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New research shows that a proposed climate intervention method could unexpectedly alter the atmosphere's ozone layer, with potential global effects.

A potential strategy to cool the planet – known as Marine Cloud Brightening (MCB) – could have a significant, and until now overlooked, impact on the middle atmosphere and the ozone layer, according to a new study led by researchers at NOAA's Chemical Sciences Laboratory (CSL) and published in Science Advances.

MCB involves spraying fine particles like sea salt into low-lying ocean clouds to make them brighter and more reflective, sending more sunlight back into space and reducing global temperatures. The technique has been considered relatively benign compared to more "invasive" climate intervention proposals like Stratospheric Aerosol Injection (SAI), which involves dispersing reflective particles directly into the middle atmosphere at altitudes 9-16 miles high.

But this new study challenges the perception that impacts of interventions in the lower atmosphere would stay in the lower atmosphere. In this case, the ozone layer could be significantly impacted by marine cloud brightening deployments some 20 km below it.

"MCB, or in fact any other troposphere-based intervention, isn't just affecting surface climate," said lead author Ewa Bednarz, a researcher with the University of Colorado Boulder CIRES affiliated with CSL. "We found it could drive a cascade of dynamical and chemical changes that could reach all the way into the stratosphere, affecting ozone levels in both the lower and middle atmosphere."

Using the UK Earth System Model (UKESM), the research team started with a high-end global warming scenario and simulated a large-scale MCB deployment to lower temperatures to those found in a moderate global warming scenario. The findings were surprising: the impacts of such an MCB deployment changed atmospheric composition and circulation throughout the tropospheric and stratosphere, which increased ozone in some regions of the atmosphere while decreasing it in others.

Winners and Losers in the Ozone Layer

Ozone is a fundamental component of our atmosphere: in the middle atmosphere, its presence shields living beings from harmful ultraviolet (UV) radiation; when produced near the surface however, often as a byproduct of various atmospheric pollutants, it can be harmful for humans and ecosystems.

In the tropics, the simulated MCB scenario caused an increase in total ozone column (the sum of tropospheric and stratospheric ozone) of up to 5% due to weakened upwelling of ozone-poor air from lower, tropospheric, altitudes and increased ozone production from elevated nitrogen oxides (NOx) produced by lightning.

But at the same time, further north, the story was different. Over the Northern Hemisphere's mid-latitudes, MCB led to a drop in total column ozone of about 3%. This decrease was most pronounced in winter and spring, and was driven by the MCB-induced changes in stratospheric circulation and ozone transport.

The Southern Hemisphere showed a more mixed response. While MCB decreased total column ozone in mid-latitudes during austral winter and spring, it also surprisingly boosted ozone over Antarctica during springtime due to changes in the polar vortex that limited the halogen-based ozone destruction that causes the seasonal Ozone Hole.

Unexpected Atmospheric Chain Reactions

The research highlights how a troposphere-level intervention like MCB could trigger a cascade of atmospheric changes that extend all the way to the middle atmosphere. MCB could cool the surface and upper troposphere, but this in turn would alter wind patterns and affect the strength of the large-scale stratospheric overturning circulation, known as the Brewer-Dobson Circulation. These effects in turn alter how ozone and other gases are transported vertically and across latitudes.

Adding to the complexity, the MCB strategy simulated in the study also triggered a La Niña-like cooling pattern in the eastern Pacific. That pattern further disrupted planetary wave activity and stratospheric transport and mixing, as well as strengthened the Northern Hemisphere polar vortex that is known to promote ozone depletion.

"Once you perturb the surface temperatures, both globally and regionally, it affects the entire atmospheric system," said Bednarz. "These are not isolated layers. They are deeply interconnected."

Beyond dynamics, MCB also influenced the chemistry of the atmosphere. Lightning activity increased in the model due to changes in clouds and precipitation, driving more NOx and ozone production in the tropical upper troposphere and lower stratosphere.

Meanwhile, colder temperatures led to drier air across the troposphere and stratosphere, which also favors ozone preservation in the tropics by reducing water vapor-driven destruction pathways.

More Questions Than Answers

The study doesn't offer a verdict on the viability of MCB, but it highlights an important issue: the assumption that impacts of tropospheric interventions are limited to the troposphere is flawed.

This study expands the horizons of MCB research by highlighting impacts that extend beyond the troposphere; impacts that will require further study to understand the complex chemical and dynamical responses to MCB implementation in the Earth system.

"Our role as scientists is to provide holistic assessments of climate interventions that include more than just their impact on surface temperature." said co-author Daniele Visoni from Cornell University. "Different climate intervention strategies will produce different interactions that may ripple through the entire Earth system, all of which need to be thoroughly evaluated."

The MCB scenario used in the study was intentionally "extreme" and not necessarily representative of a potential real-world deployment. But even so, the authors argue, it highlights plausible risks that must be studied more thoroughly.

As global pressures mount to find climate solutions, interest in climate intervention approaches like MCB has grown. But this study makes clear that the Earth's systems are not easily compartmentalized. Obtaining a full picture of what an intervention could do will require thinking about the system as a whole, including careful modeling, monitoring, and understanding.

"The middle atmosphere, especially the ozone layer, is not an exception, and must be part of that conversation." said Bednarz.