A pyrocumulonimbus cloud, a looming thunderstorm generated by an intensely hot wildfire, has been observed with radar in France for the first time. These deadly “fire clouds� of rising hot air, smoke and moisture, which accelerate blazes with strong winds and soaring embers, are expected to become more common in Europe as wildfires there get bigger.  

“You have this tilted column that is sucking air and then propagating embers in front, and it also creates lightning. That’s another way of lighting up fuels in front of the fire, so these cells are unstoppable,� says at the University of Corsica, France. “You cannot stop a thunderstorm, and neither can you stop a firestorm, a strong pyrocumulonimbus.�

Alongside huge blazes in Spain, France has been suffering a wildfire in the Gironde region, which President Emmanuel Macron has called the country’s most severe crisis since the second world war. The fire has burned more than 420 square kilometres since last week, coming within 15 kilometres of the city of Bordeaux and forcing around 220,000 people to evacuate. It generated a pyrocumulonimbus cloud on 24 July and another on 25 July, according to regional officials. Embers and lightning ignited new fires around the original inferno.

“We faced another firestorm, the same incredible, unprecedented, unknown phenomenon as the one we saw on Friday, when we realised we were entering a new dimension,� Gironde prefect Sophie Brocas told journalists.

It is a miracle this pyrocumulonimbus-fuelled fire hasn’t claimed any casualties, Filippi says.

Sometimes the “fire-breathing dragon of clouds�, pyrocumulonimbus are typically generated by massive wildfires in the forests of North America, Siberia and Australia. They’ve spurred on infamous fires like the that killed 173 people in Australia in 2009 and the conflagration that in 2016. Canada’s record-breaking wildfires in 2023 142 of these storms.

In Europe, however, there have been only a handful of confirmed pyrocumulonimbus in Spain and Portugal, including one during Portugal’s devastating Pedrógão Grande fire in 2017, which destroyed hundreds of homes and killed 66 people. But that appears to be changing.

“You have to have two things happening at the same time: an extreme heat source and an unstable atmosphere that interacts with the fire,� says at the European Centre for Medium-Range Weather Forecasts. “If… the heat from the fire increases, like it’s projected to increase from our studies, then yes, you should see more of these events.�

This firestorm begins to take shape when temperatures as high as 800°C send a plume of smoke into the sky. As the hot air rises and cools, water vapour condenses onto the ash to form pyrocumulus clouds, like the fluffy white clouds you might see on a summer day, only brown or grey.  

If the plume is hot enough and the atmosphere is unstable enough, the cloud can keep rising to reach the upper troposphere at 10,000 metres, the cruising altitude of a passenger plane. It sometimes reaches the stratosphere, as much smoke as a moderate volcano. It becomes a pyrocumulonimbus when the static electricity from colliding ice particles sparks thunder and lightning.

Although rain from the cloud often evaporates in the hot air, this kind of thunderstorm nonetheless creates its own weather system. The cloud’s updraft pulls in air, fanning the flames with winds reaching 160 kilometres per hour and accelerating the fire’s spread by up to five times, according to Filippi.

Meanwhile, strong downdrafts create erratic gusts that carry embers off to start new fires. Dozens of lightning strikes can blazes as far as 100 kilometres ahead of the main fire. It can also create fire tornadoes of twisting flames. As the winds, embers and lightning strengthen and expand the wildfire, it releases more heat and smoke, a feedback loop that can amplify the rising cloud for many hours.

Both wind shear higher in the atmosphere and erratic gusts at the ground can take the fire in unpredictable directions, making it nearly impossible to contain its acceleration or plan targeted evacuations.

“The fire front is constantly shifting. We cannot fight it directly. It’s a David-versus-Goliath scenario,� Eric Brocardi of the National Firefighters Federation of France local media about the Gironde fire on 2 August.  

Wildfire plumes have expanded in height since 2003 in the western US, creating more pyrocumulonimbus clouds, an increase that’s projected to continue.

Pyrocumulonimbus will have more chances to occur in Europe as well, as the fastest-warming continent experiences more extreme heat, such as the record-breaking heatwave in June, and wildfires. could also contribute, such as when a wet spring last year boosted vegetation growth on the Iberian peninsula, only for record summer heat to dry it out. This led to Europe’s worst-ever wildfires, including a in Spain.

“We need to adapt, so get prepared,� Filippi says. “Burn-proof and reduce the amount of fuel around your house.�

Short-term geoengineering to brighten clouds over the eastern Pacific Ocean could limit the damage caused by El Niño and save the global economy trillions of dollars, although there could be winners and losers from the disruption of natural cycles.

The El Niño climate phase occurs when easterly winds weaken, allowing warm water built up in the western Pacific to slosh back across the central and eastern parts of the ocean. That heats the atmosphere and raises global temperatures, with losses to economic growth estimated in the trillions of dollars.

What could become a very strong or “super� El Niño is now developing in the eastern Pacific. But climate modelling has suggested that, in the future, a geoengineering method called marine cloud brightening might be able to cut this warming short.

The technique involves spraying tiny droplets of seawater into the air below low-lying stratocumulus clouds, where moisture condenses onto them. The clouds become whiter thanks to the increase in the number of droplets, reflecting more sunlight back to space.

Shading part of the eastern Pacific called the Niño 3.4 region via cloud brightening could interrupt the feedback loops that cause an El Niño to develop. Cooler sea surface temperatures would strengthen the trade winds to again blow warm water back into the western Pacific. More cool water would then well up from the depths of the eastern Pacific, further cooling surface temperatures, and so on.

“You can basically stop the dominoes from falling early when you do marine cloud brightening,� says at the University of California, San Diego, who worked on the study. “We’re kicking the cycle in the other direction.�

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Wan and her colleagues got the idea from the “black summer� of catastrophic bush fires in Australia in 2019-2020, which were followed by La Niña, the opposite phase of El Niño that lowers global temperatures. Research has that drifting smoke particles brightened clouds and cooled the eastern Pacific, intensifying and prolonging the “triple dip� La Niña that began in 2020 and persisted through three winters, rather than just one or two.

The study modelled what cloud brightening could have done to the super El Niño events of 1997-1998 and 2015-2016. It found that nine months of spraying seawater would have nearly halved warming of the Niño 3.4 region, from 2°C or more to a little over 1°C. It would have ended the El Niño by January, shaving several months off the events.

The hypothetical cloud-brightening mission would have been massive, involving an estimated 2400 ships and delivering an amount of seawater spray that isn’t possible with current nozzle technology. But it would have turned a super El Niño into a moderate one.

Wan says she was surprised how well it seemed to work, given that it could only be started in June, once El Niño had clearly begun developing.

at the University of Exeter, UK, warns that these results might not translate to the real world, where warming seas typically start dissipating low-level clouds, leading to further warming and dissipation through a feedback loop.

“In a model with a stronger cloud feedback, you would have to do more aerosol injection,� he says. “The experiments seem to be at the limit of what can be done.�

Wan admits this approach could have unexpected consequences, since the model only projected the impact over two-year periods. In both simulations, La Niña started earlier after El Niño subsided, and in the 2015-2016 case, this subsequent cooler phase became stronger. That could be bad news for regions like the Horn of Africa, where strong La Niñas have, in the past, disrupted rainfall and .

But she says the idea is worth further research. Unlike geoengineering aimed at reducing global temperatures for the long term, short-term geoengineering like this could avoid the risk of “termination shock�, where any disruption to the spraying of low-level seawater or stratospheric aerosols could allow years of pent-up warming to come roaring back.

“This study is opening up doors for a completely new target for geoengineering research, which is climate variability and things like El Niño,� says Wan. “It’s potentially very powerful, because you’re not locked into these long-term risks.�