001 | Standardizing the Science: How Researchers Created a Common Framework for Evaluating Marine Cloud Brightening
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Standardizing the Science: How Researchers Created a Common Framework for Evaluating Marine Cloud Brightening
📘 Summary of: "A protocol for model intercomparison of impacts of marine cloud brightening climate intervention" from Geoscientific Model Development, Volume 17 - 2024, by Philip Rasch, Haruki Hirasawa, Mingxuan Wu, Sarah Doherty, Robert Wood, Hailong Wang, Andy Jones, Jim Haywood, and Hansi Singh, PhD.
The marine cloud conundrum
Imagine trying to reduce a fraction of the sun’s warming by making some of Earth’s clouds a little more reflective. This idea is called marine cloud brightening (MCB): a proposed climate intervention that involves spraying tiny salt particles into low-lying ocean clouds to make them reflect slightly more sunlight back to space, a phenomenon that has often been observed with pollution particles from ships. The salt and pollution particles (also frequently called aerosols) serve as nuclei for the formation of cloud droplets, and creating additional cloud droplets in clouds can make them more reflective (brighter).
But, it is very difficult to build computer models that accurately represent interactions between aerosols and clouds — and that model their effects on climate. Research teams have made different choices through different models in their attempts to assess how changing aerosols and clouds might affect the planet — and these choices produce different outcomes. These varied results are hard to compare and contrast, making it challenging to advance the science in this field.
Designing an effective comparison
A team of 18 researchers, led by scientist Philip Rasch from the University of Washington, decided to take on the challenge. They created a standardized modeling protocol that allows scientists worldwide to simulate marine cloud brightening in a consistent way, allowing findings to be compared.
Their protocol, called MCB-REG, identifies six specific ocean regions where clouds are particularly susceptible to brightening, such as areas off the coasts of California, Chile and Namibia. The researchers then designed a three-stage process: first, calibrate each climate model to produce the same amount of increased sunlight reflection from each region. Then, evaluate impacts on variables like temperature and precipitation from sea salt added in individual regions. Finally, combine multiple regions to understand how the effects add up in the computer model representing a virtual climate system.
Figure 1: Regions selected for simulation of sea salt addition using the protocol.
Models have major differences
The researchers knew that global climate models would represent MCB differently. Because these models operate at large scales and relatively low resolutions, they do not identically capture the small-scale processes that determine how clouds respond to aerosols. As a result, the same amount of injected sea salt can yield very different cooling effects depending on the model. For example, simulations with three major global climate models, the US National Center for Atmospheric Research CESM2, the Department of Energy E3SMv2, and the UK Met Office UKESM1, produced widely varying salt requirements: CESM2 needed about 7.5 teragrams per year to achieve the target cooling, while E3SMv2 required nearly seven times more and UKESM1 about ten times more.
The protocol was designed with these limitations in mind. It does not require models to simulate the behavior of aerosols or clouds in the same way, or the amount of cloud brightening produced by a given amount of sea salt. Instead, it leverages what global models do well — simulating how the climate system responds to changes in reflected sunlight. This approach enables the modeling teams to compare impacts and responses in a structured and consistent way.
Consistent patterns emerge
Despite model differences in sensitivity to sea salt, simulations using the protocol revealed remarkably consistent patterns in climate effects. Brightening clouds in the southeast Pacific triggered a "La Niña-like response" in the models, producing cooler sea surface temperatures there that rippled across the entire Pacific Ocean. Meanwhile, simulated interventions in the southeast Atlantic consistently produced weaker global cooling than other regions. Of all regions tested, the Pacific Ocean would yield the strongest cooling effects, according to the models.
Perhaps most intriguingly, the researchers discovered they could predict the climate effects of brightening multiple regions simultaneously by simply adding up the effects from individual regions. This "synthetic" approach worked remarkably well, suggesting that future climate models could rapidly test thousands of different intervention scenarios without running full simulations, which are computationally expensive.
A lift-off point for future research
The MCB-REG protocol represents a step in enabling broader scientific analysis of the effects of marine cloud brightening on the climate system and the direction for model advances. By advancing a common framework for computer simulations, researchers can identify robust results across the different models and which findings might be specific to an individual model.
As researchers work to understand climate interventions, robust protocols like this one provide the scientific community with the standardized tools needed to collaboratively advance understanding and evaluation of proposed interventions using rigorous computer modeling.
Since the paper’s publication in Nov 2024, the authors have advanced simulation approaches for the MCB-Reg protocol with follow-on papers coming soon. Stay tuned for future Paper Cuts editions.
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✍️ Want to dive deeper? Check out:
Chen, Y., Haywood, J., Wang, Y., Malavelle, F., Jordan, G., Peace, A., Partridge, D. G., Cho, N., Oreopoulos, L., Grosvenor, D., Field, P., Allan, R. P., and Lohmann, U.: Substantial cooling effect from aerosol-induced increase in tropical marine cloud cover, Nat. Geosci., 17, 404–410, https://doi.org/10.1038/s41561-024-01427-z, 2024.
Richter, J. H., Visioni, D., MacMartin, D. G., Bailey, D. A., Rosenbloom, N., Dobbins, B., Lee, W. R., Tye, M., and Lamarque, J.-F.: Assessing Responses and Impacts of Solar climate intervention on the Earth system with stratospheric aerosol injection (ARISE-SAI): protocol and initial results from the first simulations, Geosci. Model Dev., 15, 8221–8243, https://doi.org/10.5194/gmd-15-8221-2022, 2022
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🎬 Marine Cloud Brightening Research Program at the University of Washington describes their scientific research on generating sea salt aerosols and modeling their behavior at local and other scales.