Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Abstract

Abstract. A simple heuristic model is described to assess the potential for increasing solar reflection by augmenting the aerosol population below marine low clouds, which nominally leads to increased cloud droplet concentration and albedo. The model estimates the collective impact of many point source particle sprayers, each of which generates a plume of injected particles that affects clouds over a limited area. A look-up table derived from simulations of an explicit aerosol activation scheme is used to derive cloud droplet concentration as a function of the sub-cloud aerosol size distribution and updraft speed, and a modified version of Twomey's formulation is used to estimate radiative forcing. Plume overlap is accounted for using a Poisson distribution, assuming idealized elongated cuboid plumes that have a length driven by aerosol lifetime and wind speed, a width consistent with satellite observations of ship track broadening, and a depth equal to an assumed boundary layer depth. The model is found to perform favorably against estimates of brightening from large eddy simulation studies that explicitly model cloud responses to aerosol injections over a range of conditions. Although the heuristic model does not account for cloud condensate or coverage adjustments to aerosol, in most realistic ambient remote marine conditions these tend to augment the Twomey effect in the large eddy simulations, with the result being a modest underprediction of brightening in the heuristic model. The heuristic model is used to evaluate the potential for global radiative forcing from marine cloud brightening as a function of the quantity, size, and lifetime of salt particles injected per sprayer and the number of sprayers deployed. Radiative forcing is sensitive to both the background aerosol size distribution in the marine boundary layer into which particles are injected and the assumed updraft speed. Given representative values from the literature, radiative forcing sufficient to offset a doubling of carbon dioxide ΔF2×CO2 is possible but would require spraying 50 % or more of the ocean area. This is likely to require at least 104 sprayers to avoid major losses of particles due to near-sprayer coagulation. The optimal dry diameter of injected particles, for a given salt mass injection rate, is 30–60 nm. A major consequence is that the total salt emission rate (50–70 Tg yr−1) required to offset ΔF2×CO2 is a factor of five lower than the emissions rates required to generate significant forcing in previous studies with climate models, which have mostly assumed dry diameters for injected particles in excess of 200 nm. With the lower required emissions, the salt mass loading in the marine boundary layer for ΔF2×CO2 is dominated by natural salt aerosol, with injected particles only contributing ∼ 10 %. When using particle sizes optimized for cloud brightening, the aerosol direct radiative forcing is shown to make a minimal contribution to the overall radiative forcing.

Simulating Aerosol Lifecycle Impacts on the Subtropical Stratocumulus‐to‐Cumulus Transition Using Large‐Eddy Simulations

“…Based on theoretical analyses from previous studies (e.g., Diamond et al, 2020;Wood, 2021), we decomposed the contributions of the Twomey effect and cloud adjustments to albedo and SW CRE. For both the L06 and L10 cases an increase in aerosols relative to the baseline case leads to an increase in the SW CRE due to the Twomey effect.…”

The Shortwave Radiative Flux Response to an Injection of Sea Salt Aerosols in the Gulf of Mexico

“…The present study is designed to uniquely examine the TOA shortwave radiative response of MCB and MSB in the Gulf of Mexico; nonetheless it is useful to compare these results with those of Wood (2021) and other studies (e.g., Ahlm et al, 2017;Alterskjaer et al, 2012;Latham et al, 2008;Stjern et al, 2018) that show that MCB can be employed to offset the radiative forcing consistent with a doubling of CO 2 (2xCO 2 ) from the pre-industrial level (3.7 W m −2 , Ramaswamy et al, 2001). Specifically, Wood (2021) details a pathway to offset the global 2xCO 2 forcing that requires about 9% of the ocean to be seeded with Aitken mode particles to achieve a seven-fold increase in CDNC in the perturbed clouds and a corresponding increase in cloud albedo of 0.16.…”

“…Previous studies show that the radiative perturbations from sea salt injection are sensitive to the mean diameter of the injected particles as well as the amount (e.g., Connolly et al, 2014;Partanen et al, 2012;Wood, 2021). To evaluate the diameter sensitivity, we design four experiments to compare to a control simulation (CTRL), one in which the additional sea salt flux contains Aitken mode particles (with a diameter distribution between 39 and 156 nm, EXP_AIT), one with accumulation mode particles (between 156 and 625 nm, EXP_ACC), one with large accumulation mode particles (between 625-2,500 nm, EXP_ACC_LG), and one with coarse mode particles (between 2,500-10,000 nm, EXP_COARSE).…”

“…Collectively, these studies show the potential of MCB to offset, in full or partial, the global mean radiative forcing consistent with a doubling of CO 2 from the pre-industrial levels (3.7 W m −2 , Ramaswamy et al, 2001). Wood (2021) details multiple pathways to offset the radiative forcing from a doubling of CO 2 levels by deriving an equation that is a function of the fraction of ocean to be seeded, the unperturbed cloud albedo, the change in cloud albedo, and the ratio of cloud droplet number concentration (CDNC) between the perturbed and unperturbed clouds. Wood (2021) discusses a scenario involving seeding the full fraction of the ocean where low clouds are present and are approximately 1,000 km away from land (about 18% of the ocean surface) to achieve a CDNC ratio of about 3 and a change in cloud albedo of about 0.09, with respect to an unperturbed cloud with an albedo of 0.50.…”

“…Wood (2021) details multiple pathways to offset the radiative forcing from a doubling of CO 2 levels by deriving an equation that is a function of the fraction of ocean to be seeded, the unperturbed cloud albedo, the change in cloud albedo, and the ratio of cloud droplet number concentration (CDNC) between the perturbed and unperturbed clouds. Wood (2021) discusses a scenario involving seeding the full fraction of the ocean where low clouds are present and are approximately 1,000 km away from land (about 18% of the ocean surface) to achieve a CDNC ratio of about 3 and a change in cloud albedo of about 0.09, with respect to an unperturbed cloud with an albedo of 0.50. Seeding only half of this ocean area requires the CDNC ratio to be about 7 and a change in cloud albedo of about 0.16, with respect to an unperturbed cloud with an albedo of 0.50.…”