By Rebecca Hersher, NPR
The oceans are extremely warm right now. Worldwide, average ocean temperatures were in record-breaking territory for 15 months straight since last April.
That’s bad news on multiple fronts. Abnormally hot ocean water helps fuel dangerous hurricanes, like Hurricane Ernesto, which is expected to rapidly gain strength this week in the Atlantic, and like Hurricane Debby, which dumped massive amounts of rain along the East Coast of the U.S. last week. And when the water gets too hot, fish and other marine species also struggle to survive. For example, the ocean water near Florida is so warm that it’s threatening coral reefs.
So, why are the oceans so hot right now?
Let’s start with what we know: Climate change is broadly to blame. Humans continue to burn fossil fuels that release heat-trapping gasses into the atmosphere, and most of that extra heat is absorbed by the oceans. Ocean temperatures have been steadily rising for decades.
The cyclic climate pattern El Niño is also partly to blame. When El Niño is happening, there’s warmer water in part of the Pacific, and that generally means the Earth is slightly warmer overall. In 2023 and the first part of 2024, El Niño was happening and it caused global average temperatures to rise, including in the oceans.
“The two primary things are obviously global warming and El Niño,” says Andrew Dessler, a climate scientist at Texas A&M. But that’s where the certainty ends, because the oceans are even warmer than scientists expected from those two trends.
“Think of it like, the house was burglarized, and you have video of those two suspects doing it. And the question is: Is there somebody else helping them?” Dessler explains.
It seems like there probably was another suspect. And over the last 18 months or so, a few major theories have emerged about what it might be. Testing those theories is slow, laborious work for scientists, but after months of crunching the numbers, some early answers are emerging.
Suspect #1: Pollution from ships is probably part of the answer
One reason that ocean temperatures started to spike last year is that ships stopped releasing so much pollution into the air.
In 2020, new international shipping regulations went into effect that required ships to use slightly cleaner types of fuel. The new fuel still releases planet-warming gasses like carbon dioxide, but it releases a lot less pollution into the air.
That’s good news for the overall health of humans and other animals – air pollution, particularly the sulfur-heavy pollution released by dirty shipping fuel, leads to serious illness. “This saves lives,” says Stephen Smith, an expert on air pollution and climate change at Pacific Northwest National Laboratory.
But all that pollution was also reflecting some of the sun’s heat, because sulfur helps clouds form and those bright, white clouds reflect sunlight. When the extra sulfur from ship pollution went away, scientists wondered if more of the sun’s heat would be absorbed by the oceans.
Teasing apart how much of an effect cleaner air over the oceans might have on ocean temperatures is surprisingly difficult. That’s because there are still lots of unanswered questions about how air pollutants affect clouds, which in turn reflect sunlight. “It’s a complicated system and it’s going to take some time to sort that out, but people are trying to do that,” says Smith.
The most cutting-edge research in the field does suggest ocean temperatures may have increased slightly in some parts of the world as sulfur pollution from ships decreased. In major shipping lanes where pollution from ships decreased significantly since 2020, there are fewer so-called ship tracks – long, thin clouds that form with the help of sulfur pollution, and look kind of like plane contrails – according to a new study published this week.
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 is absorbed, the study finds. “This could be contributing to the warm temperatures we’ve seen in the last couple years,” says Andrew Gettelman, a climate scientist at Pacific Northwest National Laboratory and one of the authors of the forthcoming study.
The good news is that this type of warming, from cutting dangerous air pollution, isn’t caused by humans releasing new, additional greenhouse gas emissions. Humans already caused this warming, but weren’t feeling it because air pollution was protecting us.
The Hunga Tonga-Hunga Ha'apai eruption happened in 2022. An underwater volcano sent sulfur, ash and water vapor into the atmosphere.GOES-17/NOAA/NASA
Suspect #2: A massive 2022 volcanic eruption probably didn’t drive extra ocean warming
When ocean temperatures started surprising scientists last year, one of the theories was that a massive volcanic eruption in 2022 might be partly to blame.
The Hunga Tonga-Hunga Ha'apai eruption happened off the coast of the Pacific island of Tonga. An underwater volcano erupted, sending sulfur, ash and water vapor into the atmosphere.
Usually, volcanic eruptions temporarily cool the Earth slightly, because the sulfur and ash in the atmosphere spread around the world and reflect extra heat from the sun.
But because the Hunga-Tonga eruption happened underwater, it spewed a lot of water vapor into the atmosphere as well. Unlike ash, water vapor absorbs heat from the sun. “Water is a greenhouse gas,” Dessler explains.
A handful of scientists publicly wondered if all that water vapor might be trapping extra heat, and contributing to off-the-charts ocean temperatures.
Recent research suggests that the answer is “No.”
“I was very skeptical of the warming effect,” says Dessler, who is an expert on how water vapor in the stratosphere affects the Earth’s climate. He points out that, despite the size of the eruption, it changed the total amount of water in the atmosphere very little. But, to know for sure, he and scientists at NASA had to analyze reams of data from satellites and other sources.
In July, they published a study showing there was no evidence that the eruption led to overall warming. If anything, like past eruptions, it contributed to slight cooling. “The bottom line is that this had a very tiny impact on the climate,” says Dessler.
But other scientific analyses about the potential role of the eruption on global temperatures are still underway, says Gavin Schmidt, a climate scientist at NASA who was not involved in the newly published study. “I’d caution against assuming that a single [research] paper will end up being the consensus,” he says.
Suspect #3: Don’t blame the sun
Since the heat in the oceans originally comes from the sun, our local star is one place to look for answers about abnormal temperatures here on Earth.
The amount of energy coming from the sun changes a little bit over the course of an 11-year solar cycle. “As the sun’s output gets brighter and dimmer by about 0.1% over its 11-year cycle, the Earth’s global temperatures increase and then decrease by a little less than 0.1 degree [Celsius],” explains Gregory Kopp, a solar physicist at the University of Colorado, Boulder.
But that tenth of a degree Celsius change doesn’t account for the abnormally hot ocean temperatures of the last couple years. “The sun isn’t causing the recent record-breaking sea-surface temperatures,” Kopp says. The ocean is simply too large to immediately heat up or cool down in response to changes in the sun. “The sea contains so much heat energy that it doesn’t respond on the relatively short solar-cycle timescales,” Kopp explains.
So, in the quest to understand current record-breaking ocean temperatures, the sun doesn’t offer any answers.
Suspect #4: The potential role of the “weird”
The upshot is that scientists don’t know for sure what’s driving ocean temperatures into record-breaking territory. In addition to climate change and El Niño, cleaner air seems to be playing a role.
The last possibility is what Dessler calls “weirdness.”
The Earth’s climate is incredibly complex, and there’s some natural variability in temperatures on short time scales like one or two years. A few extra tenths of a degree of heat in the Atlantic due to nothing more but natural variability could account for a couple of record-breaking years, when it’s layered on top of warming from climate change and El Niño.
“My guess is, in the end, it’s just going to be internal variability,” Dessler explains. “Like, something weird happened! Because the climate’s always doing something weird.”
Gettelman agrees that normal year-to-year temperature variability is an important factor.
“We're going to see the planet warm in fits and starts,” he explains. For example, there was a period in the 2010s when the Earth didn’t warm very much. “People were saying ‘Oh, global warming's over,’” remembers Gettelman. “It wasn't. It was just a transient thing, and we now may be recovering from that.”
The real concern for climate scientists isn’t so much what happens to temperatures in a given year or two, but whether the overall warming trend is accelerating. If the oceans don’t cool off somewhat in the coming months, that would suggest that the Earth is heating up very quickly.
“I think people are starting to get a little worried that we are warming at the high end of what [climate models predicted],” says Gettelman.
“We’ll see,” Dessler agrees. “The next few months will tell us if we’ve really broken the climate.”
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