2025 News & Events

Over the past decade, there has been increasing scientific discussion about the risks and benefits of intentionally cooling Earth's surface temperature by increasing the reflectivity of the atmosphere. Two methods have emerged as the most potentially viable.

One method, stratospheric aerosol injection (SAI), involves dispersing microscopic particles between 7 and 30 miles high in the atmosphere to reflect a small portion of sunlight back into space. The other method, marine cloud brightening (MCB), involves seeding low-level marine clouds with sea salt particles to make the clouds more reflective and reduce the amount of sunlight that can reach the waters below.

One well-established characteristic of atmospheric particles is that while they reflect a small fraction of incoming sunlight, a much larger portion of sunlight that is not reflected is largely diffused, or scattered forward in different directions. Now, a new NOAA study published in the journal Geophysical Research Letters has found that this diffusion of sunlight from particles in the stratosphere could indirectly make marine clouds thousands of feet below more reflective, or in essence brighter.

"This effect could increase the reflection of sunlight from a cloud by as much as 10%, which is much larger than we anticipated," said lead author Jake Gristey, a research scientist with the University of Colorado CIRES, also affiliated with LASP and NOAA CSL.

The effect turned out to be large enough that Gristey initially thought something must be wrong with his computer model code. After carefully double-checking his calculations and reproducing similar results with slightly different model set-ups, he realized he could be onto something.

Research on climate intervention thus far has been dominated by modeling studies that investigate the impacts of either SAI or MCB almost entirely independently. Consequently, the impact of SAI on cloud brightness has not been directly examined. Following an informal discussion with fellow scientists in CSL, Gristey was inspired to do some initial model calculations to investigate potential connections between the two methods.

Gristey used an energy transfer model to simulate the scattering direction and path traveled by individual solar photons after encountering multiple cloud droplets within a “theoretical” cloud. By varying the angle of the incoming photons, he was able to determine a very strong relationship between incidence angle and the depth that photons traveled into a cloud.

A photon entering a cloud from directly overhead can penetrate twice as deep as a photon entering at a steeper angle. The deeper a photon travels into a cloud, the less likely it is to get reflected and turned back upwards.

"As any experienced marksman or boxer can tell you, if you strike something at a glancing angle, it's far more likely to simply deflect off," Gristey said. "The same is true for a photon."

Next, he zoomed out and used a 1-D model to simulate an idealized SAI scenario in which a substantial layer of uniform aerosols are dispersed evenly in the stratosphere about 12 miles above a marine cloud deck. Without the aerosol layer, the sunlight entering the cloud deck is almost entirely from directly overhead with only 6.4% diffuse radiation. With the aerosol layer, this drastically increases to 59.1% diffuse radiation.

With the enhanced diffuse sunlight, the cloud reflectivity increases by about 10%. If SAI were to be deployed above a region with cloud cover, diffusion-brightening could actually provide a larger cooling effect than the stratospheric aerosols themselves under certain conditions. Since clouds cover around two thirds of the Earth on average, this scenario would occur frequently.

What this means in practical terms, explained Gristey, is that implementing SAI "could indirectly cause an additional or 'bonus' marine cloud brightening effect, which would substantially increase the overall cooling effectiveness of SAI."

"Our assessment of diffusion-brightening could have profound implications for a potential implementation of solar radiation management," said CSL research scientist and co-author Graham Feingold, whose work focuses on clouds and aerosols. "Evaluation of solar radiation management requires a complete understanding of knock-on effects like these across the entire Earth system."

At the direction of Congress, NOAA is leading a multi-year research program to investigate, detect, monitor, and assess natural, inadvertent, or intentional events that would alter the Earth's radiation budget by increasing the reflectivity of the stratosphere or marine clouds. NOAA does not condone or endorse any climate intervention approach or technique. NOAA is not conducting outdoor experiments or planning for outdoor experiments.