A novel geoengineering approach involving marine cloud brightening over the eastern Pacific Ocean could offer a way to mitigate the severe global impacts of powerful El Niño events, according to a recent modelling study. Researchers suggest that by making clouds whiter and more reflective, it might be possible to reduce the significant global temperature increases and economic damage associated with these climate phases.
El Niño occurs when easterly trade winds weaken, allowing warm water from the western Pacific to spread across the central and eastern parts of the ocean. This process heats the atmosphere, leading to a rise in global temperatures and estimated economic losses in the trillions of pounds. With a potentially very strong, or 'super' El Niño, currently developing in the eastern Pacific, the urgency of understanding potential mitigation strategies is heightened.
The technique, known as marine cloud brightening, involves deploying tiny droplets of seawater into the air beneath low-lying stratocumulus clouds. These droplets provide nuclei for moisture to condense upon, increasing the number of droplets within the clouds. This makes the clouds appear whiter, enhancing their ability to reflect sunlight back into space and effectively shading the ocean below.
Dr. Jessica Wan, from the University of California, San Diego, a lead researcher on the study, explained that by shading a specific area of the eastern Pacific, known as the Niño 3.4 region, the method could disrupt the natural feedback loops that drive El Niño's development. Cooler sea surface temperatures would in turn strengthen trade winds, pushing warm water back west and allowing cooler water to well up from the ocean depths, further reinforcing the cooling effect. Dr. Wan noted, "You can basically stop the dominoes from falling early when you do marine cloud brightening. We’re kicking the cycle in the other direction."
The researchers drew inspiration from the 'black summer' bushfires in Australia during 2019-2020, which were followed by a prolonged La Niña – the cooler counterpart to El Niño. Previous research indicated that smoke particles from these fires may have brightened clouds and cooled the eastern Pacific, contributing to the intensification and extension of a 'triple dip' La Niña. The current study modelled the potential impact of cloud brightening on past super El Niño events in 1997-1998 and 2015-2016, finding that nine months of seawater spraying could have nearly halved the warming in the Niño 3.4 region, reducing it from over 2°C to just above 1°C and potentially ending the El Niño several months earlier.
However, the proposed scale of such an intervention is substantial, requiring an estimated 2,400 ships and a volume of seawater spray beyond current nozzle technology capabilities. Dr. Mat Collins from the University of Exeter, UK, cautioned that these model results might not directly translate to real-world conditions, where warming seas can already cause low-level clouds to dissipate, creating a further warming feedback loop. While the study suggests marine cloud brightening could turn a super El Niño into a moderate one, Dr. Wan acknowledged that unexpected consequences are possible, including the potential for an earlier and stronger subsequent La Niña, which could have adverse effects on regions like the Horn of Africa, where strong La Niñas have historically led to famine. Despite these challenges, the researchers believe the concept warrants further investigation.