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 widely-used 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 resulting 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 ΔF2xCO2 is possible but would require spraying over 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) required to offset F2xCO2 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 F2xCO2 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.

Global reduction in ship-tracks from sulfur regulations for shipping fuel

“…In addition to explicitly forming detectable ship-tracks, ship-emitted aerosols can also affect clouds in other ways. For example, as ship-tracks evolve and dissipate with time, they appear as cloudy pixels that are hard to separate from background clouds ( 2 , 15 , 22 ). Also, when background clouds are polluted, detection of ship-tracks becomes less likely, although ship-emitted aerosols still affect clouds.…”

“…The aerosol indirect forcing partially counterbalances the radiative forcing caused by greenhouse gases. Existing estimates of aerosol indirect forcing from ship-emitted aerosols range from -0.06Wm -2 to -0.6Wm -2 (10)(11)(12)(13)(14)(15). In addition to directly contributing to aerosol indirect forcing, ship-tracks have also been studied to understand aerosol indirect effects in general because they are idealized laboratories where aerosol effects can be clearly separated from effects of meteorology and other factors (16).…”

“…A strongly negative aerosol indirect forcing implies a climate that is highly sensitive to forcing by greenhouse gases as it makes the observed climate change the outcome of a small net positive forcing. In addition, ship-tracks can be viewed as inadvertent cloud brightening geoengineering experiments whose better understanding is necessary to consider deliberate experiments ( 4 , 15 , 18 ).…”

Cloud Microphysical Implications for Marine Cloud Brightening: The Importance of the Seeded Particle Size Distribution

“…Marine cloud brightening (MCB) is a geoengineering approach to increasing the reflection of incident shortwave radiation by enhancing the albedo and lifetime of clouds, via the seeding of additional particles into these clouds (Latham and Smith 1990;Latham et al 2012). Marine stratocumulus are the primary target for such an approach since they cover consistently wide swaths of the subtropical oceans, providing ample potential for MCB (e.g., Wood 2012). Macroscopically, MCB can be understood as an attempt to increase the relative cloud radiative effect (rCRE), that is, the fraction of shortwave radiation reflected by clouds (Betts 2007).…”

“…While the first three terms on the right-hand side represent potential microphysical impacts on MCB, which are of primary importance for this study, the last term represents the saturation of A c for t g, which indicates that additional brightening of already very reflective clouds is ineffective. Note that this term is usually negligible for shallow clouds like the marine stratocumulus targeted in MCB, for which A c , 0.5 (e.g., Wood 2012). [Formally, this is equivalent to the approximation A c ' t/g, due to which the d ln(rCRE)/d ln(N aero )j saturation term vanishes].…”