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Refreezing the Arctic Region: What is Marine Cloud Brightening, MCB


Several routes for refreezing are being developed. One involves the manipulation of sea ice to increase the overall rate of growth  during the early winter. Two different approaches have been cited which have not received in-depth research: the breaking up of newly formed sea ice in the winter in order to increase the thickness of some areas whilst consequently exposing more sea water to cold air which could increase the overall rate of formation of ice whilst also providing zones of thicker ice which could potentially remain frozen over a complete summer; the spraying of sea-water onto the top of ice, thereby causing more ice to form. Both of these approaches require detailed modelling of saline water with ice and Cambridge has expertise in sea-ice assessment. Prof Grae Worster in the Department of Applied Mathematics has a wealth of experience in modelling of sea ice and will be supporting a review of ideas over the course of summer 2021 by a summer intern based at CCRC. The intern will also be guided by Dr. Peter Wadhams, one of the most experienced Arctic ice scientists who has been at Cambridge for many years and now joins us in the CCRC.

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MCB is a process developed by two British scientists, Latham and Salter, but not much progress has been made to date, although it has been trialled to shield the Great Barrier Reef. It has a number of characteristics which are potentially extremely important:  the process can be terminated within about two weeks if there were any negative consequences.  The material used to brighten clouds to reflect sunlight back into space are crystals of salt derived from evaporated droplets of seawater: the spraying of seawater up into the air, similar to the action of wind on the ocean surface, is relatively uncontroversial. There are a number of critical research questions which remain unanswered, and Cambridge is collaborating with Stephen Salter; he has worked in the area for a number of years and is a retired academic from Edinburgh. The effectiveness of MCB will depend upon whether the droplet size range can be relatively constrained – if they are too large then the wavelength scattering is ineffective since the clouds will be dark. The design of the nebulizer is critical and needs development. There are also challenges with regards to the design of anti-fouling of the overall system, requiring fluid dynamical design development to provide sufficient pressure variations to clear solid material from the facility and maintain flow. The development of the MCB technical system will be complemented by further modelling in collaboration with Alan Gadian (Leeds), Rob Wood (Washington) and Doug MacMartin (Cornell). Hugh Hunt has established relationships with Rob and Doug, and Stephen has introduced CCRC to Alan. The ship designed by Salter uses ocean currents to generate all the energy needed to spray salt water droplets into the atmosphere.  The design, similar to that used in the biggest ocean sailing races, needs to be fully established and a prototype constructed.  We plan to help to take the design to this stage and beyond.


We note that these approaches are very different from Solar Radiation Management, as developed particularly by Dr. David Keith in the USA (project SCOPEX).  This was also the subject of a research exercise by our Deputy Director, Dr. Hugh Hunt (project SPICE).  The potential negative impacts of adding aerosols to the stratosphere as a means of lowering global temperatures, including even a small likelihood of recreating ozone depletion layers in the stratosphere and of the loss of monsoons, for example in India, have led to a negative reaction to these experiments, let alone to upscaling the process.  The feasibility and possible low cost of this process does however offer the potential to reduce global temperatures at a lower cost: if other actions are not successfully carried soon out it is possible that at some time in the future humanity may depend on it, but currently field-scale experimentation is proving challenging due to the reaction of the public.