The Radiative and Cloud Responses to Sea Salt Aerosol Engineering in GFDL Models
Geoengineering News
Abstract
Marine cloud brightening is a proposal to counteract global warming by increasing sea salt aerosol emissions. In theory, this increases the cloud droplet number concentration of subtropical marine stratocumulus decks, increasing cloud brightness and longevity. However, this theoretical progression remains uncertain in coupled climate models, especially the response of liquid water path and cloud fraction to aerosol seeding. We use the GFDL CM4 climate model to simulate marine cloud brightening following the published G4sea-salt protocol, in which sea salt aerosol emissions are uniformly increased over 30 S–30 N in addition to standard forcings from a SSP2-4.5 future warming scenario. The perturbed radiative and cloud responses are temporally stable though spatially heterogeneous, and direct scattering by the added sea salt predominates over changes to cloud reflectance. In fact, feedbacks in the coupled simulation lead to a net warming, rather than cooling, response by clouds.
Key Points
Temporally stable climate response to increased sea salt aerosol in GFDL’s AM4 and CM4 models following the G4sea-salt protocol
Dominant role of direct aerosol effects in both models as the indirect aerosol–cloud effects are counterbalanced by cloud feedbacks in CM4
Uncertain spatial radiative and cloud responses necessitating further constraining to yield detailed mechanistic understanding
Plain Language Summary
With calls for climate action rising, some countries and groups may be looking at counteracting global warming. As reducing emissions of greenhouse gases remains elusive, and while the results of climate change manifest in extreme events and weather records, state or private actors may look for active engineering solutions which remain hypothetical and not fully scientifically understood. Using premier climate models at NOAA GFDL, we examine one form of climate engineering, marine cloud brightening, aimed at increasing radiation reflected back to space by increasing sea salt aerosol emissions in the marine tropics. We find the climate response to a protocol of this scheme temporally stable over the time period of the simulation, though spatially uncertain. Moreover, the response is largely dominated by effects resulting from the direct interactions between aerosol particles and solar radiation, and not via clouds. Our results paint a more nuanced picture than previous studies and as such raise more questions and uncertainties about proposals for marine cloud brightening, at least through the prism of state-of-the-art climate models.
Annual global-means shortwave radiation imbalance atop the atmosphere (SWR TOA) from the fixed-sea surface temperature simulation (2020–2030) and the coupled simulation (2020–2090). The all-sky (All) radiation is decomposed into the conventional clear-sky (Clear) and the difference between all-sky and clear-sky (All − Clear) as well as into the Ghan (2013) components: the cloud radiative (Cloud), surface albedo (Surface), and direct radiative (Direct) effects.
As in Figure 1 but spatially averaged over 2020–2030 for AM4 and 2035–2065 for CM4, with hatched regions corresponding to statistically insignificant differences.
The regional (NP, SP, and SA) and global cloud properties and the Ghan radiative components.
Source: AGU
Geoengineering News
Poster's note: Reviewer 2 did a podcast on the paper: "The Radiative and Cloud Responses to Sea Salt Aerosol Engineering in GFDL Models."
Title of podcast: Does MCB actually work? MahfouzReviewer 2 does geoengineering
Some links to listen to the podcast:
https://podcasts.apple.com/us/podcast/does-mcb-actually-work-mahfouz/id1529459393?i=1000602560752
https://open.spotify.com/show/2KSB1lU18qh5gYIRDYPJMb
Description:
"Mahfouz explains some big problems with MCB to @geoengineering1. Can we fill in the knowledge gaps, before it's too late to use MCB? Paper: "The Radiative and Cloud Responses to Sea Salt Aerosol Engineering in GFDL Models" (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2022GL102340)"