is.
HERSHER: If you know how far away the water is, you can figure out how high it is relative to the land. Sentinel-6 is the latest in a string of satellites that do this kind of measurement going back to the '90s. But those missions were somewhat ad hoc, and scientists couldn't always be sure that there would be a next mission when the current one ended, which is a nightmare when you're trying to understand how the climate is changing over time, which is why they are really excited that this time, the satellite will be up there for five years and then another identical satellite will launch to do another five years - so a decade of reliable data. LuAnne Thompson studies oceans at the University of Washington.
LUANNE THOMPSON: I use that data every day in my research.
HERSHER: Thompson has been studying how the oceans have been changing for decades. She says obviously sea level rise is tangibly important to people who live on the coasts, but ocean changes affect everyone. What happens in the ocean doesn't stay there. For example, currents and ocean temperatures affect weather and fish populations.
THOMPSON: We can also use the sea level measurements to understand how currents are changing, how the ocean is storing heat.
HERSHER: And hotter oceans can drive more powerful hurricanes. And scientists use sea level data from satellites to figure out exactly how hot the oceans are getting, too, because water gets bigger as it gets hotter.
THOMPSON: So by knowing the sea level, we have an indication of how much the ocean has expanded because of warming.
HERSHER: Josh Willis of NASA says the Sentinel-6 satellite is crucial because climate change is happening fast. In the past, scientists had to make do with less data about the oceans. But now, with the Earth rapidly warming, climate scientists need as much information as possible about what's happening around the globe.
WILLIS: Sea level is continuing to rise, and we can't stop measuring it. Every year, every decade, we're remaking the climate and raising sea levels higher and higher.
HERSHER: Sentinel-6 is scheduled to launch on November 21 from California. Rebecca Hersher, NPR News.
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