As carbon dioxide (CO2) emissions continue to rise, interest in geoengineering is growing as governments, researchers, and environmental advocacy groups seek ways to limit the worst effects of climate change. These large-scale climate interventions could affect rainfall, agriculture, and ecosystems around the world, and their potential benefits and risks are equally important.
A new study by climate scientists at the University of California, Santa Barbara suggests that not all geoengineering strategies produce the same results. One of the proposed methods could significantly disrupt one of Earth’s most important climate cycles, while another appears to have little effect. The survey results are the future of the earthhighlights why the researchers say all proposals need to be carefully evaluated before being implemented.
“We need to be careful in implementing geoengineering proposals before we fully understand what will happen,” said lead author Chen Xing, a doctoral student in the UCSB Bren School of Environmental Science and Management.
Why is El Niño important?
Xing and Bren graduate student Cali Pfleger started the project to better understand how geoengineering affects marine ecosystems. This question quickly led them to investigate El Niño Southern Oscillation (ENSO), a major driver of ocean and atmospheric conditions.
ENSO is a naturally occurring climate cycle that repeats every two to seven years. It moves warm ocean water into the tropical Pacific Ocean, influencing weather around the world. During El Niño events, warm ocean water moves toward the west coast of the Americas, often bringing rainy winters to California. During a La Niña event, ocean temperatures remain further west and monsoon rains intensify in parts of South and Southeast Asia.
Comparison of two climate cooling strategies
Researchers studied two geoengineering approaches aimed at cooling the Earth by reflecting more sunlight back into space. Both rely on releasing particulates into the atmosphere, but differ in the materials used and the altitude at which they are released.
One method, known as marine cloud brightening (MCB), involves spraying sea salt particles less than two kilometers above the ocean surface. These particles create clouds that are smaller and contain more droplets, making them brighter and more reflective.
The second method, called stratospheric aerosol injection (SAI), releases far more sulfate particles into the atmosphere. These particles are spread evenly across the planet, blocking some of the sunlight over a larger area.
Surprising effects on El Niño
Brightening of oceanic clouds is often proposed on the eastern side of oceanic basins due to the strong cooling potential. However, the southeastern Pacific Ocean also plays an important role in maintaining ENSO.
The simulation results revealed some unexpected results. “Deploying an MCB in the subtropical eastern Pacific dramatically reduces ENSO amplitude by approximately 61%,” the authors write.
“It’s difficult to change ENSO this quickly,” said study co-author Samantha Stevenson, an advisor to Xing and Pfleger.
The reason lies in how the brightness of ocean clouds changes the local weather. Bright clouds cool the ocean surface below, while also reducing rainfall because smaller cloud droplets are less likely to combine with raindrops. As cold, dry air spreads over the central Pacific Ocean, evaporation decreases, atmospheric circulation weakens, and winds along the equator strengthen. These changes increase the upwelling of cold water, further cooling the ocean surface.
Together, these effects dramatically weaken ENSO.
Researchers had expected the brightening of ocean clouds to affect the climate, but it didn’t have that much of an effect.
Although we thought the proposal could have an impact, “we did not expect that two-thirds of the ENSO variance would be eliminated,” Xing said. He added that the bottom line is simple: “Do not conduct MCBs over the eastern Pacific Ocean, as this could cause a very strong chain reaction due to the disappearance of ENSO.”
Why stratospheric aerosols behave differently
The second geoengineering strategy had very different results. Stratospheric aerosol injection had little measurable effect on ENSO.
Researchers believe the difference is due to where the particles are released. Brightening of ocean clouds causes particles to concentrate in one specific area near the surface. By comparison, sulfate particles injected into the stratosphere were spread over a much wider area, creating a more uniform cooling effect with less impact on the tropical Pacific Ocean.
Still, Stevenson emphasized that this finding should not be interpreted as a complete refutation of brightening in oceanic clouds.
“We’re not saying all MCBs are going to crush ENSO. We’re just saying that if you do it in this particular region, this is what will happen,” she said.
He noted that ocean cloud brightening could be exploited elsewhere, but much greater effort would likely be required to achieve the same degree of global cooling.
Climate risks extend beyond temperature
The researchers also note that choosing not to intervene comes with its own risks. If climate change is left unchecked, ecosystems, natural climate cycles, and human societies are expected to be destroyed. Scientists still don’t know exactly how ENSO itself will respond to continued global warming, adding further uncertainty.
“Nothing can match the rate at which ENSO changes in these MCB experiments,” Stevenson said. “Even under climate change, it will not naturally decline by 60% in 10 years.”
As more sunlight is reflected from the Earth, photosynthesis may also decrease. This will reduce the productivity of crops, forests, and seaweed. Understanding these effects is especially important because algae form the basis of the marine food web and produce about 70% of the oxygen in Earth’s atmosphere.
In future studies, the researchers plan to investigate how different geoengineering strategies may affect marine ecosystems.
Understand the trade-offs
This study shows that geoengineering can’t be judged solely by how much it cools the Earth. Different approaches have the potential to achieve similar global temperature reductions with dramatically different regional climate impacts.
“The two interventions can achieve the same global warming goals and have very different impacts on local climates,” Stevenson said. “The most important question is: Have we considered all possible outcomes?”
