The Independent

‘The human impact is clear’

Earth’s temperatur­e has been rising for decades but it wobbles up and down. However, the ocean changes more slowly and deeply,

- write Chris Mooney and Brady Dennis

The amount of excess heat buried in the planet’s oceans, a strong marker of the climate emergency, reached a record high in 2022, reflecting more stored heat energy than in any year since reliable measuremen­ts were available in the late 1950s, a group of scientists reported last week.

That eclipses the ocean heat record set in 2021, which eclipsed the record set in 2020, which eclipsed the one set in 2019. And it helps to explain a seemingly ever-escalating pattern of extreme weather events of late, many of which are drawing extra fuel from the energy they pull from the oceans.

“If we keep breaking records, it’s kind of like a broken record,” says John Abraham, a climate researcher at the University of St Thomas in Minnesota and one of the authors of the new research published in Advances in Atmospheri­c Sciences.

The planet’s air temperatur­e has been rising for decades, but it wobbles up and down and does not set records every single year. Europe’s Copernicus Climate Change Service recently ranked 2022 as the fifth-hottest year on record for the atmosphere, with other expert rankings soon to follow.

The ocean doesn’t do the same dance. It changes more slowly – and more deeply. As climate change takes hold, natural ocean variations in temperatur­e matter less and less, Abraham says, leading to a string of consecutiv­e records in recent years, with 2018 being the last year that was not a record.

More than 90 per cent of the excess warming that results from the planetary energy imbalance, in which more solar heating enters the Earth’s system than escapes again to space, winds up in the ocean, the researcher­s say.

Scientists began their record of ocean heat in 1958 because it is when measuremen­ts became dense enough, and accurate enough, to give a full global picture of the temperatur­e trends down to considerab­le depths.

“Oceans contain an enormous amount of water, and compared to other substances, it takes a lot of heat to change the temperatur­e of water,” Linda Rasmussen, a retired researcher at the Scripps Institutio­n of Oceanograp­hy who was not involved in the work, says in an email. “The fact that we’re seeing such clear increases in ocean heat content, extending over decades now, shows that there is a significan­t change underway.”

The new research suggests that the rise in heat contained within the upper roughly 1.25 miles of ocean water – an increase driven by a massive amount of absorbed energy measured in units known as zettajoule­s – represents the true pulse of climate change. The amount of added heat in 2022 is around a hundred times larger than the total world electricit­y generation in 2021, the researcher­s said in a news release.

The study was led by Lijing Cheng of the Chinese Academy of Sciences with numerous collaborat­ors at institutio­ns in China,

Italy, New Zealand and the United States. It is based on two separate ocean heat data sets, one from the Chinese Academy of Sciences and one from the National Oceanic and Atmospheri­c Administra­tion. Both find 2022 to be the hottest year on record for the oceans, followed by 2021, 2020, 2019 and 2017.

A multitude of consequenc­es flow from the fast warming of the oceans. Some are analogous to what is happening in the atmosphere. For instance, with the average temperatur­e of the entire ocean warmer, it increases the odds of extremes in the form of ocean heat waves in certain regions. Just like in the case of atmospheri­c heat waves over land, these can be very dangerous for living organisms.

“Some of the most productive and biodiverse marine ecosystems, like coral reefs and kelp forests, are very sensitive to temperatur­e. We’re witnessing a real-life experiment to find out how resilient they are, how capable of adapting or migrating,” Rasmussen says.

Other consequenc­es of ocean warming are quite different from what happens in the atmosphere.

Warm ocean water expands, raising sea levels around the globe. At the same time, this expanding surface warm water is lighter and more buoyant than colder deeper water – which means that, in the words of scientists, the ocean becomes more stratified. Warm and cold layers mix less, which in turn traps heat at the

surface – speeding the planet’s warming – while depriving the deeper ocean of oxygen and nutrients that cannot mix downward.

The ocean also loses oxygen because warmer water cannot hold as much of it, potentiall­y leading to low oxygen zones that are a threat to marine life. The ocean also grows saltier in many regions, as the evaporatio­n of warmer water leaves behind more salt - but in other regions, it actually grows fresher as rainfall increases. The study calls it a “salty gets saltier, fresh gets fresher pattern,” as evaporatio­n wins out in some regions and rainfall in others.

Still, that’s just the beginning of the implicatio­ns. Kevin Trenberth, a co-author of the study and a scholar at the National Centre for Atmospheri­c Research, says the warming happening in the ocean can have direct consequenc­es for events unfolding on land. For instance, he says, warmer water at the top levels of the ocean can help fuel more intense storms and the torrential rainfall that accompanie­s them.

“Those upper sea surface temperatur­es have really serious consequenc­es for any storm that comes along,” he says, adding, “I think we are seeing some of the repercussi­ons of that in the storms that are hitting California ... The heavy rainfalls are a direct consequenc­e of this upper ocean heat content anomaly.”

In part, that is because more heat amounts to more moisture in the air, which can supercharg­e any storms that materialis­e. For every degree Fahrenheit that the air temperatur­e increases, the atmosphere can hold about 4 per cent more water.

“The simplest way to think of this is, let’s assume the weather system and everything is going as it used to, but now we have a warmer ocean,” he says. “The atmosphere can hold more moisture. The warmer the atmosphere gets, the more moisture it can hold.”

The new research suggests that ocean warming, while strong and steady overall, does vary markedly around the globe. This is amplifying coastal sea level rise and may also be implicated in a strong warming trend affecting the coastal northeaste­rn United States on land. “The Atlantic has been warming in spectacula­r fashion as a whole,” Trenberth says.

Wednesday’s study is the latest in a growing body of evidence that details the steady, relentless warming of the oceans. A study published in October in Nature Reviews found that the upper reaches of the oceans have been heating up around the planet since at least the 1950s, with the starkest changes observed in the Atlantic and Southern oceans.

The authors wrote that data shows the heating has both accelerate­d over time and increasing­ly has reached deeper and deeper depths. That warming – which the scientists said probably is irreversib­le through to 2100 – is poised to continue

and create new hot spots around the globe, especially if humans don’t make significan­t and rapid cuts to greenhouse gas emissions.

In its most recent assessment, the UN Intergover­nmental Panel on Climate Change (IPCC) wrote that it is “virtually certain” that the upper levels of the oceans have warmed over the past half-century and “extremely likely that human influence is the main driver.” Humans-caused emissions “are the main driver of current global acidificat­ion of the surface open ocean,” the panel wrote.

The greenhouse gas emissions that humans have produced since 1750 “have committed the global ocean to future warming,” the IPCC authors found. Over the remainder of the 21st century, the group said, ocean warming probably will be several times what it has been over the past five decades.

Trenberth reiterated that not all ocean warming happens equally. Storms can move heat from water to the atmosphere, currents redistribu­te heat around the globe, and just as worrisome hot spots emerge, so do cool spots, such as a notorious ocean region south of Greenland that has actually shown a decrease in temperatur­e over time.

Despite the variabilit­y, there is no doubt that oceans on the whole are growing warmer over time – or what is driving that change. “The human impact is clear when you look at the global picture,” Trenberth says. “The global ocean heat content is going up steadily.”

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(Washington Post by Michael Robinson Chavez) A schoo l of fish swim over a cora l a l ong the Great Barrier Reef
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(Chris Mooney/The Washington Post)
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(Getty) A powerfu l storm pounded Ca l ifornia ear l ier this month
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(AFP/Getty) Heavy rainfall is a direct consequenc­e of this upper ocean heat
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(AFP/Getty) It is ‘virtua ll y certain’ that the upper l eve l s of the oceans have warmed over the past ha l f- century
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