The dangerous effects of rising sea temperatures

In 30 years of studying the oceans, Matthew England has learnt to understand their irregular yet constant rhythms — the cycles of wind, temperature and atmospheric changes that interact with the masses of water covering most of the Earth’s surface.

But what he has seen in the past 15 months has shocked him. Global sea surface temperatures have reached and stayed at record levels, fuelling heatwaves and melting sea ice. Temperatures in the north Atlantic waters he has been studying, including around the UK and Ireland, were described last year as “beyond extreme” by the EU’s Earth observation service.

“I was stressed by the amount of climate change, to see the pace of change, to see these marine heatwaves, the loss of sea ice,” says England, who is Scientia professor of ocean and climate dynamics at the University of New South Wales in Australia. The rate of warming went “beyond what you would typically see from steady global warming”.

And the heat has not gone away. June was the 15th consecutive month that global sea temperatures were at a record high. Forecasters fear the warm waters will fuel a historically intense hurricane season this year. Hurricane Beryl, which hit the Caribbean, coast of Mexico and Texas this month, was the earliest maximum-severity storm on record.

Scientists are now trying to understand what has driven the rapid anomalous rise in sea temperatures, why that heat has lingered and whether the world’s seas will cool again.

At the heart of these questions is a concern that the oceans may be reaching their limits in the vital role they play in protecting the planet against the worst extremes of climate change. They have absorbed 90 per cent of excess heat and about a quarter of human-caused carbon dioxide emitted during the industrial era.

“All that heat that’s going into the ocean is not going into the land surface or into the atmosphere, or into the ice caps,” says Michael Meredith, oceanographer and science leader at the British Antarctic Survey. “The ocean has been doing this huge climate favour for us for decades.”

But he adds there is no guarantee it will continue doing so at the same rate. “If this does slow in future, the consequences will be like we saw in 2023 [the hottest year on record] but much more so.”

While scientists are struggling to understand exactly why sea temperatures spiked to such highs in 2023 and continued into this year, there is broad agreement that two issues — global warming and the El Niño-Southern Oscillation phenomenon — have been key contributors.

A report from the Intergovernmental Panel on Climate Change group of scientists in 2019 said marine heatwaves had doubled in frequency since 1982, while also becoming longer lasting, more intense and more extensive.

Its research added that it was very likely that between 84 and 90 per cent of marine heatwaves in the years from 2006 to 2015 were attributable to human-caused warming.

“Like we have heatwaves in the atmosphere, we have heatwaves in the ocean. They can last for weeks or even months, but then they fade away again,” explains Meredith. “But we are seeing more of them, they are becoming more frequent and more intense.”

Sea surface temperatures break records

Last year, this trend ran into El Niño, part of a natural fluctuation in the world’s climate system in which the trade winds that blow warm water west across the Pacific weaken, raising sea and land surface temperatures and affecting weather patterns around the world. El Niño typically occurs every two to seven years, with the most recent declared in June 2023.

“During El Niño, the tropical Pacific releases a tremendous amount of heat that’s been stored below the surface [of the water] into the atmosphere,” says Michael McPhaden, a senior scientist at the US’s National Oceanic and Atmospheric Administration (Noaa).   

“It takes a few months for that heat to spread out around the globe,” he adds. “But when it does, you see an additional rise in global mean surface temperatures, both land surface temperatures and ocean surface temperatures.”

Joel Hirschi, associate head of marine systems modelling at the National Oceanography Centre in Southampton, says that while the rises in sea temperatures during 2023 and 2024 “appear quite spectacular”, the jump tallies with past patterns during strong El Niño years. In 2007-2008, global sea surface temperatures also set records for every day for a year, he notes, although from a much lower baseline.

But Karina von Schuckmann, an oceanographer at Mercator Ocean International, points out that the effect of El Niño on global sea temperatures should have become most apparent this spring. In fact, temperatures were high a year before that — indicating other factors were also at play. She adds that El Niño would not normally be expected to lead to the pattern seen in the north Atlantic during 2023 and 2024. 

McPhaden says that prior to last year’s El Niño, the world experienced a “very unusual” triple-dip La Niña — the corresponding cooling phase of the phenomenon — which masked the “rise that you would have otherwise expected from three years of unabated greenhouse gas concentration increases in the atmosphere”.

“We weren’t setting records in 2020, 2021 and 2022, simply because even though greenhouse gas concentrations continued to rise, the ocean was storing a lot of heat that would have otherwise shown up in the atmosphere,” he adds.

“Then we had this big El Niño come along in 2023 and boom, all that heat comes back out of the ocean. And now we set record after record and record after record.” 

Scientists also point to other factors that may have contributed to recent record-breaking rises. Several mention rules introduced in 2020 that promote cleaner shipping fuel; the switch has reduced levels of sulphur dioxide, a significant pollutant which also forms aerosols that thicken and brighten clouds, reflecting the sun’s rays back into space.

Research published in May argued that reduced sulphur dioxide emissions could lead to a doubling, or more, of the warming rate of seas in the 2020s compared with the rate since 1980.

Other possible contributors include one-off events such as the eruption of the Hunga Tonga-Hunga Ha’apai submarine volcano in 2022, or natural variability, says von Schuckmann.

“Unusual atmospheric conditions”, such as calm weather, have played a role in driving up sea temperatures, as well as the maximum of the solar cycle, she adds.

There may be a period of respite because of La Niña, but it’s not going to make the problem go away

Hirschi argues that the jet stream — a core of strong high-altitude winds — could also have contributed to the marine heatwave in the north-east Atlantic last year. The jet stream “swerved north of Britain and Ireland and got stuck in this particular course for more than a month”, allowing the stiller waters to its southern flank to absorb the energy of the sun.

Ultimately, von Schuckmann says, the equilibrium between the Earth’s land, oceans and atmosphere has been upset, with consequences for the water cycle, immediate weather patterns and the longer-term climate.

“In a stable climate, the amount of energy we receive from the sun is in equilibrium with the amount of energy leaving the system,” she says. “But because we have a greenhouse effect, there is less energy leaving the system and this results in an Earth energy imbalance.”

After June set another record for sea surface temperatures there were signs that the warmth was levelling off by the end of that month, especially in the Pacific.

Yet by July, global sea surface temperatures were on the rise again, with a large contribution coming from oceans other than the tropical Pacific, including the north-west Pacific, the Indian Ocean, central Atlantic and Mediterranean Sea.

The World Meteorological Organization still puts the chances of a switch to the La Niña cooling phase between August and November at 70 per cent, given the moderation in tropical Pacific temperatures and past seasonal patterns of sea temperatures.

The expectation then is that both ocean and land surface temperatures begin “to start coming down” as the shift from El Niño to La Niña takes place, says McPhaden, who has studied El Niño for 40 years. But he cautions that there will be a lag and a risk that temperatures continue at high levels this year.

He also warns that sea temperatures “will not fall back to what they were [a few years] before”, noting that they are now higher during a La Niña year than they were during the record El Niño events in 1997 and 1998.

That indicates a long-term warming trend that is unlikely to stop until greenhouse gas emissions decrease. “As long as we continue to pump heat-trapping greenhouse gases in the atmosphere, global temperatures will rise with ups and downs from one year to the next related to El Niño and La Niña.”

Meredith agrees that any levelling off or reduction in temperatures due to La Niña will be shortlived. “There may be a period of respite because of La Niña — and we should be thankful for that — but it’s not going to make the problem go away,” he says.

“La Niña is not our get-out-of-jail-free card here,” he adds. “We still have to tackle global warming to get out of this problem.”

The failure to halt rising sea temperatures is already having dire consequences for marine ecosystems.

“Living systems” have little time to adjust to the rapid changes, says Sabrina Speich, a physical oceanographer and professor at the École normale supérieure, pointing to corals as an example.

In April, Noaa said the world was undergoing its fourth global coral bleaching event and the second in just 10 years. Bleaching occurs when rising sea temperatures cause corals to expel the algae living in their tissues, turning them white.

It said bleaching-level heat stress was extensive across the Atlantic, Pacific and Indian Ocean basins, with the mass bleaching of corals in Florida, the Caribbean, Brazil, Australia’s Great Barrier Reef, the Red Sea, the Persian Gulf and in large areas of the tropical Pacific and the south Pacific.

Warmer waters hold less oxygen and more soluble carbon dioxide, making them more acidic, dissolving the calcium structure that makes up the coral and affecting marine life. Ocean acidity has risen by approximately a quarter since about 1850, the year generally accepted as marking the start of the industrial era of human-caused warming.

Rising sea temperatures can affect tourism and disrupt fishing, as some species seek cooler waters and head for higher latitudes. Warming water expands, raising sea levels and putting low-lying land in peril. They can also accelerate the melting of ice caps, releasing more water into the ocean.  

This is the commitment we make for climate change, for warming of hundreds to thousands of years

There are growing concerns that warming seas are bringing the world closer to so-called climate tipping points, where what may seem small disturbances cause disproportionately large changes in the Earth system.

In June, the British Antarctic Society said it had observed a “new and worrying way” that large ice sheets can melt, as relatively warm seawater laps at the underside of ground-based ice and accelerates its movement into the ocean. It warned that this was a possible new tipping point in Antarctic ice sheet melting that had not been taken into account in models before. 

Atmospheric systems draw their energy from the ocean, so rising water temperatures may mean storms become “more energetic, more powerful, more destructive”, says Meredith, adding that storm surges, floods and an inundation of saline water into agriculture may also become more frequent.

“That’s a big financial cost, a big human cost in terms of destruction, infrastructure and even loss of life. So, the consequences of this are very serious,” he says.

Most fundamentally, there is the question of whether the rapid spike in ocean temperatures indicates the oceans are now struggling to absorb heat and carbon dioxide at the rate they once did.

“It doesn’t take massive shifts in the rate of which [heat and CO₂ are] being absorbed by the ocean to have very noticeable consequences for us,” warns Meredith.

Other scientists strike a more cautious note. “If the rate of uptake of heat and CO₂ in the ocean slows down, atmospheric temperatures will rise even more sharply than they have to date. That is a legitimate concern,” says McPhaden.

But he argues that the “ocean has tremendous heat capacity”, noting that most of the heat so far has been stored in the upper layers but scientists are now trying to understand the role deeper layers could play.

Daily readings from satellites provide a picture of surface temperatures while a network of about 3,000 robotic floats, known as Argo, “give very good readings” for the upper 2,000 metres of the ocean, says Meredith. But below that layer, data is “much more sparse”.

“There’s an awful lot of ocean below that 2,000-metre layer. We need to know how that works too, even though most of the heat is in the upper ocean,” says Meredith.

While the upper layers communicate with the atmosphere and can release heat, von Schuckmann argues that warming the deeper layers of the oceans will have long-term consequences. “This is the commitment we make for climate change, for warming of hundreds to thousands of years.”

Even if the lower layers take up some of the heat, it still will not be enough, warns Meredith. “There are a handful of places in the world where water is made dense enough that it sinks down from the surface right down to the bottom layers,” says Meredith, taking heat and carbon dioxide in the process.

“But it won’t be anywhere near enough to counteract what we are seeing in the upper ocean.”

Hirschi says the record sea temperatures of the past year should not come as a surprise, given what humanity continues to inflict upon the planet.

“It is not an extra reason to worry,” he says. “This is global warming. This is what global warming looks like.”

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