The oceans are extremely warm right now. Worldwide, average ocean temperatures were in record-breaking territory for 15 months straight since last April.
That鈥檚 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 , 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 .
So, why are the oceans so hot right now?
Let鈥檚 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 Ni帽o is also partly to blame. When El Ni帽o is happening, there鈥檚 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 Ni帽o was happening and it caused global average temperatures to rise, including in the oceans.
鈥淭he two primary things are obviously global warming and El Ni帽o,鈥� says Andrew Dessler, a climate scientist at Texas A&M. But that鈥檚 where the certainty ends, because the oceans are even warmer than scientists expected from those two trends.
鈥淭hink 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?鈥� Dessler explains.
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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, 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鈥檚 good news for the overall health of humans and other animals 鈥� air pollution, particularly the sulfur-heavy pollution released by dirty shipping fuel, leads to serious illness. 鈥淭his saves lives,鈥� 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鈥檚 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鈥檚 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鈥檚 because there are still lots of unanswered questions about how air pollutants affect clouds, which in turn reflect sunlight. 鈥淚t鈥檚 a complicated system and it鈥檚 going to take some time to sort that out, but people are trying to do that,鈥� 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 鈥� long, thin clouds that form with the help of sulfur pollution, and look kind of like plane contrails 鈥� according to a .
Without those reflective ship track clouds, more of the Sun鈥檚 energy does, indeed, seem to be making it to the surface of the ocean, where it is absorbed, the study finds. 鈥淭his could be contributing to the warm temperatures we鈥檝e seen in the last couple years,鈥� 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鈥檛 caused by humans releasing new, additional greenhouse gas emissions. Humans already caused this warming, but weren鈥檛 feeling it because air pollution was protecting us.
Suspect #2: A massive 2022 volcanic eruption probably didn鈥檛 drive extra ocean warming
When ocean temperatures started surprising scientists last year, one of the theories was that a 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 , 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. 鈥淲ater is a greenhouse gas,鈥� Dessler explains.
A handful of 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 鈥淣o.鈥�
鈥淚 was very skeptical of the warming effect,鈥� says Dessler, who is an expert on how water vapor in the stratosphere affects the Earth鈥檚 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 showing there was no evidence that the eruption led to overall warming. If anything, like past eruptions, it contributed to slight cooling. 鈥淭he bottom line is that this had a very tiny impact on the climate,鈥� 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. 鈥淚鈥檇 caution against assuming that a single [research] paper will end up being the consensus,鈥� he says.
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Suspect #3: Don鈥檛 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. 鈥淎s the sun鈥檚 output gets brighter and dimmer by about 0.1% over its 11-year cycle, the Earth鈥檚 global temperatures increase and then decrease by a little less than 0.1 degree [Celsius],鈥� explains Gregory Kopp, a solar physicist at the University of Colorado, Boulder.
But that tenth of a degree Celsius change doesn鈥檛 account for the abnormally hot ocean temperatures of the last couple years. 鈥淭he sun isn鈥檛 causing the recent record-breaking sea-surface temperatures,鈥� Kopp says. The ocean is simply too large to immediately heat up or cool down in response to changes in the sun. 鈥淭he sea contains so much heat energy that it doesn鈥檛 respond on the relatively short solar-cycle timescales,鈥� Kopp explains .
So, in the quest to understand current record-breaking ocean temperatures, the sun doesn鈥檛 offer any answers.
Suspect #4: The potential role of the 鈥渨eird鈥�
The upshot is that scientists don鈥檛 know for sure what鈥檚 driving ocean temperatures into record-breaking territory. In addition to climate change and El Ni帽o, cleaner air seems to be playing a role.
The last possibility is what Dessler calls 鈥渨eirdness.鈥�
The Earth鈥檚 climate is incredibly complex, and there鈥檚 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鈥檚 layered on top of warming from climate change and El Ni帽o.
鈥淢y guess is, in the end, it鈥檚 just going to be internal variability,鈥� Dessler explains. 鈥淟ike, something weird happened! Because the climate鈥檚 always doing something weird.鈥�
Gettelman agrees that normal year-to-year temperature variability is an important factor.
鈥淲e're going to see the planet warm in fits and starts,鈥� he explains. For example, there was a period in the 2010s when the Earth didn鈥檛 warm very much. 鈥淧eople were saying 鈥極h, global warming's over,鈥欌�� remembers Gettelman. 鈥淚t wasn't. It was just a transient thing, and we now may be recovering from that.鈥�
The real concern for climate scientists isn鈥檛 so much what happens to temperatures in a given year or two, but whether the overall warming trend is accelerating. If the oceans don鈥檛 cool off somewhat in the coming months, that would suggest that the Earth is heating up very quickly.
鈥淚 think people are starting to get a little worried that we are warming at the high end of what [climate models predicted],鈥� says Gettelman.
鈥淲e鈥檒l see,鈥� Dessler agrees. 鈥淭he next few months will tell us if we鈥檝e really broken the climate.鈥�
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