< Climate change is one reason for hotter oceans. But there are others

EMILY KWONG: You're listening to Short Wave from NPR. Hey, Short Wavers. Emily Kwong here. We are in the middle of hurricane season. And Hurricane Francine has hit Louisiana. Last week, it dumped rain all across the South. And forecasters are expecting even more hurricanes and tropical storms in the next few weeks. So we have brought in Rebecca Hersher from NPR's Climate Desk to talk about it. Hey, Rebecca.

REBECCA HERSHER: Hey.

KWONG: OK, so what, scientifically speaking, is happening this hurricane season?

HERSHER: Well, you know, these kinds of storms, they form over the ocean. So a big part of what's happening is the super warm water in the Atlantic and the Caribbean is affecting them. Like, last week when Francine was headed toward Louisiana, it was moving over water that was in the upper 80s.

KWONG: Oh, that's balmy. Like a warm bath.

HERSHER: Yeah, kind of gross. Like, warmer than I like my swimming pools.

KWONG: Yeah. But is this unusual for this part of the Caribbean?

HERSHER: Yeah, it's like two or three degrees Fahrenheit warmer than usual, which is like one or two degrees Celsius. It might not sound like a lot, but a few extra degrees can help storms get really powerful. Because, you know, heat is energy. And abnormally warm water is causing all of these other problems, too. Like, coral reefs off Florida's coast are dying. Fisheries aren't doing very well in a lot of areas because the fish don't thrive in warmer water. And I've been talking about the ocean water near the Eastern US. But, you know, this isn't a local problem. This is a worldwide problem. Average global ocean temperatures have been in record breaking territory for most of the last year and a half.

KWONG: So why? Is it just climate change? Can we just make this the shortest episode ever and call it what it is?

HERSHER: Yep, that's it. What if I was like, yep, it's climate change? You're listening to Short Wave from NPR.

KWONG: So but is it climate change?

HERSHER: Yeah, yeah. So climate change is the main reason for the super hot ocean temperatures. But it's not the only reason. And that's why we will have a whole episode. I've been following the scientific quest that's been underway to sort of figure out, what are the other reasons? Like, the supporting characters.

KWONG: Today on the show, why are the oceans weirdly hot this year? We know climate change is a big part of it. But what about volcanoes, the sun, or container ships?

HERSHER: Those are all actual theories.

KWONG: I assume, because it's what's written in my script. We're going to find out more. I'm Emily Kwong. That's Rebecca Hersher. You're listening to Short Wave from NPR.

[MUSIC PLAYING]

KWONG: All right, Rebecca. So we know climate change is the main reason that the oceans are so hot. And this is because oceans soak up most of the extra heat that humans are trapping on Earth with greenhouse gases.

HERSHER: Exactly. And that's why the oceans have been steadily warming for decades. And then on top of that general upward trend, you have El Nino/La Nina cycles, right? So when La Nina is happening, global temperatures are a tiny bit cooler. When El Nino is happening, it's the opposite. Global temperatures are a tiny bit warmer.

KWONG: We just finished with El Nino, right?

HERSHER: Exactly. So that partly accounts for the warm ocean temperatures last year and this year. But it doesn't fully account for it. Here's how climate scientist Andrew Dessler put it.

[AUDIO PLAYBACK]

ANDREW DESSLER: Well, I mean, I think the two primary things are obviously global warming and El Nino. Think of it like, you know, the house is burglarized. You have video of those two suspects doing it. And the question is, is there another suspect? Is there somebody else that was helping them?

[END PLAYBACK]

KWONG: The intrigue. OK, so we're going to play basically Clue.

HERSHER: Yes.

KWONG: To figure out, who are these other suspects? There are four. What are they?

HERSHER: OK, they're a volcano, pollution from ships, the sun, and a fourth suspect that I'm just going to call the weird suspect for now.

KWONG: I love a mystery. OK, let's go through this. So suspect number one is a volcano. I remember talking about this with you back in June. There was this theory that the huge underwater Hunga Tonga volcano eruption may have caused extra warming because it put so much water vapor into the atmosphere.

HERSHER: Yep. And since we last talked about this earlier in the summer, there have been some pretty big studies trying to test that hypothesis.

KWONG: OK. Well, what did they find?

HERSHER: That theory probably isn't right. If anything, it looks like that eruption caused slight cooling. Because in addition to water vapor, which traps heat, it also put a bunch of gases and ash into the atmosphere, too, which reflect heat. Andrew Dessler is one of the people who's been looking into this theory.

[AUDIO PLAYBACK]

DESSLER: I'll just say the bottom line is this had a very tiny impact on the climate.

[END PLAYBACK]

HERSHER: Now, I will say, it's not like this is a done deal. This is still something that other scientists are looking into. Something that scientists are still arguing about. But as more and more studies have been done, it is looking less and less likely that this volcano is, like, the suspect we're looking for in our hot oceans investigation.

KWONG: I'm bummed. I liked how strange this theory was.

HERSHER: I know. I know you had a personal attachment to it.

KWONG: Yeah. Yeah, that, like, the magma from our core could be heating up our planet. None of this is a good thing. But OK, so what is the next suspect?

HERSHER: Let's talk about the sun next.

KWONG: Yes, big star, source of heat.

HERSHER: Big star. Yeah, this is a very simple theory. So basically, since the heat in the oceans mostly comes from the sun, maybe the extra heat in the oceans right now is because of some little change in the sun. Because, you know, the sun has cycles, kind of like the seasons. It gets very slightly brighter or dimmer on an 11-year cycle. Which causes the earth's temperature to fluctuate by about a 10th of a degree.

KWONG: OK, that sounds potentially promising. So is the sun's cycles one of the reasons the oceans are abnormally warm?

HERSHER: I talked to a couple scientists who study the solar cycle, and they were basically like, the ocean is too large to immediately heat up or cool down in response to cyclic changes in the sun. So no, there's really no evidence that the sun is the cause of what we're seeing in the oceans right now. Basically, they were like, do not blame the sun for this.

KWONG: All right. Not the sun in the candlestick in the sky, not the volcano with the noxious gas in the study. We are flying through this. What is suspect number three?

HERSHER: This one is more promising. So let's talk about international shipping.

KWONG: Yes. Big container ships that go from port to port.

HERSHER: Exactly. Ocean shipping, specifically. So starting in 2020, there were new international shipping regulations that require ships to use slightly cleaner types of fuel. So the new fuel, it still releases planet warming pollution like carbon dioxide. But it releases a lot less other air pollution, in particular, sulfur, that's really bad for humans' and animals' health.

KWONG: Oh. Well, that sounds great. Like, these new rules maybe saved a lot of lives by cleaning up the air.

HERSHER: Yeah, they definitely saved a lot of lives. But-- this is a big but-- all that pollution was weirdly doing a good thing for the environment. It was reflecting some of the sun's heat. Because sulfur helps clouds form, and clouds are generally bright white. Right, they reflect sunlight. So when the extra sulfur from the ship pollution went away, scientists were wondering if maybe more of the sun's heat would end up being absorbed by the oceans.

KWONG: That is such an irony. I mean, that because there are fewer clouds over the ocean, because there's less sulfur to make those clouds, it actually caused it to be hotter.

HERSHER: Yeah.

KWONG: Maybe. That's the theory?

HERSHER: That was the theory. That was the theory.

KWONG: Was it right?

HERSHER: Yeah, it seems like it was right. The most cutting edge research does suggest that ocean temperatures may have increased slightly in some parts of the world as sulfur pollution from ships decreased.

KWONG: Wow.

HERSHER: So one study found that since the new regulations went into effect, there are fewer of these long thin clouds called ship tracks. So think of them like the thin clouds you see behind airplanes.

KWONG: Yeah.

HERSHER: And without those reflective ship track clouds, more of the sun's energy does indeed seem to be making it to the surface of the ocean where it's absorbed. Yeah, it's wild. Andrew Gettleman at the Pacific Northwest National Laboratory is the lead author of that recent study.

[AUDIO PLAYBACK]

ANDREW GETTLEMAN: This could be contributing to the warm temperatures we've seen in the last couple years.

[END PLAYBACK]

KWONG: OK, so shipping might be an actual culprit here. But a minor accomplice, obviously, to the big baddies of climate change and El Nino. You said there's one more suspect under consideration, the weird suspect.

HERSHER: Yeah.

KWONG: What's that?

HERSHER: The weird suspect. I mean it literally. So all the scientists I talked to said some version of this. That the climate is weird. Meaning it's really complex, and that in any given year, temperatures might be a tiny bit higher or lower than expected just because of variability.

KWONG: That is weird, OK. What's an example of that?

HERSHER: Well, so for example, there was this period in the 2010s when the Earth didn't seem to be warming much.

[AUDIO PLAYBACK]

GETTLEMAN: The planet really didn't warm at all. And people were saying, oh, global warming is over. And it wasn't. It was just transient things. We now may be recovering from that. Kind of goes up and down quite a bit.

[END PLAYBACK]

KWONG: OK. So this is just the normal way that things warm. Like, the general trend is that the planet, yes, is steadily getting hotter because of climate change. It's just not a perfectly smooth line up year after year.

HERSHER: Yeah. Andrew Gettleman described it as warming in fits and starts, which I thought was, like, really helpful for understanding it.

KWONG: Right, OK. So if warming is happening in fits and starts, maybe even this moment we're living in will just be a fit of heat that happened for a couple years and then cooled.

HERSHER: Yeah, exactly. That kind of variability might be partly to blame here. That's what we're experiencing, maybe.

KWONG: So all told, four suspects, two declared innocent.

HERSHER: Yeah, volcano and the sun, totally off the hook.

KWONG: And two are looking more guilty. Shipping pollution and the weird fits and starts of warming probably play some role in the ocean temperatures we're seeing now. So Rebecca, does this help us understand what the future holds? Are the oceans going to stay this warm? Or is it all temporary?

HERSHER: No one knows for sure, honestly. There's been some relative cooling just in the last couple months. The streak of record setting ocean temperatures ended. But it's not like the ocean all of a sudden went back to a normal temperature. It's still way hotter than it was even a few years ago. If shipping pollution and natural variability are to blame, you know, which it sort of looks like they might be, then this extra warming we've seen in the oceans should be temporary, right? The shipping rules went into effect years ago. It will still take a few more years for us to see all the warming effects of having less sulfur pollution. But at some point, scientists say we will reach a new equilibrium on that front. And of course, natural variability changes over time, by definition. It's not permanent. But the only way to know for sure is to see what happens in the coming months and years.

KWONG: Yeah. And certainly there's so many people's livelihoods who depend on this. I mean, I think of what you were saying earlier about fisheries. You know, they're planning months, years in advance for fishing seasons, wondering if the fish will be there.

HERSHER: Exactly.

KWONG: All right. So what are scientists going to do in the meantime as they wait for this to play out?

HERSHER: Oh, you know, they'll keep studying it and fighting with each other about it. In fact, they're planning to discuss all of this science at the annual meeting of the American Geophysical Union in December. There will be a big nerdy session about this with scientists from all over the world, talking about all of these suspects.

KWONG: It sounds amazing, actually. Well, thank you for reporting on this, and we hope to hear about the AGU in the winter when it happens.

HERSHER: Great, thanks.

[MUSIC PLAYING]

KWONG: Short Wave is turning five on October 15th. And to celebrate our podcast-aversary, we are taking questions from listeners, but only those listeners who are five years old. That's right. For our fifth birthday, we're handing the reins over to some of our youngest Short Wavers. Ask us anything. How do submarines work? Why is the sky blue? Record your question and your name in a voice memo and ask your parents to email it to us at shortwave@npr.org. And listen to our answers on October 15th. See you then. This episode was produced by Rachel Carlson. It was edited by our showrunner Rebecca Ramirez and fact-checked by Rebecca Hersher. The audio engineer was Kwesi Lee. Beth Donovan is our senior director, and Collin Campbell is our senior vice president of podcasting strategy. I'm Emily Kwong. Thank you for listening to Short Wave from NPR.

Copyright © 2024 NPR. All rights reserved. Visit our website terms of use and permissions pages at www.npr.org for further information.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.