Simulations suggest that brightening marine clouds could weaken climate extremes.
There could be a way to geoengineer El Niño to do less damage, scientists say.
Adding aerosols to the atmosphere of a particular part of the Pacific Ocean can increase and brighten clouds in the region, creating a cooling effect. New computer simulations show it can trigger atmospheric changes this could reduce the strength of an El Niño event – and the extreme weather conditions that accompany it, researchers report July 8 in Scientific advances.
The idea was sparked by fires, says Jessica Wan, a climate scientist now at the University of Chicago. In the aftermath of the 2019-2020 Australian wildfireshuge waves of particles rose into the atmosphere and then floated over the subtropical southeastern Pacific Ocean. The fires lit up the clouds above the ocean, and this brightening helped trigger a multi-year La Niña eventthe reverse of an El Niño.
This “opportunistic experiment” demonstrated how changing clouds in a particular region can alter broad climate patterns, says Wan, who conducted the research while at the Scripps Institution of Oceanography in La Jolla, California.
The team wondered whether it would be possible to geoengineer a similar effect to mitigate the impacts of El Niño, the “warm” phase of a multi-year climate pattern known as El Niño – Southern Oscillation. El Niño episodes are brief, usually lasting less than a year, but can be deadly and costly. Past El Niño events have brought heat waves to land and oceans and inundated some parts of the world with torrential rains and flooding, while others plunged into severe drought.
Adding aerosols specifically to enhance eastern subtropical Pacific clouds would be a targeted version of a kind climate geoengineering called marine cloud brightening, or MCB. MCB proposes that certain aerosols – particularly sea salts – injected into the atmosphere could brighten cloudier regions of the ocean, making them whiter and more reflective, which would reflect more solar radiation back into space to help cool the planet.
To simulate how the MCB might dampen El Niño, the researchers focused on two strong El Niño events: 1997-1998 and 2015-2016. The team then identified where fire particles were densest in the southeast Pacific Ocean and targeted those regions in their computer simulations for aerosol injections.
“We wanted to try to go all in and see what happened,” Wan says. The team simulated a massive injection concentration of around 500 particles per cubic centimeter. We also varied the timing of this cloud burst, seeing what happens if the injections occur at the very beginning of an El Niño, or near its peak, as well as the duration of the injections.
Reduce the heat of El Niño
The researchers simulated how marine cloud brightening could reduce the intensity of past El Niño episodes, like the one that lasted from 2015 to 2016. Cloud brightening through continued aerosol injection over the eastern subtropical Pacific Ocean (boxed region) dramatically altered the path of El Niño after its onset in May 2015 until the following spring, the simulations showed. In April 2016, sea surface temperatures were significantly colder in the simulation (right) compared to actual conditions (left).
All injections simulated El Niños weaker than actual events. But the extent of the weakness depends on timing, the team found. For the 2015-2016 event, for example, particle injection from June to the following February resulted in the most significant cooling. But starting these injections in December – essentially at the eleventh hour – led to the slightest chill. This is likely because by then, El Niño dynamics are well underway and any cooling is more localized, the team suggests.
Using the MCB to directly target large El Niño events “is really interesting and very novel,” says Daniele Visioni, a climatologist at Cornell University who was not involved in the study. And “the fact that it looks like it might work is a very good indication that it’s worth thinking about.”
Earth officially entered its final El Niño phase in June. Computer simulations of current conditions in the Pacific suggest that it has the potential to become a “super El Niño”. MCB is far from being on the menu for mitigating this year’s El Niño, Wan says—there are big hurdles, including technical constraints and sociological barriers, like who should determine whether these interventions are worth possible negative climate consequences.
Many researchers remain wary of the idea of tinkering with the climate. “There are many, many unanswered questions and uncertainties about the viability of the MCB,” says James Haywood, a climate scientist at the University of Exeter in England who was not involved in the new study. Previous research by Haywood and colleagues simulate the effects of MCB found that cooling in the eastern Pacific could produce a “mega La Niña” several times stronger than previously observed, he says.
La Niña is generally considered the milder sister: overall, it brings cooler temperatures and milder weather events. “But the impacts of El Niño and La Niña are heterogeneous” on the planet, and not everyone suffers or benefits from them, Wan says.
Visioni notes that “this is by no means the definitive answer… But it’s important to have these kinds of studies that keep the door open. Considering that big El Niños produce a lot of damage, I think it’s worth asking the question.”
