Marine stratocumulus clouds off the California coast. Photo: Pascal Polonik

Artificial Climate Controls Might Become Ineffective – Because of Climate Change

Cloud brightening cooling strategy stops working in models when natural systems respond to relentless warming

One geoengineering proposal for alleviating the extreme heat effects of global warming, the brightening of marine clouds to reflect more solar energy to space, could work in present-day conditions but may become ineffective in the future, according to a research team led by UC San Diego’s Scripps Institution of Oceanography.

The problem is that if the warming of the planet continues at its current pace, the geoengineering strategy not only stops working in computer simulations, it could actually start increasing heat stress. The study, published today in the journal Nature Climate Change, is the first to demonstrate the diminishing and even reversal of benefits of a cloud brightening strategy as climate conditions change. 

Graduate student Jessica Wan and colleagues modeled what could happen if so-called marine cloud brightening were attempted on a regional level. In actual practice, this would likely entail the spraying of reflective aerosols into stratocumulus clouds over the ocean.

Their model showed that the method would work, lowering the risk of dangerous heat exposure by more than 50% across the Western U.S. In the world of 2050, though, the strategy starts to backfire. A major avenue of ocean circulation, known as the Atlantic Meridional Overturning Circulation, or AMOC, triggers different responses as heat builds.

Simulations showing artificial cloud brightening effects in the present day and in 2050.
Simulations showing artificial marine cloud brightening effects in the present day (top) and in 2050, when initial cooling gives way to regional warming. 

“Our study provides the first evidence that regional climate interventions that appear promising for climate risk management today might become ineffective as the climate continues to change,” said study co-author Kate Ricke, Wan’s advisor and a climate scientist with appointments at Scripps Oceanography and UC San Diego’s School for Global Policy and Strategy. “Remarkably, they could even end up increasing risk. Marine cloud brightening, in particular, has the potential to tap into some of the complexities in the climate system that we don’t have a perfect understanding of because it can make an artificially large change to the radiation balance concentrated over a relatively small area.”

Geoengineering in any form remains a controversial prospect for offsetting harms caused by global warming through various proposed strategies, including marine cloud brightening. For this study, the researchers said they opted to create scenarios based on regional cloud brightening schemes rather than global-scale initiatives, which at present are considered by the geoengineering community to be too infeasible to pursue. 

To date, real-world experiments have been sparse. Australia launched a government-funded regional marine cloud brightening field experiment in 2020 in an effort to save the Great Barrier Reef. Earlier this year, the University of Washington started an experiment in Alameda, Calif., in which researchers sprayed sea salt into the air as a preliminary step on the way to planned larger-scale marine cloud brightening tests. The city shut the project down temporarily to examine potential health risks it posed and determined the project posed no danger, but the city council still unanimously voted to end the experiments altogether. 

“The recent outdoor tests in Alameda were a significant moment for the field because it was really the first time many people had ever heard of marine cloud brightening,” said Wan. “The substantial governance challenges of global-scale interventions are well documented, but I think this small-scale experiment and our modeling study highlight the challenges scientists and decision-makers will face for regional-scale deployment as well.”

The team includes Matt Luongo from Scripps Oceanography and colleagues from the National Center for Atmospheric Research (NCAR) in Colorado and the School of Global Policy and Strategy at UC San Diego. The National Science Foundation funded the work, which was undertaken as part of Ricke’s NCAR Early Career Faculty Innovator award that builds collaborative connections between NCAR and interdisciplinary researchers.

The researchers’ model simulated two marine cloud brightening schemes carried out over different regions of the northeastern Pacific Ocean that they believe would be especially responsive to artificial brightening, one in the subtropics near California and one in the mid-latitudes near Alaska. Both were designed to reduce risk of extreme heat in the western continental U.S. The simulations showed that the western U.S. would receive cooling benefit from both schemes initially, but more so from the remote Alaska scheme than the proximate one offshore California, by tapping into teleconnections, links in the climate system between geographically distant parts of the world. 

Stratocumulus clouds are formed at elevations between 1,200 and 6,500 feet. Photo: Pascal Polonik
Stratocumulus clouds off San Diego. Photo: Pascal Polonik

This result demonstrates that the regional effects of marine cloud brightening are not always intuitive. Because the cooling is concentrated in particular areas, it can induce changes to the large-scale atmospheric circulation with unexpected consequences. 

For instance, while the schemes reduce heat in the western U.S. particularly effectively under present-day conditions, they also reduce extreme heat elsewhere in the world.  Under mid-century warming, most climate models project that the AMOC will slow substantially, leading to regional cooling over the North Atlantic Ocean. Marine cloud brightening in the North Pacific induces a strong local cooling that accelerates the global ocean circulation, leading to a large-scale atmospheric response that ultimately increases heat risks in several regions around the world, particularly over Europe.

“We are still a long way out from any viable global implementation of solar geoengineering, but smaller scale interventions might become more attractive as the planet warms,” said Wan. “Our results demonstrate what could happen under a worst case scenario; that is, an approach that works as designed initially but later fails due to physical conditions in the future. More work is needed to characterize these worst case scenarios so we can be proactive, rather than reactive, to the consequences.”

About Scripps Oceanography

Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.

About UC San Diego

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