Scientists look beyond usual suspects such as El Niño to explain record temperatures 

Natural phenomena including a volcano and human activity such as reducing sulphur-producing fuels may have contributed to global warming

Natural phenomena, including El Niño, and human interventions such as burning fossil fuels are regarded as the chief causes of global warming. But scientists are also looking at other less frequently anticipated causes. Stock picture: David Jones/PA

Scientists are wondering if global warming and El Niño have an accomplice in fuelling this summer’s record-shattering heat.

The European climate agency Copernicus reported that July was 0.3C hotter than the old record. That’s a bump in heat that is so recent and so big, especially in the oceans and even more so in the North Atlantic, that scientists are split on whether something else could be at work.

Scientists agree that by far the biggest cause of the recent extreme warming is climate change from the burning of coal, oil, and natural gas that has triggered a long upward trend in temperatures.

A natural El Niño, a temporary warming of parts of the Pacific that changes weather worldwide, adds a smaller boost. But some researchers say another factor must be present.

The January 2022 eruption of Hunga Tonga Hunga Ha'apai volcano in Tonga may have an appreciable effect on climate. File picture: Maxar Technologies/AP

“What we are seeing is more than just El Niño on top of climate change,” Copernicus director Carlo Buontempo said.

One surprising source of added warmth could be cleaner air resulting from new shipping rules.

Another possible cause is 150m tonnes of water spewed into the atmosphere by a volcano. Both ideas are under investigation.

Cleaner air possibility

Florida State University climate scientist Michael Diamond says shipping is “probably the prime suspect”. Maritime shipping has for decades used dirty fuel that gives off particles that reflect sunlight in a process that actually cools the climate and masks some of global warming.

In 2020, international shipping rules took effect that cut as much as 80% of those cooling particles, which was a “kind of shock to the system,” said atmospheric scientist Tianle Yuan of Nasa.

The sulphur pollution used to interact with low clouds, making them brighter and more reflective, but that’s not happening as much now, Yuan said.

He tracked changes in clouds that were associated with shipping routes in the North Atlantic and North Pacific, both hotspots this summer.

In those spots and, to a lesser extent, globally, Yuan’s studies show a possible warming from the loss of sulphur pollution.

The trend is in places where it really can’t be explained as easily by El Niño, he said.

“There was a cooling effect that was persistent year after year, and suddenly you remove that,” Yuan said.

Diamond calculates a warming of about 0.1C by mid-century from shipping regulations. The level of warming could be five to 10 times stronger in high shipping areas such as the North Atlantic. 

Did the volcano do it?

In January 2022, the Hunga Tonga-Hunga Ha’apai undersea volcano in the South Pacific blew, spewing more than 150m tonnes of water into the atmosphere as vapour — a heat-trapping greenhouse gas — according to University of Colorado climate researcher Margot Clyne, who co-ordinates international computer simulations for climate impacts of the eruption.

The volcano also blasted 500,000 tonnes of sulphur dioxide into the upper atmosphere.

Holger Vomel, a stratospheric water vapour scientist at the US National Centre for Atmospheric Research, said the water vapour went too high in the atmosphere to have a noticeable effect yet, but that effects could emerge later.

A couple of studies use computer models to show a warming effect from all that water vapour.

Carlo Buontempo, director of the Copernicus Climate Change Service. File picture

One study, which has not yet undergone the scientific gold standard of peer review, reported this week that the warming could range from as much as 1.5C of added warming in some places to 1C of cooling elsewhere.

But Nasa atmospheric scientist Paul Newman and former Nasa atmospheric scientist Mark Schoeberl said those climate models are missing a key ingredient — the cooling effect of the sulphur.

Normally, huge volcanic eruptions, such as 1991’s Mount Pinatubo, can cool Earth temporarily with sulphur and other particles reflecting sunlight. However, Hunga Tonga spouted an unusually high amount of water and low amount of cooling sulphur.

The studies that showed warming from Hunga Tonga didn’t incorporate sulphur cooling, which is hard to do, Schoeberl and Newman said.

Schoeberl published a study that calculated a slight overall cooling — 0.04C.

Lesser suspects

Lesser suspects in the search include a dearth of African dust, which cools like sulphur pollution, as well as changes in the jet stream and a slowdown in ocean currents.

Some nonscientists have looked at recent solar storms and increased sunspot activity in the sun’s 11-year cycle and speculated that Earth’s nearest star may be a culprit.

For decades, scientists have tracked sunspots and solar storms, and they don’t match warming temperatures, Berkeley Earth chief scientist Robert Rohde said.

Solar storms were stronger 20 and 30 years ago, but there is more warming now, he said.

Still, other scientists said there’s no need to look so hard. They say human-caused climate change, with an extra boost from El Niño, is enough to explain recent temperatures.

University of Pennsylvania climate scientist Michael Mann estimates that about five-sixths of the recent warming is from human burning of fossil fuels, with about one sixth due to a strong El Niño.

The fact that the world is coming out of a three-year
La Nina, which suppressed global temperatures a bit, and going into a strong El Niño, which adds to them, makes the effect bigger, he said.

“Climate change and El Niño can explain it all,” Imperial College of London climate scientist Friederike Otto said. “That doesn’t mean other factors didn’t play a role. But we should definitely expect to see this again without the other factors being present.”

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