Competition response of cloud supersaturation explains diminished Twomey effect for smoky aerosol in the tropical Atlantic

[Geoengineering News]

https://www.pnas.org/doi/10.1073/pnas.2412247122

Authors

Jeramy L. Dedrick, Christian N. Pelayo, Lynn M. Russell, Dan Lubin, Johannes Mülmenstädt, and Mark Miller 

24 March 2025

Significance

Aerosol–cloud interactions (ACI) are one of the most uncertain aspects of global climate predictions, in part because there are insufficient process-specific constraints from observations. This method of decomposing the most radiatively important impact of aerosols on clouds known as the Twomey effect incorporates retrieved supersaturation from two independent sets of observations to constrain the feedback of aerosol particles on cloud properties. The method quantifies the reduction of the Twomey effect at high aerosol concentrations. While previously observed, this diminishing of the Twomey effect has never been explained quantitatively by observations. In addition, the results provide the direct observational constraints on a parcel-based approach that is embedded in many climate models.

Abstract

The Twomey effect brightens clouds by increasing aerosol concentrations, which activates more droplets and decreases cloud supersaturation in response to more competition for water vapor. To quantify this competition response, we used marine low cloud observations in clean and smoky conditions at Ascension Island in the tropical South Atlantic during the Layered Aerosol Smoke Interactions with Cloud (LASIC) campaign. These observations show similar increases in droplet number for increased accumulation-mode particles from surface-based and satellite cloud retrievals, demonstrating the importance of below-cloud aerosol measurements for retrieving aerosol–cloud interactions (ACI) in clean and smoky aerosol conditions. Four methods for estimating cloud supersaturation from aerosol–cloud measurements were compared, with cloud scene-based and parcel-based methods showing sufficient variability for a strong dependence on both aerosol accumulation number concentration and cloud-base updraft velocities. Decomposing aerosol-related changes in cloud albedo and optical depth shows the calculated competition response accounts for dampening the activation response by 12 to 35%, explaining the diminished Twomey effect at high aerosol concentrations observed for smoky conditions at LASIC and previously around the world. This result was consistent for independent supersaturation retrievals by cloud scene-based droplet number and cloud condensation nuclei and parcel-based multimode size-resolving Lagrangian methods. Translating aerosol effects to local radiative forcing with clean conditions as a proxy for preindustrial and smoky conditions for present-day showed that the competition response reduces cooling from the Twomey radiative forcing by 12 to 35%, providing an essential process-specific constraint for improving the representation of aerosol competition in climate model simulation of indirect aerosol forcing.

------------

PRESS RELEASE: Why Aren't Clouds as Bright as We Thought?—New work suggests dimmed prospects for popular geoengineering concept

https://scripps.ucsd.edu/news/why-arent-clouds-bright-we-thought

Author

Robert Monroer

31 March 2025

A new study is helping scientists clarify how clouds can affect climate, while also dimming the prospects for some proposed geoengineering ideas.

Jeramy Dedrick and Lynn Russell of UC San Diego’s Scripps Institution of Oceanography led a team of researchers who used observations from the tropical South Atlantic Ocean to quantify the ways that aerosols affect cloud brightness.  Aerosols are natural particles such as dust or sea salt or sometimes human-produced pollutants. Such particles provide the structure for clouds that form every day around the world.

One way aerosols affect clouds is the role they play in activating droplets within the updrafts that form clouds. The name for it is the Twomey effect. That effect plays a role in how much clouds can cool Earth’s surface since the brightness of clouds bounces a certain amount of solar radiation back into space.  Dedrick and Russell’s team found through analysis that the maximum potential for that to happen is about 30 percent less than some climate models predicted.

Russell said catching this was a matter of using observations to account for some missing physics.

“Since global models cover the entire earth, they don’t include all of the detailed distributions of updrafts and aerosol particles that are needed, so this work shows how observations can be used to make model estimates more accurate,” said Russell, a climate scientist at Scripps Oceanography.

Climate researchers who improve understanding of cloud dynamics by simulating them in computer models can use new information from this analysis – which includes physics that occurs on scales too small to represent in global models – to constrain what is in the realm of possible scenarios. The task will be further aided by ongoing regional studies, such as EPCAPE, a field project installed on Scripps Pier and other locations around La Jolla, Calif. in 2023. As with EPCAPE, the U.S. Department of Energy Atmospheric Radiation Measurement facility supported the work.

This clarity will also help researchers understand how feasible artificial efforts to control planetary climate can be.  One solution popular in science circles is to enhance the brightness of clouds through human intervention. 

Russell said the data suggest that the strategy might not be as effective as thought because what happens in real clouds is not the same as what happens in models of clouds.

Dedrick pursued this question as part of his PhD in the Climate Sciences Curricular Group at Scripps Oceanography.  

“It was incredibly exciting to tackle this research question because it brought together everything I had worked on during my PhD,” Dedrick said. “This study not only tied together the complexities of aerosol-cloud interactions but also shed light on their broader implications for climate, which has been a central motivation in my research.”

Study authors include Christian Pelayo and Dan Lubin from Scripps Oceanography and Johannes Mülmenstädt from the Pacific Northwest National Laboratory and Mark Miller from Rutgers University.  

Source: UC San Diego