AYESHA RASCOE, HOST:
Summertime brings trips to the beach.
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RASCOE: And while most of us like to sit by the ocean, NPR's Short Wave podcast is diving in, with this story from producer Hannah Chinn on how the ocean and the atmosphere affect each other.
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HANNAH CHINN, BYLINE: If you've ever walked along the beach and picked up a seashell, chances are it was brought to you by an ocean current. Those currents are like rivers and streams, but in the sea instead of on land. They're driven by temperature, salinity, coastlines, wind patterns and even the rotation of the Earth. And as they wind through the ocean, they take things with them, like seashells or fish - or even humans.
LYNNE TALLEY: People have needed to know surface currents ever since they were sailing, you know, thousands of years ago. It's very, very important for navigation.
CHINN: This is physical oceanographer Lynne Talley. She's a professor at the Scripps Institution of Oceanography at the University of California, San Diego. And she told me that one big surface current that affects us was first mapped by Benjamin Franklin because he was trying to solve the problem of mail. See, airmail wasn't an option at the time, so everything from England had to come via boat over the Atlantic Ocean. That could take multiple weeks, and Franklin wanted to speed that up.
TALLEY: So he noticed that his cousin in Rhode Island, who was a whaling captain, could get across the Atlantic Ocean a whole lot faster than the British official postmaster ships. And that's because his cousin knew about the Gulf Stream.
CHINN: The Gulf Stream is a big current that flows from the Gulf of Mexico north along the eastern coastline of the United States and then towards Northern Europe. And for a long time, the Gulf Stream and the majority of these surface currents have stayed the same, but now they're shifting. An oceanographer named Shang-Ping Xie, who's a colleague of Talley's at the Scripps Institution, has used predictive models to show that in a warming climate, more than three-quarters of the ocean's surface currents will significantly accelerate by the end of the century. That's because the ocean absorbs a lot of the heat that humans produce with greenhouse gases. And that heat, Xie told me, doesn't diffuse evenly. It tends to sit on the top.
SHANG-PING XIE: You know, the ocean is a layer of warm water floating over a body of deeper, cold water.
CHINN: As that top layer warms, water molecules within the surface currents move faster and further apart, meaning they're going to take up more space. That'll result in intensified pressure differences, pushing parts of the ocean to speed up.
XIE: That speed up of the currents is associated with tremendous changes of the ocean temperature and salinity. So fish definitely is going to tell the difference because, you know, a lot of fish - they migrate among spawning and feeding and the nursery grounds.
CHINN: There is one key exception to this overall surface acceleration trend. And that's the AMOC, or Atlantic Meridional Overturning Circulation. It's a system of surface currents and deep currents, kind of like a big ocean conveyor belt that moves warm water north and cold water south. And instead of speeding up, the AMOC is slowing down, which could change a whole host of things - increase storm severity, cause sea level rise, make Western Europe and a lot of the Northern hemisphere way colder, change monsoon season and rainfall near the equator. Scientists don't know all the potential effects yet. That's in part because when it comes to climate science, the ocean is understudied.
XIE: Climate change is often presented as a atmospheric problem. There are just much fewer studies on how the ocean responds.
CHINN: But Xie says we need those additional studies because even though the ocean isn't our home, we can't survive climate change without it.
Hannah Chinn, NPR News.
RASCOE: This story is a part of Short Wave's Sea Camp series. Follow the podcast and subscribe to their newsletter at npr.org/seacamp to learn more. Transcript provided by NPR, Copyright NPR.
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