13 October 2021
This summer's barrage of extreme weather around the globe – including record heat waves, wildfires, and flooding – have amplified calls for urgent action to address climate change. The view that rapid, drastic reductions in greenhouse gas emissions are needed is now the scientific consensus. More controversial are calls for investigating geoengineering techniques that may cool the planet quickly by reflecting sunlight away from Earth's surface.
One technique, called marine cloud brightening, would seek to make low-level clouds over the ocean more reflective and longer-lived by injecting them with small particles of salt generated by spraying seawater into the air. Theoretically, water vapor would collect on the surface of these salt particles creating additional cloud droplets that would reflect more sunlight back out to space.
This may be easier said than done, according to new research published in the Journal of Atmospheric Sciences by NOAA's Chemical Sciences Laboratory and CIRES researchers. Using a sophisticated computer model that accurately simulates the miniscule changes in cloud droplets, the researchers found that both the size of the cloud seed particles and the number of particles injected are crucial for successfully brightening the clouds to reflect more sunlight. To make matters even more complicated, the optimal particle size is likely case (or cloud) dependent.
"It's really not so easy as just spraying seawater up and hoping for the best," said lead author Fabian Hoffmann, a researcher at Germany's Ludwig-Maximilians-Universität and a CIRES Visiting Fellow in CSL at the time of this research. "There are complex microphysics at play. If your particles are too large or too small, too many or too few, you could get little or no cloud brightening, or even less reflective clouds, as a result."
Like Goldilocks and her bowls of porridge, the authors found that there is an ideal particle size that is 'just right' for increasing the reflectivity (or albedo) of marine clouds. Particles that are too large can form large droplets; these can collide with each other falling out of the cloud as rain. Even a tiny number of large particles (as few as one in a million) can increase rainfall and break up the cloud layer. Conversely, very small particles may either not form cloud droplets at all, or may cause an existing cloud to evaporate away faster. In each case, the result could be more sunlight striking the surface, not less.
All clouds are not created equal. Different types of clouds have different microphysical properties and exist in different environments, both of which need to be taken into account.
"Since there is a risk that particle size can either increase precipitation or increase evaporation, the optimal particle size and number will likely need to be matched to the target cloud," explained CSL scientist Graham Feingold, who co-authored the study with Hoffmann.
This concept of marine cloud brightening is not a new one – it was first proposed in the early 1990s by the late British physicist John Latham. However, it is receiving more attention recently with the growing international focus on climate change and warnings from scientists, including in the recently released IPCC Sixth Assessment Report, that we are not on track to limit the average temperature increase to below the 2°C threshold. This has prompted a spate of new research, including this current study, focused on taking a detailed and concentrated look at whether such an approach is even technically (and ethically) feasible.
Whether or not humans could, or should, ever modify clouds on a scale that might help alleviate climate change, doing this research has given Hoffmann a newfound appreciation for cloudy days. "I grew up in Northern Germany and always hated those gray skies," he said. "I have now come to appreciate how important they are in regulating the Earth's temperature."
Hoffmann, F., and G. Feingold, Cloud microphysical implications for Marine Cloud Brightening: The importance of the seeded particle size distribution, Journal of Atmospheric Sciences, doi:10.1175/JAS-D-21-0077.1, 2021.
Marine cloud brightening (MCB) has been proposed as a viable way to counteract global warming by artificially increasing the albedo and lifetime of clouds via deliberate seeding of aerosol particles. Stratocumulus decks, which cover wide swaths of Earth's surface, are considered the primary target for this geoengineering approach. The macroscale properties of this cloud type exhibit a high sensitivity to cloud microphysics, exposing the potential for undesired changes in cloud optical properties in response to MCB. In this study, we apply a highly detailed Lagrangian cloud model, coupled to an idealized parcel model as well as a full three-dimensional large-eddy simulation model, to show that the choice of seeded particle size distribution is crucial to the success of MCB, and that its efficacy can be significantly reduced by undesirable microphysical processes. The presence of even a small number of large particles in the seeded size spectrum may trigger significant precipitation, which will reduce cloud water and may even break up the cloud deck, reducing the scene albedo and hence counteracting MCB. On the other hand, a seeded spectrum comprising a large number of small particles reduces the fraction of activated cloud droplets and increases entrainment and evaporation of cloud water, which also reduces the efficiency of MCB. In between, there may exist an aerosol size distribution that minimizes undesirable microphysical processes and enables optimal MCB. This optimal size distribution is expected to be case dependent.