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Oceans are heating up, and El Nino will make it worse

The latest readings from more than 4,000 buoys around the world show record-breaking sea-surface temperatures from January to March this year
Last Updated : 15 May 2023, 21:01 IST

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Even if you hate the beach, live inland and aren’t bothered by dwindling fish, the latest spike in ocean temperature matters to you. The ocean is like a huge closet where we’ve been able to store 90 per cent of the excess heat trapped by greenhouse gases. That closet is now stuffed. The latest readings from more than 4,000 buoys around the world show record-breaking sea-surface temperatures from January to March this year.

And we’re on the cusp of an El Nino event — a shift in wind patterns and ocean currents that opens the closet door and lets heat and energy spill into our atmosphere. That’s where the heat buried in the ocean comes back to haunt us.

The changes in sea-surface temperatures don’t sound like much — right now they’re about 0.2 degrees C above normal. But it sounds like a lot when expressed as energy added to the system that includes our ocean and atmosphere — 40 zettajoules, or sextillions of joules. That’s the equivalent of hundreds of millions of atomic bombs. It’s energy that gets trapped in the system by an increasing concentration of greenhouse gases.

That energy can manifest as heat waves or storms, said Kevin Trenberth, a climatologist I met a few years ago at a meeting in Princeton on the El Nino phenomenon. He was then at the National Centre for Atmospheric Research in Colorado, and is now an honorary academic at the University
of Auckland.

While the latest news has focused on new surface measurements, he said, the warming goes deeper. Last year he co-authored a paper showing the ocean is warming down to 2,000 metres. The heat doesn’t stay there, thanks to the alternating patterns of wind and currents known as El Nino and its opposite counterpart, La Nina.

La Nina has prevailed during the last three years. During this phase, Trenberth said, trade winds push warm equatorial water westward across the Pacific where it piles up by Indonesia. Every two to seven years, that switches to the El Nino pattern. The easterly winds die down enough that the warmer water can slop back toward the Americas.

El Nino events can be gentle or fierce, depending on whether strong westerly winds start helping that warm water slosh back across the Pacific, said Kim Cobb, an oceanographer and climatologist at Brown University. But big ones happen roughly every 10 to 15 years. Real whoppers, she said, have occurred in 1972-73, 1982-83, 1997-98 and 2015-16.

While El Nino is born in the Eastern Pacific, the change in patterns redistributes heat around the planet — disturbing the entire global atmospheric circulation, she said. The cycle of El Nino and La Nina has probably been going on for millions of years, since our continents and oceans reached in their current configuration.

Now, however, with rapid overall warming, the cycle might become stronger, and is already creating more extreme conditions. “When we think about what sets the thermostat of our planet, it really is our oceans,” she explained.

Trenberth said that even during the La Nina phase we’ve seen hot spots in the Pacific that have led to trouble: “This was the region that was feeding the atmospheric rivers that flowed into California and produced all the prodigious rains and even snows in the southwestern parts of the United States.”

Even though there’s more capacity for heat deep in the ocean, the water heats up at the surface first, and since cold water is heavier, that can create regions of unhealthy stillness, where normal circulation stops and blobs of stagnant oxygen-depleted ocean grow. That’s why scientists have been observing alarming dead zones in the Gulf of Mexico, which are made worse by nitrate runoff from farms.

“It affects the phytoplankton, the zooplankton, the fish, the marine mammals and seabirds,” Trenberth said. Warming oceans also lead to marine heat waves, which can destroy kelp forests and grasses that provided a home for fish and marine mammals.

Trenberth uses the phrase global heating to describe what’s going on. Global warming implies increasing atmospheric temperatures, while global heating suggests extra energy sloshing around the whole system that is our atmosphere and oceans.

He recently found some old VHS tapes of his interviews in the 1990s, and sent a 1997 interview from the MacNeil/Lehrer News Hour, where he talks about the danger of global heating. Watching it 26 years later, not much has changed in the basic underpinnings of the science. But the climate certainly has.

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Published 15 May 2023, 17:43 IST

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