By now, most people know very well that 2023 was the hottest year on recordwhich is 1.48°C warmer than the pre-industrial average from 1850 to 1900. This beat the 2016 record of 1.25°C.
In climate terms, an increase in warming of 0.23°C is considerable and climatologists are still trying to understand why this happened.
Is 2023 just a big blow in the upward trend of global warming? Maybe. Scientists are trying to identify all the potential contributors.
But one of the undeniably contributing factors is the continued warming of our oceans.
Last year our oceans were the warmest on record. This was the first year in which the average sea surface temperature (SST), that is, the temperature of the top meter of water, exceeded 1°C above pre-industrial levels.
Then there is the ocean heat content (OHC), which is the temperature 2,000 meters below the surface. This figure also reached a record high in 2023. And this is particularly concerning because OHC is a critical climate indicator. Our oceans, which cover more than 70 percent of the planet, store more than 90 percent of Earth’s excess heat.
How much heat did our oceans absorb last year?
To put things into perspective, Zeke Hausfather, a research scientist at Berkeley Landan independent, non-profit climate analysis organization, said we have added 15 zettajoules of energy to the oceans compared to 2021. This amounts to a trillion trillion joules, or the equivalent of 25 times the entire energy used by humanity.
“It’s a pretty staggering number,” he said.
And the heat content of the oceans continues to increase. The more CO2 we release into the atmosphere, the more the oceans absorb.
We could be forgiven for not paying as much attention to the oceans as they should. After all, as humans we observe global heat as it occurs on the surface, because that is where we live, that is what we experience.
But for climatologists, the oceans constitute an essential indicator.
“For climate scientists thinking 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. “That’s because they cover two-thirds of the planet, they’re very deep, and they’re made of water, not air. And water has a high heat capacity.
“So they are like the great heat sink of the planet.”
Looking towards 2023
Looking only at the oceans, many different factors contributed to 2023’s record-breaking heat.
First there was El Niño. Even if it wasn’t a great event, as it was in 2015-2016, it was still quite important. This warming of the central Pacific Ocean tends to produce an increase in global temperature. But there’s usually a lag, meaning warmer temperatures aren’t visible until the year after an El Niño begins.
“This year has been unusual, even for an El Niño. If we look right now, globally, (the SST is) about… 21.0°C. Based on the SST data, that’s about 0 .2°C above where we were at this time in 2016, which is the last big El Niño event,” Hausfather said. “And 2016 was, to be honest, a much bigger El Niño event. And you know, a 0.2°C increase from that year in eight years is a little bit worrying.”
But Hausfather and Donner think part of the reason is that we’re coming from a “triple dip” La Niña, El Niño’s little sister, where ocean temperatures are cooler than normal. These three years – from 2020 to 2022 – could have masked the warming, they say.
Other ocean phenomena contributed to the heat.
The North Atlantic experienced a marine heatwave, which is prolonged heat in a specific area of the ocean. This is why we saw hot tub-worthy temperatures off the coast of Florida.
And the North Atlantic wasn’t the only hot spot.
“There are marine heat waves in all basins: in the Atlantic, north and south; in the Indian Ocean, the Pacific Ocean,” said Josh Willis, a climatologist at NASA’s Jet Propulsion Laboratory. . “There are areas of surface temperature that are just unusually warm. And some of that contributed to last year’s record as well.”
“The genie doesn’t go back in the bottle”
All of this – El Niño, record sea surface temperatures, increasing ocean heat content, marine heatwaves – has far-reaching consequences.
Coral reefs are particularly sensitive to changes in ocean temperature, which cause massive coral bleaching. This is when critical algae is expelled from the living tissues of the reef, turning them from beautiful bright colors to white. The reef is not dead, but it is more susceptible to mortality.
Then there is the impact on marine life, which has economic and sustainability challenges.
“The ocean ecosystem is really essential to life on Earth, including ours,” Willis said. “And it’s under enormous pressure. And it’s pressure that most of us don’t see.”
And there is the most pressing problem for humans: sea level rise.
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 “gets higher.”
And sea level rise cannot be reversed.
THE The recent report of the Intergovernmental Panel on Climate Change noted that sea level rise is “inevitable for centuries, if not millennia, due to continued warming of deep oceans and melting ice caps, and that sea levels will remain high for thousands of years.”
“The genie doesn’t go back in the bottle. Once sea levels rise, the chances that we can get them to drop again are very, very low,” Willis said.
Although we may not be able to reverse the trend, there is a solution to prevent the situation from getting worse: abandon fossil fuels.
“Everything is absorbing more heat because we continue to add greenhouse gases to the atmosphere,” Donner said. “If we want it to stop absorbing more heat, we need to stop that.”