Cloud geoengineering could push heatwaves from US to Europe

A cloud-modifying technique could help cool the western US, but it would eventually lose its effectiveness and, by 2050, could end up driving heatwaves around the planet towards Europe, according to a modelling study.

There is growing interest in alleviating the severe impacts of global warming by using various geoengineering techniques. These include marine cloud brightening (MCB), which aims to reflect more sunlight away from Earth’s surface by seeding the lower atmosphere with sea salt particles to form brighter marine stratocumulus clouds.

Small-scale MCB experiments have already taken place in Australia on the Great Barrier Reef and in San Francisco Bay, California. Proponents hope this approach could be used to reduce the intensity of extreme heatwaves in particular regions as the climate continues to get hotter.

Katharine Ricke at the University of California, San Diego (UCSD), and her colleagues modelled the impact that a possible MCB programme to cool the western US might have under present climate conditions and projections for 2050.

The team modelled the impact of MCB in two locations in the northern Pacific Ocean: one in temperate latitudes and another in sub-tropical waters. The modelling applied MCB for 9 months out of every year for 30 years, essentially altering the long-term climate.

The researchers found that under present-day climate conditions, MCB reduces the relative risk of dangerous summer heat exposure in parts of the western US by as much as 55 per cent. However, it dramatically reduces rainfall, both in the western US and in other parts of the world such as the Sahel of Africa.

They also modelled the impact MCB would have in 2050, in a predicted scenario where global warming reaches 2°C above pre-industrial temperatures. Under these conditions, the same MCB programme was ineffective and instead dramatically warmed almost the entirety of Europe, except the Iberian peninsula. Ricke says the modelled temperature increase was especially large in Scandinavia, Central Europe and Eastern Europe.

These far-reaching impacts were caused by changes to large-scale atmospheric currents leading to unexpected consequences.

Team member Jessica Wan at UCSD says a big takeaway is that the impacts of regional MCB aren’t always intuitive. “Our results provide an interesting case study illustrating the unexpected complexities in the climate system you can uncover through regional geoengineering because of the highly concentrated perturbation to a small part of the planet.”

The MCB experiments that have taken place so far in Australia and California haven’t been of a sufficiently large scale to cause detectable climate effects, but they suggest that regional geoengineering could be closer to reality than previously thought, says Wan. “We need more regional geoengineering modelling studies like this work to characterise these unintended side effects before they have a chance to play out in the real world.”

Ricke says another issue is that if countries start to rely on these methods while they are still effective, it may discourage action to reduce carbon emissions. Then, when the geoengineering stops working, the world would be locked into an even more dangerous trajectory, she says.

“Lock-in is a major concern people have about geoengineering approaches in general because there will be opportunity costs associated with pursuing these approaches,” says Ricke. “In a world like the one we simulate, what other risk management approaches would we have invested in developing if we hadn’t pursued MCB?”

Daniel Harrison at Southern Cross University in Australia is the project lead of the research looking into whether MCB could be used in the future as a tool to mitigate heatwaves in the Great Barrier Reef region.

He says the scenarios modelled by the new paper’s authors are “completely unrealistic and extreme”. “It’s a huge poke to the global climate system, so of course there will be consequences,” he says.

The project Harrison is researching would involve MCB over much shorter time periods and in a fraction of the area modelled by Ricke’s team, he says.

John Moore at the University of Lapland in Finland says there is an urgent need for more research on solar geoengineering to explore the possible outcomes more thoroughly, including the impact on low-income countries and Indigenous peoples in the Arctic.