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A recent rapid warming of the world’s oceans has alarmed scientists who are concerned it will add to global warming.
This month, the global sea surface hit a new record high temperature. It has never heated up so much, so quickly.
Scientists don’t fully understand why this happened.
But they fear that, combined with other weather events, global temperatures could reach a worrying new level by the end of next year.
Experts believe that a strong El Niño weather event, a weather system that warms the ocean, will also occur in the coming months.
Warmer oceans can kill marine life, lead to more extreme weather and raise sea levels. They are also less efficient at absorbing the greenhouse gases that warm the planet.
A major new study, released last week with little fanfare, highlights a worrying development.
In the last 15 years, the heat stored by the Earth has increased by 50%, with most of the extra heat going to the oceans.
This is having real-world consequences: not only did the overall temperature of the oceans hit a new record in April this year, but in some regions the difference over the long term was huge.
In March, sea surface temperatures off the east coast of North America were as much as 13.8C above the 1981-2011 average.
“It’s still not clear why there’s such a rapid change and such a huge change,” said Karina Von Schuckmann, lead author of the new study and an oceanographer at the Mercator Ocean International research group.
“We’ve doubled the heat in the climate system in the last 15 years. I don’t mean whether it’s climate change, or natural variability or a combination of both, we don’t know yet. But we see this change.”
One factor that could affect the level of heat entering the oceans is, curiously, a reduction in pollution from ships.
In 2020, the International Maritime Organization put in place a regulation to reduce the sulfur content of fuel burned by ships.
This had a rapid impact, reducing the amount of aerosol particles released into the atmosphere.
But the aerosols that litter the air also help reflect heat back into space: their removal may have caused more heat to enter the waters.
What are the impacts of ocean warming?
The average surface temperature of the world’s seas has increased by about 0.9°C compared to pre-industrial levels, with only 0.6°C in the last 40 years.
This is less than the increase in air temperature over land, which has risen by more than 1.5°C since pre-industrial times. This is because much more energy is needed to heat water than on land and because the oceans absorb the heat far below their surface.
Even this seemingly small average increase has significant real-world consequences.
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Species loss: More frequent and intense marine heat waves lead to mass mortality of marine life. This is especially bad for coral reefs.
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More extreme weather: Increased heat in the upper ocean surface means hurricanes and cyclones can harvest more energy. This means that they become more intense and long-lasting.
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Sea level rise: Warmer waters take up more space — known as thermal expansion — and can greatly accelerate the melting of Greenland and Antarctica glaciers as they flow into the oceans. This raises global sea levels, increasing the risks of coastal flooding.
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Less ability to absorb CO2: Oceans currently absorb about a quarter of greenhouse gas emissions. Warmer waters have a lower capacity to absorb CO2. If the oceans absorbed less CO2 in the future, more would accumulate in the atmosphere, further warming the air and oceans.
Another major factor that worries scientists is the meteorological phenomenon known as the El Niño Southern Oscillation.
For the past three years this natural event has been in a colder phase called La Niña and has helped keep global temperatures in check.
But researchers now believe a strong El Niño is brewing that will have significant implications for the world.
“The Australian Bureau’s model goes strongly for a strong El Niño. And it has trended like this and all the climate models have trended like this towards a stronger event,” said Hugh McDowell of Australia’s Bureau of Meteorology.
McDowell cautioned that forecasts at this point in the year are less reliable. Other researchers are more optimistic.
A coastal El Niño has already developed off the coasts of Peru and Ecuador, and experts believe a full-blown event with implications for global temperatures will follow.
‘If there is another El Niño, we will probably have an additional 0.2-0.25°C of global warming,’ said Dr. Josef Ludescher, of the Potsdam Institute for Climate Research.
“The impact on temperature is attenuated a few months after the peak of any El Niño, so this is why 2024 is likely to be the warmest on record.”
“And we could, we’ll be close to 1.5C days and maybe we’ll exceed it temporarily.”
El Niño will likely disrupt weather patterns around the world, weaken the monsoon and threaten more bushfires in Australia.
But there are more fundamental concerns that as more heat goes into the ocean, the waters may be less able to store the excess energy.
And there are concerns that the heat held in the oceans won’t stay there.
Several scientists contacted about this story were reluctant to write down the implications.
One spoke of being “extremely worried and completely stressed out.”
Some research has shown that the world is warming in jumps, where small changes over the years and then there are sudden jumps, like rungs on a ladder, closely related to the development of El Niño.
There is some hope in this scenario, according to Karina Von Schuckmann. Temperatures could drop again after the El Niño subsides.
“We still have a window in which we can act and we should use it to mitigate the consequences,” he told BBC News.
Graphics by Erwan Rivault.
