The oceans are talking to us, and it's not a pleasant conversation

By now, most people are well aware that 2023 was the hottest year on record, coming in at 1.48 C warmer than the pre-industrial average from 1850-1900. This beat out 2016's record of 1.25 C.

In climate terms, an increase in warmingby0.23 C is considerable and climate scientists are still trying to figure out why it happened.

Was 2023 just a big blip on the upward trend of global warming? Perhaps. Scientists are trying to tease out all the potential contributors.

But one of the things that is inarguably a contributor is the continued warming of our oceans

Last year, our oceans were the hottest on record. It was the first year in which the average global sea surface temperatures (SST) that is, temperaturesof the upper metre of water surpassed 1 C compared to pre-industrial levels.

Then there's the ocean heat content (OHC), which is the temperature 2,000 metres below the surface. That, too, was at a record high in 2023. And this is of particular concernbecause OHC is a critical climate indicator. Our oceans, which cover more than 70 per cent of the planet, store more than 90 per cent of Earth's excess heat.

Just how much heat did our oceans take in last year?

To put it in perspective, Zeke Hausfather,a research scientist at Berkeley Earth, a non-profit, independent climate analysis organization,said that we added 15 zettajoules of energy to the oceans compared to2021. That's the equivalent toa billion trillion joules or the equivalent of 25 times of all the energy used by humanity.

"It's a pretty mindbogglingly big number," he said.

And that ocean heat content is continually rising. The more CO2 we pump into the atmosphere, the more the oceans will absorb.

We may be forgiven for not paying as much attention to the oceans as we should. After all, as human beings, we look at global heat as it occurson the surfacebecause that's where we live,that's what we experience.

But for climate scientists,the oceans are a critical indicator.

"For climate scientists who think about the climate system as a whole the oceans are the most important thing," said Simon Donner, a climate scientist and professor at the University of British Columbia. "It's because they cover twothirds of the planet, they're very deepand they're made up of water, not air.And water has got a high heat capacity.

"So they are like the planet's great heat sink."

Looking at 2023

Taking a look at just the oceans, there were many different contributors to 2023's record heat.

First, there was an El Nio. Though it wasn't a super event, as it was in 2015-16, it's still been fairly significant. This warming of the central Pacific Ocean tends to produce an increase in the global temperature. But typicallythere is a lag, meaning the warmer temperatures aren't seen until the year following the start of an El Nio.

"This year has been unusual, even for an El Nio. If we look at right now, globally, [the SST is] about 21.0. C. By SST data, that's about 0.2 C above where we were at this time in 2016, which is the last big El Nio event," said Hausfather. "And 2016 was, to be honest, a much bigger El Nio event. And so you know, a 0.2 C increase relative to then in eight years is a little worrying."

But Hausfatherand Donnerbelieve part of this may have been because we came from a "triple-dip" La Nia, the little sister to El Nio where ocean temperatures are cooler than normal. Those three years from 2020 through 2022 could have masked the warming, they say.

There were other ocean phenomena that contributed to the warmth.

The North Atlantic experienced a marine heat wave, which is a prolonged warmth in a specific area of the ocean. That's why we saw hot tub-like temperatures off the coast of Florida.

And the North Atlantic wasn't the only hot spot.

"There are marine heat waves in basins all over: in the Atlantic, both north and south; [the] Indian Ocean, Pacific Ocean," said Josh Willis, a climate scientist with NASA's Jet Propulsion Laboratory. "There are these surface temperature areas that are just unusually warm. And some of that has also added to [last] year's record."

'The genie doesn't go back in the bottle'

All of this ElNio, the record-breaking sea surface temperatures, the increase in ocean heat content, the marine heat waves has far-reaching consequences.

Coral reefs are particularly sensitive to ocean temperature changes, which cause mass coral bleaching events. This is when critical algae is forced out of the reef's living tissue, turning them from beautiful bright colours to white. The reef isn't dead, but it is more susceptible to mortality.

Then there's the impact on marine life, which has economic and sustainability challenges.

"The ocean ecosystem is really critical for life on Earth, including ours," Willis said. "And it's under enormous pressure. And it's pressure that most of us don't see."

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And there's the most pressing issue for humans: rising sea levels.

Willis noted that it takes a long time to add heat to the ocean and a lot of heat to change its temperature. As the oceans warm, the water "literally stands taller."

And rising sea levels can't be reversed.

The Intergovernmental Panel on Climate Change's recent report noted that, sea level rise is "unavoidable for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and sea levels will remain elevated for thousands of years."

"The genie doesn't go back in the bottle. Once the sea levels have risen, the odds of us being able to make them fall againare very, very small," said Willis.

While we may not be able to reverse it, there is one solution in order to prevent things from becoming far worse: moving away from fossil fuels.

"Everything's absorbing more heatbecause we keep adding greenhouse gases to the atmosphere," Donner said. "If we want it to stop absorbing more heat, we need to stop that."