Physical science research needed to evaluate the viability and risks of marine cloud brightening

Abstract

Marine cloud brightening (MCB) is the deliberate injection of aerosol particles into shallow marine clouds to increase their reflection of solar radiation and reduce the amount of energy absorbed by the climate system. From the physical science perspective, the consensus of a broad international group of scientists is that the viability of MCB will ultimately depend on whether observations and models can robustly assess the scale-up of local-to-global brightening in today's climate and identify strategies that will ensure an equitable geographical distribution of the benefits and risks associated with projected regional changes in temperature and precipitation. To address the physical science knowledge gaps required to assess the societal implications of MCB, we propose a substantial and targeted program of research-field and laboratory experiments, monitoring, and numerical modeling across a range of scales.

Fig. 1.. Marine cloud brightening proposals using ship-based generators.

Aerosol particle generators would ingest seawater and produce fine aerosol haze droplets with an equivalent dry diameter of approximately 50 nm. In optimal conditions, many of these haze droplets would be lofted into the cloud by updrafts, where they would modify cloud microphysics processes, such as increasing droplet number concentrations, suppressing rain formation, and extending the coverage and lifetime of the clouds. At the cloud scale, the degree of cloud brightening and surface cooling would depend on just how effectively the droplet number concentrations can be increased, droplet sizes reduced, and cloud amount and lifetime increased. On the left are shown details of the key aerosol, cloud, dynamics, and radiation processes in the marine boundary layer that are the foundation of MCB in shallow liquid clouds that reside close to the Earth’s ocean surface (104). The strong coupling between these processes presents interesting challenges and opportunities for understanding the outcomes of seawater haze injections into these clouds.

Fig. 2.. Primary microphysical pathways between system variables in response to marine cloud brightening.

Blue arrows indicate pathways along which clouds are optimally brightened and red arrows indicate counterproductive pathways that offset cloud brightening. Gold text boxes indicate the processes that drive the changes in variables. Plus (+) and minus (−) signs indicate the expected response of the receiving variables. The separation of cloud microphysics (i.e., “drop concentration”) and cloud macrophysics (“cloud water and cloud fraction”) represents the impact of injected aerosol directly via aerosol-cloud interactions and cloud adjustments. The two competing pathways between drop concentration and cloud water and cloud fraction reflect the documented possibility of both desirable (suppression of drop coalescence; blue) and undesirable (droplet evaporation; red) responses. The desirable pathway is characterized by increased drop concentration, larger cloud water and cloud fraction, and, if some precipitation does fall and evaporate below the cloud, resuspension of aerosol into the atmosphere. The undesirable pathway suffers from drop evaporation, precipitation, and removal of aerosol to the surface—all of which reduce drop concentration, cloud water, and cloud fraction. Note that all of these microphysical processes act in clouds, and a major challenge would be to seed in optimal conditions and with optimally sized aerosol particles so as to enhance brightening.

Fig. 3.. How MCB fits into the atmospheric component of the Earth system.

The MCB box subsumes processes and pathways as depicted in Figs. 1 and 2. Gold boxes indicate the changes (Δ) in key variables along the pathways indicated. MCB modifies the radiation budget by increasing aerosol emissions, albeit in a very targeted manner, potentially offsetting some of the radiative effects of greenhouse gasses. Radiation influences and responds to the atmospheric/oceanic state (Δ in temperature, pressure, and circulation), which together with changes in the background aerosol, sets the stage for changes in cloud susceptibility and the potential for MCB cooling. The feedback loop between radiation and the atmospheric/oceanic state illustrates how MCB might influence regional temperature and precipitation patterns.

Fig. 4.. An integrated approach to a marine cloud brightening research program comprising laboratory facilities, field experiments, and modeling.

Laboratory facilities such as cloud chambers together with observations at a range of scales will help improve the representation of physical processes in models. Parcel models, large eddy models, and cloud-resolving models will inform the global model activities to improve the reliability of regional climate responses. The Earth view image is courtesy of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT).