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Measuring sea-level rise

Green Daily

In climate news today...

As the planet warms, the increase in sea levels threatens to uproot more than a third of the world's population. Understanding exactly how fast and by how much they're rising is crucial information for policy makers trying to protect communities that live near the coast, and for scientists modeling the effects of global warming. 

In order to predict how sea level will rise in the future, scientists need data over a long period of time. Earth observation satellites are the best tool we have to do so. We've been launching them into space for nearly three decades, where they orbit the earth and track data that can be used to estimate sea-level rise. But the technology they employ has remained largely the same over the years, according to NASA scientist Josh Willis. 

On Saturday, Willis and his team launched the Sentinel-6 Michael Freilich satellite. The U.S. and Europe worked on the project together in the hope Sentinel-6 will be able to measure sea levels covering 90% of the world's oceans, down to a few centimeters, for the next 10 years. Comparing the data it gathers with that of its predecessors dating back to the early 90s could help researchers predict sea-level rise over the next few decades. 

"It turns out we're great at predicting global warming, but we're terrible at predicting sea level rise," said Willis. "The best way to predict it right now is to look at the last 10 or 20 or 30 years and try and extend that out into the future."

Scientists have relied on the same method to gauge sea-level rise for decades. A radar at the bottom of the satellite shoots radio waves to the surface of the ocean and measures how long it takes the waves to hit the water and come back. The radar is supported by GPS and other positioning systems to pinpoint the location of the satellite. Comparing data over time shows how much the height of the water has changed.

There are a few new gadgets on Sentinel-6 that will help fine-tune the process. For one, its radar will be able to read data closer to the coast than before, and in finer detail. Previous instruments looked at larger areas of water at once, making it difficult to differentiate between water and land along coastlines. 

Sentinel-6 also has a more stable radiometer than previous satellites, which should help provide more accurate readings. Radar waves slow down when they hit water — the more water, the slower they move. The radiometer looks down through the atmosphere to see how much water is in the air, so scientists can correct for that when timing how long the waves take to reach the water and return.

The new satellite will also have a radio occultation receiver, which is a GPS antenna that looks forward and watches GPS satellites appear over the horizon as it flies along. The device measures the change of temperature and humidity of a slice of the atmosphere, which can help in predicting the weather.

Sentinel-6 is state of the art technology for the particular orbit it will travel, said Craig Donlon, principal scientist for oceans and ice at the European Space Agency, which was also involved in the project. "Sea level is really something that we have to be aware of," he said. "It's one of these sinister, global-scare phenomena that contributes to eating away at coastal defenses, eroding our very society."

Launching Sentinel-6 will allow scientists to gather 10 years of new data. The satellites are built to last between five and seven years, so a second identical model will launch in 2025. Along with sea-level rise, the two satellites will also track ocean circulation, atmospheric pressure and humidity. 

Scientists are already working on the next technological leap in measuring water-level change over time. The U.S., France and Canada are planning to launch the SWOT satellite, which stands for Surface Water and Ocean Topography, in the next two years. It will take the first global survey of Earth's surface water, and put it "in HD," Willis said. -- by Elizabeth Elkin

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