Dimming the Sun Would Help Lower the Risks of El Niño

This year’s El Niño is shaping up to be among the strongest on record, and it’s set to create chaotic weather around the world.

A new study suggests that there could be a way to mitigate some of the impacts of future El Niños and global warming: dimming the sun.

El Niño develops naturally in the tropical Pacific every few years, caused by weakened trade winds that push heat from the ocean toward the coast of South America. This tilts the odds toward higher-than-average global temperatures, as well as droughts in some regions, intense rains and floods in others, and more cyclones in the Pacific. Piled on top of warming driven by burning fossil fuels, a strong El Niño can mean hundreds of billions in economic losses.

The new study argues that deflecting solar energy could cool the ocean and help moderate El Niño events before they become too strong, staving off the worst impacts.

“El Niño is one of these things where something happens in the tropical Pacific, and then it rearranges the way the entire global atmosphere is holding energy that year,” says Katherine Ricke, a coauthor of the study published Wednesday in the journal Science Advances and a climate scientist at UC San Diego and the Scripps Institution of Oceanography. “It’s an ultimate pressure point in the climate system.”

Ricke and her coauthors looked at using marine cloud brightening, or MCB, as a way to dim the sun in the Pacific. The technique entails spraying seawater into marine clouds to enhance the clouds’ reflectivity. While some pilot projects and randomized controlled trials have tested the technique’s efficacy, they’ve only been on very small scales.

MCB is one of a few different solar geoengineering methods intended to reflect sunlight back into space. Other methods, like using planes to inject aerosols into the stratosphere, can only work globally. But MCB has the potential to be a regional cooling solution.

To get around the lack of MCB experiments, researchers looked at a recent natural phenomenon that mimicked it: the catastrophic 2019-2020 Australian bushfire season. More than 10,000 bushfires raged across the country, producing almost 1 million metric tons of smoke. That represents one of the largest inputs of smoke into the stratosphere that humans have observed with satellite technology.

While the effects of this massive amount of smoke were complex, previous research shows it helped trigger a rare triple-dip La Niña—the opposite phase of El Niño—thanks in part to reflective particles in the smoke.

This event, Ricke says, enabled her and her coauthors to finally address a question they’d had for years about whether regional interventions can help relieve the pressure events like El Niño put on the global climate system. The researchers created a model based on the MCB effects of the Australian bushfires, and ran it against two different historic El Niño events to observe its effects. The modeling showed that lowering the amount of sunlight reaching the Pacific’s surface would have significantly reduced the magnitude of those El Niño events and their global impact.

Geoengineering techniques have traditionally been viewed as a method to cool the entire planet, acting as a counterbalance to humans’ use of fossil fuels—albeit an extremely controversial one. The new study makes the case that some forms of geoengineering would be better used to target regional events, like El Niño. Doing so has the potential to avoid—or at least lower the risk—of the compounding effects of El Niño piled on top of rising temperatures due to human activity.

“The idea of having to sustain geoengineering indefinitely gives a lot of people pause—we all understand that cooperation at that magnitude would be hugely complicated in the world we live in,” Ricke says. “This is a totally different way to think about geoengineering.”